Microstructure Manipulation And Cryogenic Temperature Mechanical Behavior Of Hf-Zr-Ti-based High-entropy Alloys

Body-centered-cubic(BCC)high-entropy alloys(HEAs)typically contain group ⅣB,ⅤB and ⅥB transition metals with equal or near-equal atomic ratios,exhibiting high strength and hardness,good wear resistance,corrosion resistance and radiation resistance at room temperature.Recently,the equiatomic TaNbHfZrTi BCC HEA has been reported to exhibit high yield strength with no ductile-to-brittle transition(DBT)at 77 K,indicating a broad application prospect at cryogenic temperatures.However,the systematic study on the mechanical properties and deformation mechanism of this BCC HEA at cryogenic temperature is still missing.In addition,this BCC HEA show low plasticity and insufficient work hardening ability at room temperature,thus limiting its practical cryogenic uses.In light of this challenge,effects of alloying elements and heat treatment on the microstructure,phase constitution,phase stability,mechanical property at both room and cryogenic temperatures,and deformation mechanism of Hf-Zr-Ti-based HEAs were studied in this dissertation.First,effects of chemical composition and annealing on the microstructure and mechanical properties of Hf-Zr-Ti HEAs at room temperature were investigated.With the increase of Hf content,the basal stacking fault energy(SFE)of the alloy increases but the prismatic SFE decreases,which promotes the cross slip of<a>dislocations from the basal plane to prismatic plane and facilitates the proliferation of<c+a>dislocations through the dislocation reaction,which finally enhanced the work hardening ability of the Hf-Zr-Ti HEAs.In addition,it was found that the recrystallization temperature increases with the addition of Hf content.After recrystallization,the microstructure of the alloys transformed from the Widmanstatten lamella structure to the equiaxed crystal structure,accompanied by increased plasticity and decreased strength.Clearly,the microstructure and mechanical properties of Hf-Zr-Ti HEAs can be regulated by adjusting the alloying elements and heat treatment process,which laid a foundation for the further optimization of these HEAs.Based on the above results,effects of Hf and annealing on mechanical properties and deformation mechanism of Hf-Zr-Ti HEAs at cryogenic temperature were investigated.It was found that all the samples,either the as-cast or annealed,showed appreciably increased strength and decreased plasticity at cryogenic temperature.Further analysis revealed that dislocation slip is the main deformation mechanism and no mechanical twinning or other deformation mechanisms were activated at cryogenic temperature,which leads to a deteriorated plastic deformation ability.In addition,effects of the β stabilizers,i.e.,Nb and Ta,as well as heat treatments on the phase composition,phase stability and room-temperature mechanical properties of Hf-Zr-Ti-(Nb,Ta)HEAs were systematically studied.Both Nb and Ta could stabilize the BCC structure,and the Nb showed a stronger stabilization efficacy compared with Ta.With the decrease of Nb or Ta elements,the alloy changes from single-phase BCC to BCC+HCP and finally to a singlephase HCP(hexagonal-close-packed)structure.The tendency for phase transformation of Hf-Zr-Ti-(Nb,Ta)HEAs increased with the decrease of the Nb and Ta content.The strong working hardening ability and large ductility of the HEAs with the decreased β stabilizer content originated mainly from strain-induced phase transformation and HCP twinning,which relieved local stress concentration and generated plenty of interfaces to accommodate dislocation movements.For Ta0.2NbxHfZrTi HEAs,the plasticity of the as-cast alloy was elevated from 14%in Nb0.25(7.25 at.%Nb)to 31%in Nb0.2(5.88 at.%Nb),while the yield strength decreased from 748 to 607 MPa.Finally,mechanical behavior of Ta0.2NbxHfZrTi HEAs at cryogenic temperature were systematically investigated.The BCC phase stability decreases with the decrease of temperature,and phase transformation and twinning were triggered during the low-temperature deformation,which improves the work hardening ability and plasticity.As the temperature decreases from 298 K to 77 K,the dominated deformation mechanism in Ta0.2Nb0.3HfZrTi HEA transformed from dislocation planar slip to multiple mechanisms containing dislocation planar slip,phase transitions(BCC to HCP and ω)and {332}<113>twinning.As a result,the yield strength increased from 838 to 1019 MPa,while the plasticity increased from 12 to 25%.With further increase of Nb(i.e.,Tao 2NbHfZrTi),the deformation was dominated by dislocation slip at 77 K due to the high BCC stability,thus the yield strength was further increase to 1492 MPa,but the plasticity was decreased to 14%.To sum up,this dissertation systematically studied effects of different alloying elements and heat treatments on the microstructure and mechanical properties of Hf-Zr-Ti HEAs,which is not only helpful for understanding deformation mechanisms and property optimization of BCC HEAs for cryogenic uses,but also important for expanding the application range of BCC HEAs under extreme service environment.