High-throughput Screening And Mechanical Behavior Of Body-Centered-Cubic ZrTiNb Multi-Principal Element Alloys

Next-generation high-performance structural materials are required to possess ultrahigh strength,excellent ductility,as well as low-density design.However,traditional alloy systems generally derive their properties from a dominant component,which brings great obstacles to breaking the existing performance limit.In recent years,many new alloys with promising properties are likely to be discovered near the centers(as opposed to the corners)of phase diagrams,which no longer contain a single major component,but multiple major elements and form a concentrated solid-solution structure,as referred as multi-principal element alloys(MPEAs).The new concept provides the possibility to explore desirable performances in a wide composition space,and further achieve breakthroughs in performances.In most of the studied MPEAs,the alloys dominated by bodycentered-cubic(bcc)structures possess relatively-high intrinsic yield strengths.However,the tensile ductility of bcc-MPEAs seems relatively conventional.Brittleness and short-term strain-hardening ability are their Achilles’ heels.Hence,this work tends to develop a series of low-density Zr-Ti-Nb bcc-MPEAs to meet the requirement in the complex applications,and made efforts in the following aspects:First,a method of co-sputtering deposition combined with the physical mask was applied to the parallel preparation of ternary Ti-Nb-Zr system alloys.Sixteen independent specimens with different compositions were obtained once time.The microstructures,phase structure,and mechanical properties were studied in detail.By optimizing the composition,the Zr50Ti35Nb15 alloy was selected as a target composition to investigate the performances of bulk alloy.It is found that the Zr50Ti35Nb15 bulk alloy possesses a single bcc phase and displays a good combination of yield strength of 657 MPa and tensile ductility of 21.9%.After coldrolling,the yield strength of the alloy reaches 810 MPa,and the tensile ductility remained at 10%.For the alloy subjected to recrystallization annealing,the plasticity is significantly improved to 26%and the tensile strength is comparable to that of the as-cast alloy.The underlying mechanisms are related to dislocation slip,deformation twinning,and dynamic grain refinement mechanisms during deformation.Secondly,the high-temperature deformation ability of the Zr50Ti35Nbi5 alloy was studied.Near-superplastic and superplastic behaviors are rarely observed in bcc-MPEAs.Here,the cold-rolled Zr50Ti35Nb15 alloy exhibits superior tensile ductility with a maximum elongation of up to 200%-a near-superplastic behavior-at elevated temperatures in the temperature range of 400-500 ℃.The high density of dislocations induced by the cold rolling leads to a high driving force for dynamic recrystallization,which enhances its tensile ductility.The refined and homogenous microstructure in such MEA enables grain-boundary sliding to relieve the stress concentration during the tensile test.At the later stage of recrystallization,dislocation slip further facilitates the shear strains of coarse grains.Furthermore,low-density Al element was introduced into Zr50Ti35Nb15 alloy based on the purpose of regulating chemical disorder to enhance mechanical properties and reduce the density of alloy simultaneously.The quaternary(Zr50Ti35Nb15)100-0Alx(at.%,x=10,20,30,40)alloys with a low density of less than 5.8 g/cm3 were been designed.It is found that the phase structure of the alloy changes from disordered bcc to ordered B2 phase with the increase of Al content.When the Al content reaches 40 at.%,the alloy possesses multiple phases.Compression tests show that the compressive strain of Al10 and Al20 alloys exceeds 50%,and the yield strengths are 1 GPa and 1.5 GPa,respectively.In contrast,Al30 and Al40 alloys exhibit compression brittleness.Meanwhile,the tensile properties of Al10 and Al20 alloys were tested.The results show that the as-cast Al10 alloy has a yield strength of over 1 GPa and a tensile strain of 14%,while the cold-rolled Al10 alloy with a tensile strength of 1.3 GPa and strain of 9%.The cold-rolled Al20 alloy possesses a remarkable tensile strength of 1.8 GPa and keeps a tensile strain of 8%.The significant improvement in mechanical properties can be attributed to the fact that the introduction the of Al element increases the atomic size mismatch of the alloy system,which will facilitate obtaining more significant solid-solution strengthening in these alloys.On the other hand,the mixing-enthalpy between Al and Zr\Ti\Nb element is negative,which provides a thermodynamic possibility for designing nanoprecipitation.Simultaneously,the(Zr0.5Ti0.35Nb0.15)90Al10 alloy is treated as a model composition to clarify the deformation mechanism and strain-hardening capability.Through large plastic deformation and subsequent controlled heat treatment,different microstructures and dislocation morphologies were designed in the Al10 alloy.The effective strain-hardening ability of the recovered and recrystallized Al10 alloys is achieved.It demonstrates that the semi-coherent nanoprecipitate strengthening and flexible dislocation motion modes are the key factors to achieve this remarkable breakthrough.On the one hand,through controllable heat treatment,a high-density dislocation cell structure and a dense short-rod dislocation structure are designed in the recovered and recrystallized alloys,respectively.In this case,it ensures the realization of frequent interactions between dislocations,which are conducted to obtain homogeneous dislocation microstructure during deformation.On the other hand,by inspiring the dual action of entropy and enthalpy,nanoprecipitation is obtained in this alloy,which plays an active role in enhancing strength and triggering effective strain-hardening ability.The current results not only provide a general method for developing bcc-MPEAs with excellent comprehensive mechanical properties but also prove the feasibility of obtaining effective strain-hardening capability.We expect that this design philosophy has significant implications in developing promising ultra-strong and ductile bccMPEAs superior to existing commercial alloys.