Manipulation Of Microstructure And Mechanical Property Of HfZrTi-based High-entropy Alloys Via Metastability Engineering

The single-phase body-centered-cubic(bcc)high-entropy alloys(HEAs)and those with bcc as the main phase,which ususally mainly contain group ⅣB,ⅤB and VIB transition metals,not only exhibit extremely high strength and hardness,but also show great potential for functional applications such as superconductivity,hydrogen storage,shape memory,and damping.However,plastic deformation ability of those bcc HEAs is normally poor under room-temperature loading,which makes them difficult to be processed and thus limits their practical uses.Improving their room-temperature ductility with no sacrifice of their high strength has been the main challenge and research focus in the field.Initiation of stress-induced phase transformation or twinning during plastic deformation is a common way to improve the comprehensive mechanical properties of steels and titanium alloys based on the"metastability engineering" design strategy.However,systematic research on the metastability engineering in HfZrTi-based HEAs is still mising so far.In light of this challenge,effects of the β stabilizers(i.e.,V,Nb and Ta)and heat treatment on the phase constitution,phase stability and mechanical property of HfZrTi-based HEAs were studied in this dissertation.The main purpose is to explore new solutions to address the room-temperature brittleness of bcc HEAs.First,effects of annealing temperature and duration on the microstructure and mechanical property of the hcp HfZrTi HEA were investigated.The microstructure could vary from equiaxed grains,bimodal structure,to the basket-weave structure through annealing at different temperature ranges.The bimodal alloy with a mixture of equiaxed grains and basket-weave structures exhibited an unique combination of high tensile strength(1200 MPa)and decent uniform elongation(10%).It was found that the microstructure and mechanical property of HfZrTi were mainly determined by the phase transformation process during heat treatment,which laid a foundation for the further optimization of this HEA.Second,effects of group VB elements including β stabilizers V,Nb and Ta on the microstructure of HfZrTi-based HEAs were systematically investigated.It was found that V,Nb and Ta could all stabilize the bcc structure of the HEAs,whilst the element with a higher atomic number showed a stronger bcc stabilizing efficacy.The synergistic effect of multi-components in HEAs enhanced the bcc stabilization effect of the alloying elements in comparison with binary titanium alloys.Addition of V,Nb and Ta transited the microstructure of the solution-treated HfZrTi-based HEAs from a single hcp phase to multiple phases of hcp/α" phase plus metastable bcc phase with ω precipitation,and finally to a single bcc phase.Particularly,the addition of V led to formation of the HfV2 Laves phase at grain boundaries of the bcc phase.Meanwhile,it was found that the volume fraction of ω precipitation decreased with the increase of the β stabilizers,but Ta showed the greatest tendency to inhibiting the ω phase.In addition,influences of cooling rates after the solution treatment on the microstructure stability and mechanical performance of HfZrTi-based HEAs were studied.A slow cooling rate could promote elemental segregation and phase decomposition in HEAs,thus leading to variations in mechanical property and deformation mechanism.The structural stability sequence of the studied four HEAs is HfZrTiNb>HfZrTiTaNb>HfZrTiTa0.5>HfZrTiTa.As the cooling rate decreased from water quenching to furnace cooling,the plasticity of HfZrTiTaNb and HfZrTiTa halved while the tensile strength increased by nearly 300 MPa.It was found that the difference in electronegativity and melting point among the constituent elements determined the phase decomposition potency of the studied HEAs,which could guide the heat treatment process of such HEAs.At last,effects of V and Nb concentration on the tensile property and deformation mechanism of the solution-treated HfZrTi-based HEAs were investigated.Besides,the brittle as-cast HfZrTiTa was ductilized by reducing the content of Ta via metastability engineering.It was found that the phase transformation potential of the HfZrTi-based HEAs increased with the decrease of the β stabilizers.The strong working hardening and large ductility of the HfZrTibased HEAs with the decreased β stabilizer content originated mainly from straininduced phase transformations including multiple martensitic transformations and martensite reorientation.The stress-induced phase transformation relieved the local stress concentration and generated plenty of phase interfaces,which greatly limited the dislocation movement.The plasticity of the as-cast HfZrTiTax was elevated from less than 5%in Tal(25.00 at.%Ta)to 30%in Ta0.4(11.76 at.%Ta),accompanied with a slight decrease in tensile strength.To sum up,this dissertation first investigated influences of composition and heat treatment process on the microstructure of the HfZrTi-based HEAs and then regulated deformation mechanism and mechanical properties of the HEAs through the concept of metastability engineering,which provides new insights on the ductilization of brittle bcc HEAs and the development of high-performance HEAs.