Investigation On The Microstructure,Mechanical Properties And Deformation Mechanisms Of Refractory Medium/high Entropy Alloys

High-entropy-alloying alloys based on multiple-principal elements,including medium-entropy alloys(MEAs)and high-entropy alloys(HEAs),exhibit the unique chemical and physical properties of HEAs,such as local chemical composition heterogeneity,thermodynamic high entropy effect,structural lattice distortion effect and kinetic sluggish diffusion effect.Refractory MEAs/HEAs(RMEAs/RHEAs)with high strength,radiation resistance,high temperature softening resistance,etc.,have great potential application value in the field of high temperature structural materials.However,the development of RHEAs/RMEAs also have problems such as high ductilebrittle transition temperature and high density.Meanwhile,the sudies on the correlation of microstructure design,mechanical properties and deformation mechanism is lacking.Thus,it is impossible to effectively feedback to optimize the design of alloy composition and structure,which has also become the barrier to optimize the room temperature plasticity,low density and high strength of RMEAs/RHEAs.Based on the above-mentioned problems of RHEAs/RMEAs,low-density plastic RMEAs dominated by solid solution strengthening and precipitation strengthening,respectively,were firstly developed.Their correlations between the microstructures,mechanical properties and deformation mechanisms were deeply understood in this paper.Above studies could feedback the alloy optimization.Finally,the plastic RHEAs with excellent specific strength at medium-high temperatures were developed and their high-temperature deformation mechanisms were clarified.The main content of this paper is as follows:1.Mo/W/B alloying on the microstructure,mechanical properties and deformation mechanisms of TiZrNb MEA have been investigated.It is found that with the addition of Mo/W,the alloy structure transitions from a single-phase BCC solid solution to a dual-phase BCC solid solution composed of a W-rich phase and a Zr-rich phase,and the compressive strengths of the alloy are significantly improved at 298 K and 1273 K.Due to the difference in moduli between W-rich phase and Zr-rich phase in TiZrNbMo0.3W0.3 alloy,stress concentration at the interface during deformation at 1273 K leads to the crack initiation and extension to W-rich phases,causing them to split and break,resulting in the flow stress of the alloy decreasing sharply after yielding.The slow and sustained decrease in flow stress with increasing strain is associated with dynamic recrystallization on the Zr-rich side at the interface between the two phases.B-doping significantly improves the strength and plasticity of TiZrNb MEA.Compared with 0 ppm-TiZrNb,the yield strength and elongation of 500 ppm-TiZrNb are increased by 19.0%and 48.7%,respectively.With the increase of B,the deformation of the alloy transitions from strain-softening to strain-hardening.The long-range steplike extension of the slip band and the planar slip of dislocations dominate the strainsoftening and premature necking fracture of the 0 ppm-TiZrNb.While 500 ppm-TiZrNb has excellent strain hardening ability.Firstly,the B-doping refined grains effectively limit the long-range extension of the slip band and promote the uniform deformation;secondly,the interstitial solid solution of B into the matrix plays a role in pinning the dislocations,which promotes the cross-slip and entanglements of dislocations.2.The microstructures,mechanical properties and deformation mechanisms of Ti1.5AlxZrNb MEAs at room temperature has been investigated systematically.It is found that the as-cast Ti1.5ZrNb alloy and Ti1.5Al0.3ZrNb alloy have a single-phase BCC structure,while the Ti1.5Al0.5ZrNb alloy has a nano-scale B2 phase+BCC phase structure.Compared with Ti1.5ZrNb alloy,the yield strength of Ti1.5Al0.3ZrNb alloy and Ti1.5Al0.5ZrNb alloy increased by 28.9%and 59.4%,respectively.While the elongation of the alloy deteriorated drastically with the increase of Al content,and the alloy underwent the transition from strain-hardening to strain-softening.After-60%,cold rolling and annealed at 1073 K-1 h,873 K-1 h,the structure of Ti1.5Al0.5ZrNb alloy is composed of B2 matrix and antiphase domain boundaries(APBs).And its mechanical properties and deformation characteristics are the same as those in as-cast conditions.The deformation of the as-cast and fully recrystallized Ti1.5Al0.5ZrNb alloy is dominated by planer-slip with the narrow dislocation channels along the<111>{110}BCC/B2,because the shear modulus of dislocations in this direction is lower than other crystal slip systems.The dislocations in the channels continuously move and shear the B2 phase,reducing the B2 order-hardening effect significantly.Thus,the dislocation slip resistance decreases,and the obvious strain softening appears.After～60%cold rolling and aging at 873 K for 5 hours,the structure of Ti1.5Al0.5ZrNb alloy has transformed into the BCC+B2+AlZr2 coherent three-phase structure,while maintaining a high dislocation density in the alloy.It exhibits excellent uniform deformability,whose uniform elongation of 5.5%is much higher than the～2%of the as-cast and fully recrystallized alloys.During the deformation process,the ωAlZr2 phases effectively hinder the movement of dislocations and play the role of pinning dislocations.At the same time,the dislocations generated during deformation interact with the reserved dislocations in the alloy,and resultant vast dislocation entanglements increase the dislocation movement resistance.The superposition of theω-AlZr2 phases and reserved dislocations acts as obstacles for the formation of longrange low-resistance movement channels of dislocations and promotes the transformation from strain-softening to strain-hardening.3.NbTiAlV-based RHEAs with excellent compressive plasticity and high specific strength have been developed,whose microstructures,mechanical properties and high temperature deformation mechanisms have also been analysised.It is found that the ascast Nb40Ti25Al15V10Ta5Hf3W2 RMPEA consists of B2 nanoprecipitates and BCC matrix.Its specific yield strength(83.2 MPa·g-1·cm3)at 1073 K is higher than that of NbMoTaW and VNbMoTaW two typical RHEAs,and the alloy maintains the intrinsic compressive plasticity.The dislocation-dominated deformation of as-cast Nb40Ti25Al15V10Ta5Hf3W2 RMPEA at 1073 K compression provides the initial strainhardening ability,and as the strain increases,the generation of microcracks and dynamic recovery accelerate the transition from strain-hardening to strain-softening.However,the dislocations are easily thermal activated and interact with each other during the 1273 K compression,resulting in annihilation of vast dislocations.Meanwhile,the continuous dynamic recrystallization controlled by diffusion consumes a large number of dislocations at the grain boundaries and promotes the continuous strain-softening of the alloy during the 1273 K compression.The as-cast Nb40Ti25Al15V10Ta5Hf3W2 RHEA deformed by 40%at 1273 K,and then annealed at 923 K,1023 K and 1123 K for different times maintaines excellent structural and mechanical stability.