Microstructure evolution and deformation mechanism ofα+βdual-phase Ti-xNb-yTa-2Zr alloys with high performance

Biomedicalβ-phase Ti-Nb-Ta-Zr alloys usually exhibit low elastic modulus with inadequate strength.In the present work,a series of newly developed dual-phase Ti-xNb-yTa-2Zr(wt.%)alloys with high performance were investigated in which the stability ofβ-phase was reduced under the guidelines of ab initio calculations and d-electronic theory.The effects of Nb and Ta contents on the microstructure,compressive and tensile properties were investigated.Results demonstrate that the designed Ti-xNb-yTa-2Zr alloys exhibit typical characteristics ofα+βdual-phase microstructure.The microstructure of the alloys is more sensitive to Nb rather than Ta.The as-cast alloys exhibit needle-likeα′martensite at a lower Nb content of 3 wt.%and lamellarα′martensite at an Nb content of 5 wt.%.Among the alloys,the Ti-3Nb-13Ta-2Zr alloy shows the highest compressive strength(2270±10 MPa)and compressive strain(74.3%±0.4%).This superior performance is due to the combination ofα+βdual-phase microstructure and stressinducedα"martensite.Besides,lattice distortion caused by Ta element also contributes to the compressive properties.Nb and Ta contents of the alloys strongly affect Young's modulus and tensile properties after rolling.The as-rolled Ti-3Nb-13Ta-2Zr alloy exhibits much lower modulus due to lower Nb content as well as moreα"martensite andβphase with a good combination of low modulus and high strength among all the designed alloys.Atom probe tomography analysis reveals the element partitioning between theαandβphases in which Ta concentration is higher than Nb in theαphase.Also,the concentration of Ta is lower than that of Nb in theβphase,indicating that theβ-stability of Nb is higher than that of Ta.This work proposes modernα+βdual-phase Ti-xNb-yTa-2Zr alloys as a new concept to design novel biomedical Ti alloys with high performance.

The dual effect of grain size on the strain hardening behaviors of Ni-Co-Cr-Fe high entropy alloys

It has been well documented that grain size plays a critical role in the strain hardening behaviors of metals and alloys.However,the influence of grain size on the strain hardening of high entropy alloys(HEAs)was not fully understood.Here,we report that the grain size not only affects the twinning-induced plasticity(TWIP)effect but also changes the dislocation-based deformation behaviors of face-centeredcubic(fcc)HEAs significantly.The strain hardening and deformation micro-mechanisms of NiCoCrFe and Ni_(2)CoCrFe were investigated using electron channeling contrast(ECCI)analysis.Our results showed that Ni_(2)CoCrFe exhibits a typical three-stage strain hardening behavior and NiCoCrFe shows the fourth stage at high strains due to the TWIP effect.For both NiCoCrFe and Ni_(2)CoCrFe,the increase of grain size leads to a transition of dislocation glide from wavy to planar mode,resulting in a low value and the recovery of strain hardening rate in stage II.The large-grain NiCoCrFe showed a higher strain hardening rate in stage IV due to the promoted deformation twinning.Combining the strain hardening behaviors of the TWIPNiCoCrFe and the mechanically stable Ni_(2)CoCrFe,we showed that the grain size influences the stage II hardening through tuning dislocation glide mode and the stage IV by tailoring deformation twinning activity of the Ni-Co-Cr-Fe HEAs.The grain size just affects stages I and III slightly in the current cases.These findings will also provide some insights into the understanding of strain hardening behaviors in other face-centered-cubic HEAs.

Elemental partitioning as a route to design precipitation-hardened high entropy alloys

Precipitation-hardened high entropy alloys(HEAs)with carefully tuned compositions have shown excellent mechanical properties,demonstrating great potential for engineering applications.However,due to the lack of precise multiple phase diagrams,the composition design of multi-principal-component HEAs still inevitably relies on the extremely time-consuming trial-and-error approach.The present study,on the basis of powerful composition quantification ability of atom probe tomography(APT)technology,proposed a framework to guide the quantitative design of precipitation-hardened HEAs.In this framework,the elemental partitioning was used as a crucial route to avoid the thermodynamic challenge of designing precipitation-hardened HEAs.As a case study,the role of Ti/Al ratio in the design ofγ-γ’HEAs was predicted through the proposed framework and then validated by experimental studies.The framework predicted that when the total content of Ti and Al is fixed,a higher Ti/Al ratio makesγ-γ’HEA stronger.APT and mechanical results agreed well with these predictions and validated the feasibility of the framework.These findings provided a new route to design the precipitation-hardened alloys and a deeper insight into the design ofγ-γ’HEA.

Strengthening of high-entropy alloys via modulation of cryo-pre-straining-induced defects

Owing to their attractive structure and mechanical properties,high-entropy alloys(HEAs) and mediumentropy alloys(MEAs) have attracted considerable research interest.The strength of HEAs/MEAs with a single face-centered cubic(FCC) phase,on the other hand,requires improvement.Therefore,in this study,we demonstrate a strategy for increasing the room-temperature strength of FCC-phase HEAs/MEAs by tuning cryo-pre-straining-induced crystal defects via the temperature-dependent stacking fault energyregulated plasticity mechanism.Through neutron diffraction line profile analysis and electron microscope observation,the effect of the tuned defects on the tensile strength was clarified.Due to the cryorolling-induced high dislocation density,mechanical twins,and stacking faults,the room-temperature yield strength of an equiatomic CoCrFeNi HEA was increased by ~290%,from 243 MPa(as-recrystallized)to 941.6 MPa(30% cryo-rolled),while maintaining a tensile elongation of 18%.After partial recovery via heat treatment,the yield strength and ultimate tensile strength decreased slightly to 869 and 936 MPa,respectively.Conversely,the elongation increased to 25.6%,The dislocation density and distribution of the dislocations were found to contribute to the strengthening caused by forest dislocations,which warrants further investigation.This study discussed the possibility of developing single-phase high-performance HEAs by tuning pre-straining-induced crystal defects.