Microstructure And Mechanical Properties Of A Low-density AlNb₂TiV Refractory High-entropy Alloy

At present,requirements for heat-resistant alloys become higher and higher in more and more fields,such as aerospace and power generation industries.The performance of conventional heat-resistant alloys has been approaching their limits.Therefore,in order to meet the needs of the rapid development of these industries,the exploitation of new heat-resistant alloys has become extremely urgent.High-entropy alloys(HEAs),as kind of new-alloy design strategy,have been widely studied and got rapid development in recent years.Different from the traditional design methods,which are based on one principal element,HEAs contain several principal elements,according to the preparation methods of(near)equiatomic ratio.Nowadays,the varieties of HEAs have been rapidly increasing,exhibiting unique performance,especially the refractory HEAs(RHEAs),which are hopeful to replace conventional superalloys and become a new generation of heat-resistant alloys.Most existing RHEAs have problems like high density,low specific strength,severe brittleness at room temperature and insufficient oxidation resistance,which limit the further development and application of this kind of alloys.Therefore,this thesis focuses on a low-density RHEA Al Nb₂Ti V,which was put forward by our research group previously.Firstly,as-cast alloys were acquired by high-vacuum arc melting and drop casting,and the microstructure,composition distribution and mechanical properties at different temperatures have been studied.Results show that the as-cast Al Nb₂Ti V alloys,with a density of about 6.19 g/cm³,lower than most reported RHEAs,whose microstructures are composed of typical dendrites and the phase structure consists of single B2 phase.Room temperature compression performance shows that the room-temperature yield strength,specific yield strength and compressive plastic strain of the as-cast Al Nb₂Ti V alloys are about 1043MPa,167MPa·cm³/g and 100%,performing excellent deformability at room temperature.In addition,the yield strength of this alloy at 600?and 800?are 694MPa and 612MPa,decreasing with the increasing temperature.When the temperature rises to1000?,the yield strength of this alloy suddenly drops to 144MPa.The deformation behavior and microstructure evolution of the as-cast alloy during high temperature thermal simulation experiments at 1000?,1100?and 1200?with strain rates of 10^-1s^-1,10^-2s^-1 and 10^-3s^-1 respectively were further studied.In the process of high-temperature deformation,the hardening phenomenon dominated by work hardening and the softening phenomenon dominated by dynamic recovery and dynamic recrystallization occur.The fitting calculation results of rheological stress show that the apparent activation energy of the alloy decreases slightly with the increase of strain,but basically stable in the range of401?375k J mol^-1,with a small variation range,especially the apparent activation energy value remaining almost constant after the strain reaches 0.5.In addition,EBSD analysis under different conditions shows that two dynamic recrystallization mechanisms occur simultaneously in the alloy,namely discontinuous dynamic recrystallization and continuous dynamic recrystallization.This study provides a reference for further composition design by using the criterion of high-entropy solid solution and obtaining a new type of heat-resistant alloy with low density,high strength and intrinsic plasticity.The research results of high-temperature deformation behavior and microstructure evolution provide theoretical support for the design of high temperature plastic deformation process of the alloy.