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In situ detection of stability limit of ω phase in Ti–15Mo alloy during heating
Abstract
Phase transitions in a single crystal of a metastable β-titanium alloy (Ti-15Mo) were investigated in situ during heating by synchrotron X-ray diffraction. The results were compared with previous measurements of electrical resistance. Single-crystalline samples allowed different crystallographic families of ω-Ti and α-Ti phases to be distinguished. The observed evolution of the intensity of the reflections of the ω phase during heating is consistent with the evolution of electrical resistance, which proves that the resistance is affected by the presence of ω-phase particles. Between approximately 673 and 833 K, both the resistance and the intensity of ω peaks sharply decrease. At 833 K, ω reflections disappear, indicating a complete dissolution of the ω phase due to achieving the solvus temperature of the ω phase in the Ti–15Mo alloy. The synchrotron X-ray diffraction experiment proved that the disappearance of the ω phase during heating of Ti–15Mo with a heating rate of 5 K min ⁻¹ occurs by its dissolution back to the β phase and not by ω → α transformation.
... On the other hand, the employment of high energy X-ray diffraction (HEXRD) available at synchrotron facilities can overcome these drawbacks. Previous works have shown the potential of HEXRD for the investigation of phase transformations in Ti-alloys [18][19][20][21][22]. To the best of our knowledge, phase transformations in UFG β-Ti alloys have not been investigated yet by HEXRD. ...
... The evolution of phase fractions evaluated from the Rietveld fit for the non-deformed and the HPT-deformed condition is shown in Fig. 4(a) and (b), respectively. The evolution of phase content in Ti15Mo alloy during heating has been studied in several previous studies [22,26,27,40,41]. Initial decrease of the content of ω ath phase is followed by significant increase of content of ω isothermal (ω iso ) reaching a maximum at around 450 °C. ...
... Initial decrease of the content of ω ath phase is followed by significant increase of content of ω isothermal (ω iso ) reaching a maximum at around 450 °C. Upon further heating, the volume fraction of ω iso decreases and finally, ω iso phase dissolves at 560 °C [22] -the dissolution is represented by dashed line in Fig. 4 similarly to Fig. 1. α phase peaks could be resolved just before the dissolution of ω phase and the maximum α phase content of approx. 15% is observed at 700 °C. ...
A metastable β solution treated Ti15Mo alloy was deformed by high pressure torsion (HPT) resulting in a severely deformed microstructure with high density of lattice defects. In order to gain insight into the kinetics of phase transformations, both non-deformed and HPT-deformed materials were studied in-situ during linear heating; phase evolution was investigated using high energy synchrotron X-ray diffraction (HEXRD) complemented by the measurement of electrical resistance. It was shown that in the non-deformed material the dissolution of the ω phase is followed by precipitation of the α phase during linear heating. In contrast, in the HPT-deformed material the nucleation of the α phase is shifted to lower temperatures, resulting in the coexistence of all three β, ω, and α phases at ~ 550 °C. Moreover, in HPT-deformed samples, the growth of α particles is accelerated due to high density of dislocations and grain boundaries. Post-mortem observations of selected samples revealed that the microstructure of the HPT-deformed material after heating up to 650 °C remains ultra-fine grained with equiaxed grains of α phase.
... It is widely accepted that these particles can influence the nucleation of the α phase during subsequent heat treatments, thereby providing a method for microstructural control [9]. However, the exact mechanism of α phase formation is still not well understood [10][11][12], and some of the most recent works have suggested that α phase nucleation may, in certain heating regimes, be only indirectly influenced by the ω phase or completely independent of it [6,13,14]. ...
... Let us now discuss in more detail the kinetics of phase transformations during the slower heating rate of 1.9 • C/min. ND patterns demonstrating the evolution of Ti-15Mo during heating and cooling are presented in Figure 2. The material at room temperature consists of β phase matrix and ω particles [13], although ω peaks are wide at this temperature (see the ω peaks in Figure 1a) due to the small size of the ω particles [14]. The presence of Nb peaks in Figure 2 is due to the Nb sample holder, as was explained in Section 2. The peaks of the ω phase begin to sharpen during heating around 300 • C, which is caused by coarsening of ω particles. ...
A transformation pathway during thermal treatment of metastable β Ti-15Mo alloy was investigated by in situ neutron diffraction. The evolution of individual phases α , β , and ω was investigated during linear heating with two heating rates of 1.9 ∘ C / min and 5 ∘ C / min and during aging at 450 ∘ C . The results showed that with a sufficient heating rate (5 ∘ C / min in this case), the ω phase dissolves before the α phase forms. On the other hand, for the slower heating rate of 1.9 ∘ C / min , a small temperature interval of the coexistence of the α and ω phases was detected. Volume fractions and lattice parameters of all phases were also determined.
... Crucibles with salt baths were used to maintain a more stable temperature. In Timetal LCB, the solvus of the ω phase (ω solv ) is approximately 500 • C, as demonstrated in our previous research using electric resistance measurements [39]; the ω solvus manifests itself as a sharp change in the dependence of resistance on temperature, and was also confirmed by in situ x-ray diffraction [40]. The two lower ageing temperatures used in this study lie below ω solv ; and therefore, ω particles evolution may be expected during the early stages of ageing, especially at 460 • C and to a limited extent at 490 • C. ...
Phase transformations in a metastable beta Zr–12Nb alloy were investigated by high-energy X-ray diffraction (HEXRD) measured simultaneously with thermal expansion in situ during linear heating from room temperature to 800 °C. Complementary in-situ methods of electrical resistance and differential scanning calorimetry, which were performed using the same heating conditions as in the HEXRD experiment, provided additional information on the transformation sequence occurring in the Zr–12Nb alloy. Two bcc phases with a different lattice parameter, βZr and βNb, were observed in the investigated temperature range and identified using the phase diagram of the Zr–Nb system. In the initial solution-treated condition, metastable βZr phase and athermal ω particles are present in the material. At about 300 °C, Nb-rich βNb phase starts to form in the material and the original βZr phase gradually disappears. Ex-situ observations of the microstructure using transmission electron microscopy revealed a cuboidal shape of the ω particles, which is related to a relatively large misfit between the ω and β phases. At 560 °C, ω solvus was observed, identified by an abrupt dissolution of ω particles which was followed by growth of the α phase.
Ti-8.5Mo (wt%) alloy was prepared by laser directed energy deposition (L-DED) from the mixture of cp-Ti and Ti-15Mo powder in the ratio of 50:50 using constant laser power of 600 W. Microstructure of as-printed material was characterized by microhardness mapping, light and scanning electron microscopy employing EDS technique and local X-ray diffraction measurements. In particular, the influence of the baseplate preheating to 500 °C on microstructure evolution was determined. The microstructure of the as-deposited samples consists of columnar grains orientated parallel to the building direction. Volume fraction of α, β and ω phases, and consequently also microhardness, vary for different positions within the sample due to different thermal history during the printing process. Bottom part of the sample contains increased fraction of α phase due to longer exposure to temperatures favorable for α phase precipitation. Preheating of the baseplate provides additional thermodynamic stabilization of the α phase. Numerical model, predicting the temperature evolution in each layer of the sample during deposition and consequently the ultimate phase composition, was developed. The results obtained from the model are consistent with the experimentally observed phase transformations.
The formation of non-equilibrium and transient phases in metastable beta titanium alloys during low temperature thermal treatments is currently of great interest, as they provide a potential method of controlling the size and distribution of the equilibrium alpha phase and, hence, the resulting mechanical properties. Here, for the first time, we report on the formation of a new, B2 structured phase in the Ti-Mo system. The phase was observed in electron transparent material during in situ, and following ex situ, heat treatment at 300 °C. The B2 phase was enriched in Mo compared to the surrounding matrix material and formed in regular arrays of approximately square cross-section particles interspersed by thin beta channels. Electron diffraction indicated that the lattice parameter of this new phase was smaller than that of the parent phase, leading to significant strain in the beta channels. Critically, the B2 phase was only observed in material that had been electro-polished prior to heat treatment, and, therefore, it is hypothesised that this phase forms as a result of the preparation method and thin foil effects.
Evolution of \(\omega \) phase during heating of metastable \(\beta \) titanium alloy Ti–15Mo was investigated in situ during heating by electrical resistance measurements and accompanied by transmission electron microscopy. Different heating rates were employed aiming to determine kinetics of occurring phase transformations. Sharp change of temperature dependence of electrical resistance caused by complete dissolution of \(\omega \) phase was observed at 560 \(^{\circ }\hbox {C}\) independently of heating rate. Majority of \(\omega _{\mathrm{iso}}\) particles revert back to \(\beta \) phase at 560 \(^{\circ }\hbox {C}\); therefore, they are not direct precursors of \(\alpha \) precipitation during continuous heating.
The completeness of the β→ω transformation in ω particles in a Ti–8 at.%Mo (Ti–15 wt%Mo) single crystal was investigated by measuring the X-ray diffraction maximum 20{\overline 2}2, which is forbidden in both the pure body-centred cubic β phase and the hexagonal ω phase, and also the diffraction maxima 0001, 0002 and 10{\overline 1}1, which are forbidden in the β phase and allowed in ω. From a comparison of the integrated intensities and widths of the diffraction peaks with simulations, the effective (mean) degree of the transformation was determined and the radial profile of the transformation degree in an ω particle was estimated.
Nucleation and growth kinetics of nanoparticles of hexagonal omega phase in a body-centered cubic beta titanium matrix in single crystals of beta-Ti alloys were investigated by small-angle X-ray scattering measured in situ during ageing at various temperatures up to 450 °C. The experimental data were compared with numerical simulations based on a three-dimensional short-range order model of nanoparticle self-ordering. The X-ray contrast of the particles is caused by an inhomogeneous distribution of impurity atoms (Mo, Fe and Al), whose density profile around growing nanoparticles was simulated by solving the corresponding diffusion equation with moving boundary conditions. From the analysis of the experimental data we determined the mean distance and size of the nanoparticles and confirmed the validity of the t.^(1/3) growth law following from the Lifshitz–Slyozov–Wagner theory. From a detailed comparison of the experimental data with simulations we also assessed the diffusion coefficient of the impurity atoms and its activation energy.
The effect of heating rate on the phase transformation kinetics of a Ti-10V-2Fe-3Al metastable beta titanium alloy quenched from the beta field is investigated by fast in situ high energy synchrotron X-ray diffraction and differential scanning calorimetry. The initial microstructure is formed by alpha aEuro(3) martensite and fine omega(ath) particles distributed in the retained beta-phase matrix. The phase transformation sequence varies with the heating rate as revealed by analysis of the continuous evolution of crystallographic relationships between phases. At low temperatures an athermal reversion of alpha aEuro(3) martensite into beta takes place. This reversion occurs to a larger extent with increasing heating rate. On the other hand, diffusion-driven precipitation and growth of the omega phase is observed for lower heating rates accompanying the reverse martensitic transformation. Furthermore, the results show that the stable alpha phase can form through three different paths: (a) from the omega phase, (b) from alpha aEuro(3) martensite, and (c) from the beta phase.
Nanosized particles of ω phase in a β-Ti alloy were investigated by small-angle X-ray scattering using synchrotron radiation. We demonstrated that the particles are spontaneously weakly ordered in a three-dimensional cubic array along the 〈100〉-directions in the β-Ti matrix. The small-angle scattering data fit well to a three-dimensional short-range-order model; from the fit we determined the evolution of the mean particle size and mean distance between particles during ageing. The self-ordering of the particles is explained by elastic interaction between the particles, since the relative positions of the particles coincide with local minima of the interaction energy. We performed numerical Monte Carlo simulation of the particle ordering and we obtained a good agreement with the experimental data.
Because of their unique properties of high strength, excellent hardenability and good corrosion resistance beta titanium alloys have been the subject of much attention. Recently, since cost is an increasingly important parameter, a LCB (low cost beta) titanium alloy(Ti6.8Mo-4.5Fe-1.5A1) has been developed to be used in non-aerospace new technology as high performance substitutes for more classical material. Metastablefl Ti alloys are typically known to precipitate additional phases (fi and! phases) during thermomechanical treatments. The morphology, size and distribution of these precipitates determine in large part the mechanical properties of the alloy [1]. It is known that! precipitates can be formed during quenching by a diffusionless martensitic mechanism (!ath) and during aging by a diffusion controlled process (!iso) [2]. Nevertheless, the parameters allowing the control of !iso phase crystals density are still unclear [3]. Therefore, it seems necessary to control the appearance of ! phase particles infl titanium alloys and particularly to understand the nucleation mechanism. The direct interest of such a knowledge is crucial since it was recently reported that the!iso precipitates could behave as nucleation sites of an extremely fine and dispersedfi phase at intermediate aging temperatures leading to higher yield strength (flCfi) duplex alloys [4‐6]. In this present paper a method based on an experimental new approach of the fl!! transformation is described. This method could allow an experimental control of!iso dispersion. Thefl phase decomposition during heating was studied on Ti-6.8Mo-4.5Fe-1.A1 water quenched from thefl field (1123 K). This method was developed by studying the electrical resistivity changes of samples during subsequent heat treatments.
The temperature coefficient of resistivity (TCR) of beta stabilized titanium alloys is sensitive to minor constitutional changes produced by quenching, ageing and precipitate reversion. The influence of omega and alpha phase precipitation on TCR is demonstrated for binary and ternary alloys of composition Ti80(Nb + V)20. It is concluded that TCR measurements provide a sensitive method for detecting the early stages of beta phase decomposition in these alloys.
The metastable beta Ti-6Mo-5Ta-4Fe (wt.%) alloys was synthesized by cold crucible levitation melting and then quenched in water from the beta phase field. In order to investigate the transformation sequence upon heating, thermal analysis methods such as electrical resistivity, dilatometry and neutron thermodiffraction were employed. By these methods, the different temperatures of transition were detected and solute partitioning was oberved to the beta matrix during the omega and alpha nanophase precipitation
Now in paperback, this comprehensive book is the first text devoted to the problem of understanding the electrical properties of metals and alloys. Dr Rossiter, well-known for his work on the electrical resistivity of alloys, has written a book which blends results and theory, but does not rely on a strong grounding in quantum mechanics. After an introduction to the basic ideas, the concepts of atomic and magnetic correlations and their microstructural consequences are explained. Later chapters then deal with the effects of such correlations on electrical resistivity. Examples and applications of the concepts derived are given in discrete sections, allowing the uninterrupted development of theory for each specific problem, and enhancing the value of the book for a wide range of readers from theoretical and experimental solid state physicists to metallurgists and materials scientists. Anyone with an interest in the electrical conduction process or in the application of resistivity measurements to the study of alloy configuration will find this essential reading.
The decision of Springer-Verlag to publish this book in English came as a pleasant surprise. The fact is that I started writing the first version of the book back in 1978. I wished to attract attention to potentialities inherent in selected-area electron diffraction (SAED) which, for various reasons, were not being put to use. By that time, I had at my disposal certain structural data on natural and synthetic minerals obtained using SAED and high-resolution electron microscopy (HREM), and this stimulated my writing this book. There were several aspects concerning these data that I wished to emphasize. First, it was mostly new and understudied minerals that possess the peculiar structural features studied by SAED and HREM. This could interest mineralogists, crystallo chemists, and crystallographers. Second, the results obtained indi cated that, under certain conditions, SAED could be an effective, and sometimes the only possible, method for structure analysis of minerals. This inference was of primary importance, since fine dispersion and poor crystallinity of numerous natural and synthe tic minerals makes their structure study by conventional diffrac tion methods hardly possible. Third, it was demonstrated that in many cases X-ray powder diffraction analysis of dispersed miner als ought to be combined with SAED and local energy dispersion analysis. This was important, since researchers in structural min eralogy quite often ignored, and still ignore even the simplest in formation which is readily available from geometrical analysis of SAED patterns obtained from microcrystals.
Metastable β titanium alloy Ti-15Mo was investigated in this study. In-situ electrical resistance and thermal expansion measurements conducted on solution treated material revealed influence of ongoing phase transitions on measured properties. The monotonicity of the dependence of electrical resistance on temperature changes at 225, 365 and 560 °C The thermal expansion deviates from linearity between 305 and 580 °C.
Elastic constants of single crystals of metastable β-titanium alloys with particles of both athermal and isothermal ω phase were studied by ultrasonic methods in the temperature range of 0 °C–180 °C. It was shown that the presence of these particles leads to stiffening and isotropization of the material and has also a significant impact on the internal friction. Athermal ω particles were shown to influence the internal friction more strongly than isothermal ones; on the other hand, the presence of the isothermal ω particles in the material leads to stronger stiffening of the elastic constants and also to their stronger temperature dependence. The effective isotropic elastic constants of the isothermal ω particles were calculated from the comparison of elastic constants of samples with different volume fractions of these particles.
A metastable nano-scale disordered precipitate with orthorhombic symmetry has been identified using high resolution scanning transmission electron microscopy. The phase, termed O′, is metastable, formed by a shuffle mechanism involving a {110}<1-10> transverse phonon wave in samples of Ti–26Nb–2Zr (at.%) quenched from the β phase. The addition of 2% Zr to Ti–26Nb appears to suppress significantly the stability of both the {11-2}<111> shear and 2/3 <111> longitudinal phonon wave but promotes the {110}<1-10> transverse shuffle. This results in the nano-size O′ phase being homogeneously formed in the parent β phase matrix rather than the massive α′′ phase.
A previously unidentified, nanometer scale ordered precipitate has been characterized in the metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr (Ti-5553) using conventional transmission electron microscopy (TEM) and high-angle annular dark field (HAADF) high-resolution scanning transmission electron microscopy (HRSTEM). The phase, termed Oʺ, is metastable, the ordering transformation occurring over an intermediate temperature range (e.g., 375 °C), and this ordered face centered orthorhombic precipitate may be observed co-existing with isothermal ω and α phases in the alloy. It is also noted that this transient phase may provide the diffracted intensities observed from another phase, identified tentatively in the literature as orthorhombic martensite, αʺ [1].
The results of resistivity measurements on Ti-Mo alloys of
concentrations 5-20 at.% Mo are discussed. The experimentally determined
resistivity concentration dependence, which exhibits a pronounced local
maximum near 8 at.% Mo, is compared with some calculated curves, derived
with the aid of previously determined Fermi-density-of-states
[n(EF)] and Debye-temperature (ΘD) data. On
the assumption that the total resistivity is the sum of contributions
due to thermal scattering (the ideal resistivity ρi) and
solute scattering (ρs), the measured resistivity in the
concentration range 5-15 at.% Mo is shown to be anomalously high. For a
quenched alloy of composition near the middle of the above range, viz.,
Ti-Mo (10 at.%), the temperature dependence of resistivity (dρdT) is
negative between 4 and 480 K. But at higher temperatures dρdT is
positive, and after being aged at 620 K the alloy assumes a normal
positive dρdT over the full temperature range of 4-620 K. As a
result of this investigation it is deduced that the anomalous magnitude,
composition dependence, and temperature dependence are all associated in
one way or another with an ω-phase precipitate (a small fraction
of which is reversible or "athermal") occurring in the brine-quenched
alloys within the composition range 5-15 at.% Mo. It is concluded,
however, that the observed resistivity effects do not derive from
intrinsic physical properties of the ω phase itself, but are due
instead to electronic scattering from the interfaces between the
precipitate particles and the matrix. Interfacial scattering associated
with the irreversible (isothermal) ω phase is responsible for the
anomalous isothermal resistivity, while the relatively small athermal
ω-phase component gives rise to the negative dρdT exhibited by
Ti-Mo (10 at.%).
The general features, terminology, and method of the dynamical theory of X-ray diffraction are discussed, stressing the analogy with the general theory of small oscillations of a mechanical system.
Understanding the origin of electrical properties of alloys is critical to the development of new materials. Dr. Rossiter blends theoretical and experimental results without relying on detailed quantum mechanics. After introducing the basic concepts of atomic and magnetic correlations, he explains their microstructural consequences. Later chapters deal with the effects of such correlations on electrical resistivity. Examples and applications are given in discrete sections, which allow the uninterrupted development of theory for each specific problem.
The ω phase is commonly observed in many commercial β or near-β titanium alloys on rapidly cooling from the single β phase field and also during subsequent isothermal annealing. However, the crystallographic formation mechanism for the omega particles is hitherto unclear/under discussion. The present study primarily focuses on ω precipitation within the β (body-centered cubic or bcc) matrix of simple model binary titanium-molybdenum (Ti-Mo) alloys. It provides direct experimental evidence of the formation of ω-like embryos from competing compositional and structural instabilities arising in the bcc lattice of titanium-molybdenum alloys during rapid cooling from the high temperature single β phase field. The displacive partial collapse of the {111} planes of the parent bcc structure within compositionally phase-separated regions containing several atom percent less of Mo, forming ω-like embryos, has been conclusively shown by coupling aberration-corrected high-resolution scanning transmission electron microscopy with atom probe tomography observations. Growth and coarsening of these ω-like embryos take place during subsequent isothermal annealing, accompanied with both a completion of the collapse of the {111} β planes leading to a fully-developed ω structure as well as rejection of Mo from these precipitates leading to near-equilibrium compositions.
The influence of the specimen shape on the electrical resistivity measurements was investigated both theoretically, by means of finite element method modelling, and experimentally, by measuring electrical resistivity of samples of variable shapes. Precision of electrical resistivity measurements using a four-point method for samples of arbitrary shape was critically reviewed.
There has been significant progress in regard to the effect of stability and grain size of the beta phase on deformation modes and mechanical properties of beta titanium alloys. For example, it has been shown recently, that an increase in the stability of the beta phase reduces the formation of stress-induced plates thereby decreasing the ambient temperature creep strain. Increase in the stability appears to favor deformation by slip. The grain size of the beta phase has also been shown to effect the deformation modes in tensile deformation and the extent of ambient temperature creep strain. In this paper the interrelationships between stability, microstructure, deformation modes and mechanical properties of beta titanium alloys will be critically reviewed.
Les alliages Ti 16% V, Ti 29% V, Ti 13% Mo et Ti 13% Cr forment tous la phase ω. Une comparaison des intensités des raies de diffraction des rayons X montre que la structure ω est, dans chacun de ces alliages, la même que dans l'alliage Ti 16% V.(1)
La formation de cette phase est réversible et peut être retardée par déformation à froid. Après une telle déformation, les quatre orientations de ω se développent indifféremment. Un échantillon formé des phases β + ω et α en traces, par vieillissement à haute température, contient des proportions inégales des quatre orientations de ω: ceci prouve que ω est hexagonale et non cubique. L'écart de planéité du plan central proposé par Bagaryatskii (2) est probablement inférieur à 0,005; les positions atomiques pourraient être 0 0 0— 0,495 — 0,505 au lieu de 0 0 0—— .
La formation de a est accélérée par déformation à froid et elle apparaît alors suivant un nombre limité d'orientations.
La précipitation de a est la raison du durcissement de l'alliage de Ti 29% V vieilli à 460°C. Au cours d'essais d'attaque de cristaux minces dans l'acide phosphorique, les structures β + ω et β + α se sont transformées en β + hydrure.
This review contains a comprehensive examination of all modern measurements and calculations of the temperature-dependent electrical resistivity ρ(T) for the alkali metals—and especially potassium (K)—from their melting points down to below 0.1 K. The simplicity of the electronic structures of these metals makes them unique for testing our fundamental understanding of ρ(T). At all temperatures down to a few K, ρ(T) is dominated by its electron-phonon scattering component, ρep(T). Current quantitative understanding of ρep(T) in the alkali metals is examined in detail, including effects of phonon drag at temperatures below ≃ 10 K. In the vicinity of 1 K, ρ(T) in pure, unperturbed, bulk alkali metals is predicted to be dominated by electron-electron scattering. ρee(T)=AeeT2. In disagreement with previous reviews, the authors argue that Aee is nearly constant for each alkali metal and—at least for K—also in quantitative agreement with calculation. Below 1 K, alloys based on K and lithium display both previously predicted and completely unexpected effects. Perturbations such as deformation and thinning of K wires induce unusual and interesting behaviors. An unexpected Kondo-like effect appears when K contacts polyethylene. Charge-density-wave-based predictions of contributions to ρ(T) in the alkali metals are also considered. Three appendices examine (a) what is involved in a realistic calculation of ρ(T); (b) the experimental problems encountered in high-precision measurements of ρ(T) at low temperatures and how they are solved; and (c) the most recent experimental data concerning charge-density waves in the alkali metals.
The kinetics of the formation of the omega phase in the Beta III alloy is followed during aging at 400°C after quenching from 825°C. Electron diffraction patterns indicate that the omega phase forms during the quench. The aging kinetics indicate that the omega particles that form on quenching grow to form ellipsoidal shaped particles during aging. After aging 49 hrs at 400°C the omega phase makes up greater than 90% of the volume of the material. During the first 20 min of aging a heavy grain boundary precipitate is formed which continues to grow as aging progresses.
The� (BCC) -> �(HCP)+�(BCC phase transformation kinetics of the metastable titanium alloy Ti17 was studied by in situ high energy X-ray diffraction (XRD) and electrical resistivity. The use of synchrotron radiation (E=89.12 keV,$lambda$=0.01402 nm) allowed to characterize specimens 6 mm in diameter, limiting thus oxidation effects. The� �phase transformation kinetics was characterized during continuous heating up to the� temperature range. The� �transformation was studied in isothermal condition(dwell at 708r}C or 750rC) and during continuous cooling(cooling rates of 0.05rC/s, 0.1r{C/s and 0.61rC/s). The evolutions of the phase fractions are analyzed and compared to kinetics measured by electrical resistivity, showing a good correlation between both methods. In addition, the evolutions of the cell parameters of each phase obtained from XRD patterns are analyzed in regard of the transformation kinetics.
The synthesis and phase transformation behaviors of six new β Ti alloys containing Ta, Mo, and Fe biocompatible elements were studied. The alloys had been synthesized by arc-melting furnace. The alloys were then quenched in water from the beta phase fields. The different phase transitions upon heating were detected by employing Electric Resistivity Measurements (ERM) and Differential Scanning Calorimetry (DSC). The thermally activated nanostructure was observed by using Transmission electron Microscopy (TEM). The precipitation of α inter in intra granular phase formation were successively detected upon heating from room temperature to the beta transus. The alloys can be classified as beta-metastable since different phase transformation are detected both by ERM and DSC. A coherent pseudo-binary phase diagram was proposed to explain the different phase transitions behavior upon heating.
Using atom probe tomography, the three-dimensional morphology and composition of nanoscale omega precipitates in a simple binary titanium–molybdenum alloy have been uniquely determined. The results include the average composition of the omega precipitates and the partitioning of Mo across the ω/β interfaces. These investigations also revealed compositional inhomogeneity within the nanoscale precipitates, with the presence of a Mo-rich core, presumably arising from the relatively slow rate of Mo rejection during isothermal growth of these omega precipitates.
A detailed investigation has been made of the structure of alloys of the Ti-Mo system containing up to 10wt% Mo, water-quenched from the-phase region. With increase in molybdenum content, the martensite structure changes from hexagonal () to orthorhombic () at 4 wt% Mo, and at 10 wt% Mo, the structure is completely retained . For alloy compositions -phase at the quench rates employed. There is a transition, with increase in molybdenum content, in morphology (from massive to acicular) and in substructure (from dislocations to twins). However, the transitions in crystallography, morphology and sub-structure are not directly related to one another except for an abrupt loss of dislocation substructure at the/ transition. The to crystallographic transition has the characteristics of a second order transformation, and evidence has been obtained of the existence of a spinodal within the metastable orthorhombic system. The orthorhombic martensites of Ti-6 and 8 wt% Mo decompose during quenching producing a fine modulated structure within the martensite plates, consistent with a proposed spinodal mode of decomposition.
Phase transformations during artificial and isothermal aging of Ti-6.8Mo-4.5Fe-1.5Al have been investigated over the temperature
range from 300 °C to 750 °C utilizing hardness measurements, X-ray diffraction, optical microscopy, and electron microscopy.
Artificial aging following solution treatment and water quenching initially involved growth of the athermal ω phase. This was followed by formation of the α phase, either in association with the ω phase, through homogeneous precipitation within the matrix, or through heterogeneous grain-boundary nucleation. Similarly,
isothermal decomposition of the metastable β phase resulted in the precipitation of ω phase exhibiting an ellipsoidal morphology. While precipitation of ω was immediate at 345 °C, an incubation period was observed upon aging at 390 °C. Isothermal aging above this temperature
involved direct precipitation of the α phase, either homogeneously within the β matrix or heterogeneously at β grain boundaries. The extent of homogeneous vs heterogeneous α nucleation during isothermal aging depended upon aging temperature; low aging temperatures promote homogeneous nucleation
and higher aging temperatures promote α heterogeneous nucleation. Finally, continued aging resulted, independent of aging path, in coarsening and spheroidization
of the α phase.
The omega phase is a metastable phase which forms in alloys of titanium and zirconium with most transition metals. In this paper the available data from both alloy systems on the occurrence, the structure, the mechanism of formation, and the morphology of the phase are reviewed and compared. The effect of omega phase on the mechanical behaviour and on the superconducting properties is then discussed. It is concluded that uncertainties still exist on the mechanism of the omega transformation during quenching and on the mechanism of the effect of omega on the mechanical properties.
The measurement of low‐temperature specific heat (LTSH) (0.1 K≪T≪60 K) has seen a number of breakthroughs both in design concepts and instrumentation in the last 15 years—particularly in small sample calorimetry. This review attempts to provide an overview of both large and small sample calorimetry techniques at temperatures below 60 K, with sufficient references to enable more detailed study. A comprehensive review is made of the most reliable measurements of the LTSH of 84 of the elements to illustrate briefly some of the problems of measurements and analysis, as well as to provide additional references. More detail is devoted to three special areas of low‐temperature calorimetry that have seen rapid development recently—(1) measurement of the specific heat of highly radioactive samples, (2) measurement of the specific heat of materials in high magnetic fields (18 T), and (3) measurement of the specific heat of very small (100 μg) samples. The review ends with a brief discussion of the frontier research currently underway on microcalorimetry for nanogram sample weights.
The process of transition of the cubic-body-centred modification into the hexagonal-close-packed modification of zirconium can be described by means of a combination of shearing- and dilatation-processes parallel to definite crystallographic directions. The transition has, therefore, “homogeneous” or “oriented” character. See for particulars the detailed summary at the end of the paper.
Solid-solid displacive, structural phase transformations typically undergo a discrete structural change from a parent to a product phase. Coupling electron microscopy, three-dimensional atom probe, and first-principles computations, we present the first direct evidence of a novel mechanism for a coupled diffusional-displacive transformation in titanium-molybdenum alloys wherein the displacive component in the product phase changes continuously with changing composition. These results have implications for other transformations and cannot be explained by conventional theories.
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