Microstructure Modulation And Deformation Behavior Of Multicomponent Metastable High Entropy Alloys

The first generation of high-entropy alloys(HEAs)are formed by solid solution of multiple dominant elements with equal or near-equal molar ratios.They have attracted much attention due to their simple phase composition,high strength and toughness,excellent wear,and fatigue resistance.However,the early design of equimolar composition is not a necessary factor for forming a stable single-phase structure,meanwhile,the subsequent introduction of the metastable engineering strategy breaks through the strength and ductility trade-off of the alloy,stimulating the research wave of the second generation of non-equimolar metastable HEAs.The metastable engineering strategy aims to reduce phase stability by composition design to achieve two mechanisms:phase transformation-induced plasticity and twinning-induced plasticity.Since the intrinsic stacking fault energy of the alloy related to phase stability is highly sensitive to factors such as composition,temperature,and strain rate,these factors will be directly related to the composition optimization and practical application of the alloy.However,the current research on these influence factors is not perfect.Also,the phase transformation behavior,triggering mechanism,and mechanical properties of metastable high-entropy alloys still need to be further investigated.In the present study,Gd micro-alloying Fe38-xMn30Co15Cr15Ni2Gdx(x=0,0.1,0.2 at.%,abbreviated as Gdx alloy)metastable HEAs and precipitation-strengthened Fe3sMn15Co15Cr10Ni10Ti6Al6 metastable HEA were prepared by vacuum electromagnetic induction melting,drop casting,thermal treatment and so on.By quasi-static and dynamic compression tests at room temperature,the mechanical properties of the above alloys were systematically studied.Combined with the characterization of the deformed microstructure,the effects of Gd element and strain rate on the microstructure evolution of Gdx alloys,and the effects of different thermal treatments on the microstructure evolution of precipitation-strengthened metastable HEAs were studied.It was found that the stability of the FCC phase in FeMnCoCrNi HEA decreased significantly with the solid solution of Gd element,resulting in a large number of strain-induced FCC→HCP phase transformations in quasi-static compression at room temperature.In the low strain rate range(10-3～10-1/s),the phase transformation is gradually inhibited with the increase of strain rate,which is attributed to the inhibition of strain rate-induced shear bands on the continuous growth of the HCP phase.In the dynamic strain rate range(1000～5000/s),the phase transformation is replaced by mechanical twinning due to the significant adiabatic temperature rise effect which increases the stability of the FCC phase.Under the influence of different deformation textures,the volume fraction of the HCP phase produced by phase transformation is significantly different in tensile and compressive deformation,resulting in the asymmetry of flow stress,which makes the above alloys show a significant Bauschinger effect.Massive phase transformation induced in Gd0.2 alloy inhibits the increase of pile-up density of geometrically necessary dislocation,resulting in a back stress relaxation effect.The quasi-in-situ EBSD characterization showed the obvious HCP→FCC reverse phase transformation during the loading and unloading process of Gd0.2 alloy,which proved the instability of the HCP phase.The established dynamic model points out the limitations of the classical back stress measurement method for metastable high entropy alloys.The Ni-depleted dual-phase Fe38Mn15Co15Cr10Ni10Ti6Al6 high-entropy alloy is composed of FCC metastable matrix and semi-coherent L21 nano-precipitation phase.Increasing homogenization temperature(such as 1200℃)can promote the transformation of precipitation behavior from discontinuous to continuous precipitation,resulting in significant precipitation strengthening.In quasi-static compression at room temperature,the formation of early texture delays the local concentration of strain,and the FCC→HCP phase transformation at the precipitate interface inhibits the initiation of interfacial cracks,which is beneficial to the improvement of plasticity.This chemical composition design concept containing multiple strengthening and toughening mechanisms is expected to provide insights into the development of next-generation high-performance HEAs.