Microstructure,Strengthening And Toughening Mechanisms For Ti-Zr-Nb Refractory High-entropy Alloys

The concept of high-entropy alloys(HEAs),which is a brand new alloy design strategy,was first proposed in the early 2000s.The compositional feature of HEAs can be characterized as that HEAs are composed of multicomponents and the concentrations of all constituents are equal or near-equal to each other.Ascribed to the "high-entropy effects" originated from the compositional characteristics,HEAs usually possess unique features,such as high phase stability and sluggish diffusion,et al.Based on the concept of HEAs,refractory high-entropy alloys with refractory elements are potential to high temperature applications and have attracted increasing research interest.Nowadays,the research of HEAs is still at an initial stage,and many scientific issues are urgently needed to be addressed.First,previously reported refractory HEAs usually contain large amounts of high-cost and high density elements,resulting in the HEAs being expensive and heavy,and hence hindering their industrial applications.Second,researches on predicting thermodynamic and mechanical properties for refractory HEAs by computational simulation are rarely reported,and the relationship between the phase formation and mechanical properties needs further investigation.Finally,the current refractory HEAs usually show only compression properties,but the data of tensile properties are also needed in industrial applications.Thus,development of refractory HEAs with excellent tensile properties and the investigation of strengthening and toughening methods and mechanisms are of significance for refractory HEAs.In this thesis,the microstructure,strengthening and toughening mechanism of Ti-Zr-Nb based HEAs have been systematically studied.Reliable random solid solution structure was constructed for TaNbMoW and TiZrNbHf quaternary alloys.The phase structure,thermodynamics and elastic properties have been calculated by using first-principle calculations.These calculations are hopeful to provide theoretical guidance for the compositional design and development of novel refractory HEAs.Ti-Zr-Nb-Mo-V HEAs with high specific strength and Ti-Zr-Nb-Hf HEAs with excellent tensile strength and ductility were fabricated.And the strengthening and toughening mechanisms for these HEAs were investigated.The research contents of this thesis mainly consist of the following parts:A special quasirandom structures(SQS)were successfully constructed to mimic the quaternary BCC random solid solution structure.Thermodynamic and elastic properties can be calculated based on this structure by using phonon calculation method.The Poisson's ratio of TiZrNbHf refractory HEAs was calculated to be 0.315,which indicates good ductility in this alloy.The as-cast TiZrNbHf HEA exhibits tensile yield strength of 879 MPa and elongation of 16.5%,reflecting that validity of the simulation results.The in-situ tensile experiments under synchrotron radiation revealed that no phase transformation occurred during the plastic deformation of TiZrNbHf HEA,and the elastic moduli extracted from the in-situ tensile experiments match to the calculated results quite well.Appropriate combination of Mo and V results in excellent combination of specific yield strength and elongation.Especially,TiZrNbMoV possesses the highest yield strength of 1800 MPa,while TiZrNbMo0.3V0.3 exhibits the yield strength of 1312 MPa and compressive elongation higher than 50%.The specific strength of TiZrNbMo0.3V0.3 reaches 198 MPa·m3/kg.Thus it is considered that this HEA realized a perfect combination of the lightweight and ductility.Oxygen was introduced into the ductile TiZrNbHf refractory HEA for the strengthening.It has been found that the addition of O significantly increases the strength of TiZrNbHfO HEAs,and at the same time,maintains the ductility.The HEA containing 2.0 at.%of O exhibits excellent product of strength and elongation of 36500 MPa%,which is superior to that of most of conventional alloys and all of refractory HEAs.In-situ tension experiments under synchrotron radiation revealed that no phase transformation occurs during the plastic deformation of the alloy containing 1.5 at.%of O.The excellent tensile properties are caused by the interstitial solid solution strengthening effect of O.Besides,the effect of O on the kinetics of grain growth for TiZrNbHfO refractory HEAs was investigated.The mixing effect of multicomponents made TiZrNbHf exhibiting lower grain growth rate than that of pure BCC elements,and the addition of O further decreases the grain growth rate.Moreover,the addition of O causes the precipitation of the HCP phase at 873 K,which decreases the ductility of HEAs.The alloying effects of A1 on the microstructure of refractory TiZrNbHf HEAs were systematically investigated.All TiZrNbHfA1 HEAs with the A1 contents up to 12%are composed of single BCC phase after cold rolled with 80%reduction in thickness and solution treated at 1273K for 0.5h.When aged at 873K,the HEAs with more than 7%Al,a(Al,Zr,Hf)-rich phase with hexagonal crystal structure was precipitated.The AIMD simulation results reveal that the addition of A1 causes the formation of Al-Zr short range order in liquid state,and decreases the diffusion rate of A1 and Zr.Thus,intermetallics phase would tend to form during the solidification in these high A1 containing HEAs.The addition of A1 significantly increases the strength of TiZrNbHfAl alloys,but decreases the tensile ductility.The yield strength increases linearly with Al content,while the tensile elongation of alloys dropped significantly when the Al content is higher than 7%.Atomic size mismatch and difference of(s+d)electrons of constituents both attribute to solid solution strengthening of TiZrNbHfAl HEAs.Only dislocation cells were observed in(TiZrNbHf)93Al7.Micro bands were formed in TiZrNbHf and(TiZrNbHf)95Al5,which attribute to the great tensile elongation.(TiZrNbHf)95Al5 exhibits excellent combination of strength and elongation and structural stability at 873 K.These results indicate that(TiZrNbHf)95 Al5 may be a promising prototype alloy for further development of refractory HEAs for high temperature applications.