Zengbao Jiao

The Hong Kong Polytechnic University | PolyU · Department of Mechanical Engineering

Advanced ultra-high strength steels are highly desirable for a wide range of engineering applications. Nanoscale co-precipitation strengthening in steels has received increasing attention in recent years and has become a new cornerstone for the development of advanced steels with superior combination of mechanical, welding, and irradiation properti...

Nano-lamellar materials with ultrahigh strengths and unusual physical properties are of technological importance for structural applications. However, these materials generally suffer from low tensile ductility, which severely limits their practical utility. Here we show that markedly enhanced tensile ductility can be achieved in coherent nano-lame...

Control of discontinuous and continuous precipitation is crucial for tailoring the microstructure and mechanical properties of NiAl-strengthened steels. Through a combination of atom probe tomography, transmission electron microscopy, electron backscatter diffraction, first-principles calculations, and mechanical tests, we demonstrate that Cu is ef...

The synergistic effects of Mo, Ti, and Cr on nanoscale precipitation and mechanical properties of maraging stainless steels were systematically studied using high-resolution scanning transmission electron microscopy, atom probe tomography (APT), thermodynamic and first-principles calculations, and mechanical tests. Our results reveal a notable prec...

Synergistic effects of Al and Ti on the oxidation behaviour and mechanical properties of FeCoCrNi high-entropy alloys were systematically investigated. Our results reveal that increasing the Al/Ti ratio slows down the oxidation kinetics at 700−900 °C and changes the oxide scale from TiO2 and spinel oxides to Cr2O3 and Al2O3, leading to a substantia...

Understanding of the competition between continuous precipitation (CP) and discontinuous precipitation (DP) is crucial for tailoring the microstructure and mechanical properties of precipitation-strengthened high-entropy alloys (HEAs). In this study, the effects of aging temperature, aging time, and grain size on the CP and DP behaviors of L12-stre...

The effect of pre-deformation on nanoscale precipitates and hardness of a maraging stainless steel strengthened by the co-precipitation of Ni3Ti, Mo-enriched and Cr-enriched precipitates was systematically studied using electron back scattered diffraction, transmission electron microscopy and atom probe tomography (APT). Hardness measurements showe...

The effects of pre-strain on the continuous precipitation (CP) and discontinuous precipitation (DP) as well as mechanical properties of L12-strengthened HEAs were thoroughly studied. It is found that the pre-strain has a dual effect on the L12 precipitation behavior depending on the pre-strain level. At low pre-strains, the plastic deformation incr...

Immiscible bimetallic composites are a kind of transpiration cooling material with potential in high-temperature service. Aiming at boosting their load-bearing capacity, the refractory phase was replaced with the multi-principal refractory high-entropy phase in the present work. Bi-phase metallic nanocrystalline NbMoTaW–Cu composites were fabricate...

Achieving dense coherent precipitates in 7xxx aluminum alloys during artificial aging to attain high strength-ductility synergy remains a challenging task as they are unstable and tend to dissolve or transform to the semi-coherent phase. Here, we successfully obtained high-density (1.32 × 10²⁴ m⁻³) and fine-sized (∼4.1 nm) coherent precipitates in...

To simultaneously obtain superior superelasticity and biological properties, single- and multi-layer Ti–23Nb coatings were deposited on a cold-rolled NiTi substrate using laser metal deposition (LMD). The microstructure of the single-layer coating consisted of a cellular structure with a grid size of ∼300 μm in the eutectic layer, strip structures...

Metallic alloys with high strength and large ductility are required for extreme structural applications. However, the achievement of ultrahigh strength often results in a substantially decreased ductility. Here, we report a strategy for addressing the strength-ductility trade-off by tailoring the alloy composition to control the local stacking faul...

Advanced structural materials with superior mechanical properties are of technological importance for industrial applications. Multicomponent precipitation provides a potential approach for designing high-performance alloys and has been receiving increasing attention from both academia and industry. In this Perspective, we highlight the recent adva...

In the present study, a nanoprecipitate-strengthened high-strength low-alloy steel was irradiated by high-energy Au²⁺ ions with a peak dose of ∼70 displacements per atom (dpa) at room temperature. The formation of nanoprecipitates, dislocation loops, elemental segregation and mechanical properties before and after irradiation were carefully charact...

Glass-to-glass transitions are useful for us to understand the glass nature, but it remains difficult to tune the metallic glass into significantly different glass states. Here, we have demonstrated that the high-entropy can enhance the degree of disorder in an equiatomic high-entropy metallic glass NbNiZrTiCo and elevate it to a high-energy glass...

In the present work, three kinds of precipitates with different morphologies, structures, sizes, and volume fractions were obtained via energetically-tuning the microstructures of the nano-precipitated CoCrFeNiTi0.2 high-entropy alloy (HEA). Subjected to the heavy cold rolling immediately after homogeneous precipitation, L12 structured spherical na...

The coherent precipitation-strengthened high-entropy alloys (CPS-HEAs) as a new type of structural materials are expected to possess many unique mechanical properties, such as the outstanding strength-ductility combination at cryogenic and room temperatures. Apart from this, most of their strengths can even be well retained at elevated temperatures...

Severe intergranular embrittlement has been found in a wide range of polycrystalline metallic materials in the intermediate temperature regime, setting limits on their engineering applications. In this study, we have systematically investigated the origin of such premature tensile failure in a precipitation-hardened high-entropy alloy. We highlight...

Physically vitrifying amorphous single-element metal requires ultrahigh cooling rates, which are still unachievable for most of the closest-packed metals. Here, we report a facile chemical synthetic strategy for single-element amorphous palladium nanoparticles with a purity of 99.35 at.% ± 0.23 at.% from palladium—silicon liquid droplets. In-situ t...

High-entropy alloys (HEAs) with a single-phased face-centered-cubic structure possess excellent plasticity but generally low strength. Precipitation strengthening is one of the most promising methods to improve the strength of alloys. However, plagued by a nerve-wracking fact that strength-ductility trade-off frequently limits the improvement of al...

This study presents the alloy development of a new class of L12-strengthened Co-Al-Nb-based alloys with high γ′-solvus temperatures together with superb strengths at both ambient and elevated temperatures. The L12-Co3(Al, Nb) phase was found to be in equilibrium with the γ-Co matrix and the B2-CoAl phase in ternary Co-10Al-3Nb alloys after isotherm...

We investigated the mechanical and microstructural responses of a high-strength equal-molar medium entropy FeCrNi alloy at 293 and 15 K by in situ neutron diffraction testing. At 293 K, the alloy had a very high yield strength of 651 ± 12 MPa, with a total elongation of 48%± 5%. At 15 K, the yield strength increased to 1092 ± 22 MPa, but the total...

Lithium-sulfur (Li-S) batteries are a promising next-generation energy storage technology due to high theoretical energy density, low cost and abundant reserves. However, the poor electronic conductivity of sulfur and huge volume change hindered their commercial applications. In this paper, selected as a cathode host of Li-S batteries from two Ce-M...

Alloying plays an important role in determining the phase stability and mechanical behavior of medium/high-entropy alloys (M/HEAs). In this work, the effects of Al, Ti, Mo, and W additions on the phase stability, strengthening behavior, and stacking fault energies of CoCrNi alloys are quantitatively investigated by using first-principles calculatio...

A high strength steel with a combination of ∼930 MPa yield strength and excellent low temperature toughness with an upper shelf energy of above 200 J and ductile brittle transition temperature (DBTT) of lower than −90 oC is developed. The strengthening and toughening mechanisms are investigated systematically based on the detailed characterization...

The intrinsic partitioning behavior of Al and its influence on the nanoscale precipitation of Cu-rich nanocluster-strengthened steels were investigated by using atom probe tomography (APT) and first-principles calculations. The APT results reveal that Al partitions to Cu-rich nanoclusters, which results in a slight decrease in the volume fraction o...

The present research on high chromium cast irons (HCCIs) strengthened with TiC particles was motivated by a need to improve the abrasive wear resistance of materials for mining equipment such as board hammers and liners. The effect of TiC content and heat treatment on the wear properties of eutectic HCCIs was investigated by combining the three-bod...

Control of the formation and stability of reverted austenite is critical in achieving a favorable combination of strength, ductility, and toughness in high-strength steels. In this work, the effects of Cu precipitation on the austenite reversion and mechanical properties of maraging stainless steels were investigated by atom probe tomography, trans...

Ultra-coarse grained cemented carbides are often used under conditions of concurrently applied stress and high temperature. Improvement of high-temperature mechanical performance of ultra-coarse grained cemented carbides is highly desirable but still a big challenge. In this study, it is proposed that the high-temperature compression strength of ul...

Understanding phase stability and precipitation at intermediate temperatures is crucial for tailoring microstructures and mechanical properties of L12-strengthened multicomponent alloys. In this study, the precipitate type, morphology, and distribution of (CoCrNi)100−2x(AlTi)x (x = 3, 5, and 7 at.%) medium-entropy alloys (MEAs) at 600-900 °C were s...

Nanostructured W-Cu-Cr composites were fabricated, which exhibit exceptionally high hardness (~1000 HV) as compared with those of the conventional W-Cu composites, due to the combined advantages of Cr dissolution, precipitate formation and grain refinement. Nonmonotonic variation of the wear resistance with hardness was discovered. With an appropri...

A L12-strengthened multicomponent Co-rich high-entropy alloy with superior microstructural stability was developed in this study. High-density cuboidal γ′ particles were found to embed in the matrix for the alloy subjected to isothermal aging at temperatures ranging from 800 to 1100 °C without the formation of brittle intermetallic phases neither a...

Low-carbon advanced nanostructured steels have been developed for various structural engineering applications, including bridges, automobiles, and other strength-critical applications such as the reactor pressure vessels in nuclear power stations. The mechanical performances and applications of these steels are strongly dependent on their microstru...

A series of G-phase strengthened ferritic stainless steels Fe-20Cr-3Ni-3Si-X (X = 2Mn, 1Mn-2Ti, 1Mn-2Nb and 1Mn-2Ta) are characterized after aging using experimental (microhardness, TEM and APT) and theoretical (DFT) techniques. The results indicate that the Ni16Mn6Si7 G-phase shows sluggish precipitation during aging treatment. This was attributed...

Improvement of high-temperature mechanical properties of W–Cu based composites is highly desirable but still a challenge. Here it is achieved by combined effects of solid solution, dispersed nano-precipitation and highly stabilized nanostructure in the W–Cu–Cr–ZrC composite, which takes advantage of the in-situ precipitated Zr–Cr–C nanoparticles an...

High-entropy alloys (HEAs) strengthened by coherent nanoparticles show great potentials for elevated- temperature structural applications, which however, generally suffer from a severe intergranular embrit- tlement when tested at intermediate temperatures. In this study, we demonstrated a novel “heterogenous columnar-grained”(HCG) approachthatcanef...

The strength-ductility paradox is a long-sought challenge for all engineering materials. In this study, we escaped the strength-ductility trade-off by engineering nano-scale heterogeneities carefully in the advanced nanostructured Fe-based alloys through alloying with Cu and Mn additions. We demonstrated a triple ductility enhancement by 20% togeth...

In this study, alloying effects of Mo and W refractory elements on the microstructural evolution of high-entropy alloys (HEAs) were systematically studied. High-density L12-type precipitates formed during the isothermal treatment at 800 °C. Alloying additions of Mo and W displayed different partitioning behaviors between the matrix and precipitate...

Alloys that have high strengths at high temperatures are crucial for a variety of important industries including aerospace. Alloys with ordered superlattice structures are attractive for this purpose but generally suffer from poor ductility and rapid grain coarsening. We discovered that nanoscale disordered interfaces can effectively overcome these...

Structure, crystallization behavior, and magnetic properties of as-quenched and annealed Fe81.3Si4B13Cu1.7 (Cu1.7) alloy ribbons and effects of Nb alloying have been studied. Three-dimensional atom probe and transmission electron microscopy analyses reveal that high-number-density Cu-clusters and Pre-existing Nano-sized α-Fe Particles (PN-α-Fe) are...

It is known that nanoscale precipitates strongly affect the precipitation hardening of structural materials. In this study, we report the precipitation kinetics and thermostability of Cu-rich and NiAl nanoprecipitates in nanostructured steels with Mo additions. Atom probe tomography and first-principles calculations revealed that the addition of Mo...

A novel austenite-martensite dual-phase steel with a ductility of ~30% and tensile strength over 1.4 GPa was developed. The hard martensite in the dual phase steel was strengthened through precipitation strengthening by Cu/NiAl precipitates, forming the maraging phase. The deformation mechanisms of the steel were investigated using in situ neutron...

In the present work, CoCrFeMnNiVx (x = 0, 0.07, 0.3, 0.7, 1.1) high entropy alloy films were fabricated by magnetron co-sputtering. For low contents of V, typical face-centered cubic (fcc) peaks were identified in X-ray diffraction patterns. With the increasing V content, the diffraction peaks became broadened and the formation of an amorphous phas...

Control of discontinuous and continuous precipitation of L12-ordered γʹ precipitates is crucial for tailoring microstructure and mechanical properties of coherent precipitation-strengthened high-entropy alloys (HEAs). In this study, we show that the appropriate addition of Nb not only suppresses discontinuous γʹ precipitation through grain boundary...

We developed a novel high-performance L12-strengthened high-entropy alloy (HEA) in the multicomponent Ni-Co-Fe-Cr-Al-Nb system. The phase transformation, mechanical properties and associated deformation behaviors were systematically investigated through combinational analyses involving the three-dimensional atom probe tomography (3D-APT), transmiss...

Titanium alloys have been widely used for medical devices and structural applications. However, conventional titanium alloys often suffer from low resistance to wear, particularly at elevated temperatures. Herein, an equiatomic TiMoNb compositionally complex alloy (CCA) is shown to exhibit wear resistance comparable to alumina at room temperature (...

Thermally stable high-entropy alloys (HEAs) consisting of a high density of coherent precipitates show a great potential for high-temperature applications. In this work, we systematically investigated the phase stability and coarsening kinetics of L12-type coherent precipitates in a Ni-30Co-13Fe-15Cr-6Al-6Ti-0.1B (at.%) HEA isothermally aged at 800...

Metallic materials are important for making multi-scale parts and structures because of their excellent mechanical properties and good formability. This article reviews several types of classic metallic materials for multi-scale structural applications, including aluminum alloys, magnesium alloys, copper alloys, titanium alloys, carbon steels, nano...

Metallic glass (MG), with the superiorities of unique disordered atomic structure and intrinsic chemical heterogeneity, is a new promising and competitive member in the family of environmental catalysts. However, what is at stake for MG catalysts is that their high catalytic efficiency is always accompanied by low stability and the disordered atomi...

Aging treatments at 400–550 °C are commonly used to attain a peak strengthening for the Cu-rich nanocluster-strengthened high-strength low-alloy (HSLA) steels. However, these temperatures fall within the dangerous 300–600 °C temper-embrittlement regime, leading to poor impact toughness. On the other hand, aging at temperatures above the embrittleme...

In this paper, G-phase precipitation and the resulting hardening effect on Fe-20Cr-3Ni-1Mn-3Si ferritic alloys by the addition of Ti, Nb, Ta and Zr were individually studied by electron microscopy and atom probe tomography, combined with thermodynamic and first principle calculations. The high resolution scanning electron and transmission electron...

In this paper, G-phase precipitation and the resulting hardening effect on Fe-20Cr-3Ni-1Mn-3Si ferritic alloys by the addition of Ti, Nb, Ta and Zr were individually studied by electron microscopy and atom probe tomography, combined with thermodynamic and first principle calculations. The high resolution scanning electron and transmission electron...

Alloy design based on single–principal-element systems has approached its limit for performance enhancements. A substantial increase in strength up to gigapascal levels typically causes the premature failure of materials with reduced ductility. Here, we report a strategy to break this trade-off by controllably introducing high-density ductile multi...

Alloying has proven an enabling strategy to stabilize nanocrystalline materials against grain growth, especially in cases where the solute segregates to grain boundaries and lowers their energy. Among such materials reported to date, most all are stable up to some temperature at which second phases precipitate, depleting solute from the boundaries....

Nanolattice structure fabricated by two-photon lithography (TPL) is a coupling of size-dependent mechanical properties at micro/nano-scale with structural geometry responses in wide applications of scalable micro/nano-manufacturing. In this work, three-dimensional (3D) polymeric nanolattices are initially fabricated using TPL, then conformably coat...

The Cu and NiAl nanoscale precipitation and interfacial segregation in the martensite and austenite phases within a high-strength steel were studied by atom-probe tomography (APT). In the martensite phase, APT reveals the precipitation of isolated NiAl nanoparticles and NiAl/Cu co-precipitates, indicating that NiAl nanoparticles form first in the p...

Combining high strength and good ductility is highly-desired yet challenging for conventional structural materials. Newly emerging concentrated multi-component alloys with face-centered-cubic structure provide an ultra-ductile matrix, and the precipitation hardening based on these alloys provides a very effective way to achieve a superior strength-...

The effects of welding and post-weld heat treatment (PWHT) on nanoscale co-precipitation, grain structure, and mechanical properties of an ultra-high strength steel were studied through a combination of atom probe tomography (APT) and mechanical tests. Our results indicate that the welding process dissolves all pre-existing nanoparticles and causes...