Novel Mixed-Mode Phase Transition Involving a Composition-Dependent Displacive Component
Department of Materials Science and Engineering, University of North Texas, Denton, Texas 76203, USA.Physical Review Letters (Impact Factor: 7.51). 06/2011; 106(24):245701. DOI: 10.1103/PhysRevLett.106.245701
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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- "In order to understand the evolution of microstructure during this single step ageing, both aberration corrected HRSTEM and atom probe tomography (APT) were employed here. Nag et al.   and Devaraj et al.  have pioneered the study of x-assisted a precipitation in Ti5553 and exploited the displacive–diffusional phase transformation of x phase by a combination of HRTEM and APT. However, this was carried out using different processing schedules. "
ABSTRACT: The thermo-mechanically processed powder Ti–5Al–5V–5Mo–1Cr–1Fe alloy was aged at 923 K for 1–8 h in order to investigate the effect of ageing time on the microstructure–mechanical properties relationships. The microstructures of the alloy after ageing consist of the both primary and secondary α phases with retained β matrix phase. The shift of β peaks in X-ray pattern with ageing time indicates an increase in lattice parameter distortion due to diffusion into the retained β phase of more β stabilisers with smaller atomic radius compared to the elemental Ti. The tensile test results indicate that the sample aged for 1 h has achieved the best combination of mechanical properties with ultimate tensile strength of 1194 MPa and total elongation of 14.1% among all the experimented conditions. The modified Crussard–Jaoul method is applied to characterise the work hardening behaviour of the alloy.
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ABSTRACT: We performed plane wave-based first principles calculations using the projector augmented wave (PAW) potential under the generalized gradient approximation (GGA) within the density functional theory to study the formation of ordered omega (B82-structured) Zr2Al phase in �-Zr3Al alloy. The transformation involves both replacive and displacive processes. We investigated two possible paths for the transformation where steps involving replacive (diffusive) and displacive processes occur in succession with their sequence of occurrence being different in the two paths. From this study, it was possible to show that the initial chemical ordering facilitates the displacive process leading to the transformation. It was also possible to correlate instability with respect to omega-type displacements in Zr2Al alloy with the number of Zr–Al bonds present in the unit cell. Electronic structure analysis indicated that stronger Zr–Al bonding plays an important role in the formation of chemically ordered omega phase.
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ABSTRACT: The ω transformation and its correlation with elastic properties were investigated in cold-worked Ti–36Nb–2Ta–3Zr–xO mass% alloys with low body-centered cubic (β) phase stability, known as gum metal. Analysis of the temperature dependence of the ω (hexagonal) phase formation using transmission electron microscopy and of the elastic properties of solution-treated and cold-worked alloys using resonant ultrasound spectroscopy revealed that in the solution-treated 0.36% and 0.51% O alloys, the high concentration of oxygen suppressed ω-phase formation from room temperature to a fairly low temperature of ∼13 K. However, the ω phase was formed by cold working at room temperature in the 0.30% and 0.47% O alloys. Importantly, the fraction of the ω phase clearly increased upon cooling, which indicates that the formation of the ω phase is thermodynamically favorable near and below room temperature in the cold-worked 0.30% and 0.47% O alloys. This formation of the ω phase and the low stability of the β phase related to the low electron/atom (e/a) ratio were the dominant factors determining the elastic properties near and below room temperature in the cold-worked Ti–Nb–Ta–Zr–O alloys.
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