Composition Design And Fundamental Research On Selective Laser Melting Of AlCrCuFeNi_x High Entropy Alloys

AlCrCuFeNi system dual-phase high entropy alloys（HEAs）exhibit a high strength-ductility combination.However,the AlCrCuFeNi system dual-phase HEAs prepared by selective laser melting（SLM）are easy to deviate from the equilibrium state,forming metastable phases and cracks,which makes them difficult to have high strength and ductility.In this work,via experimental research and theoretical analysis,the non-equilibrium composition of AlCrCuFeNix dual-phase HEAs was designed,the microstructural evolution and mechanisms of SLM-built AlCrCuFeNix HEAs were revealed.Also,the cracking problem of this alloy was solved and the mechanical properties were regulated,to achieve a good strength-ductility combination.The main contents and results are as follows:（1）Based on the first-principles calculation,the driving force of rapid solidification was analyzed,a phase stability model of non-equilibrium AlCrCuFeNi system HEAs was established,and the phase transition behavior of fcc and bcc phases at finite temperatures was predicted.Mixing enthalpy can explain the contribution of enthalpy to phase stability at lower temperatures（T<900 K）.At higher temperatures（T≥900 K）,configurational entropy dominates the phase stability.In the same alloy composition,the solidification driving force of bcc phase in AlCrCuFeNi system is higher than that of fcc phase,favoring the formation of bcc phase during rapid solidification.（2）The microstructures of SLM-built AlCrCuFeNix HEAs were studied,and the microstructural evolution and mechanisms were revealed.Basket-weave microstructure is formed in the as-built Ni1.0 alloy.Adding Ni induces the eutectic reaction,forming the lamellar eutectic microstructure and reducing the basket-weave microstructure.The lamellar eutectic microstructure is composed of fcc phase and spherical bcc nano-precipitates reinforced B2 phase,basket-weave microstructure is composed of B2phase and striped bcc nano-precipitates.With the increase of Ni content,the alloy changes from hypoeutectic（Ni2.0 and Ni2.5）to near-eutectic（Ni3.0）to hypereutectic（Ni3.5）.During the eutectic reaction,the constitutional supercooling induced by Cr and Fe solutes enrichment ahead of the solid/liquid interface facilitates heterogeneous nucleation and the formation of near-equiaxed grains.（3）Based on the microstructural evolution and mechanisms,the cracking behaviors of SLM-built AlCrCuFeNix HEAs were studied,the valence electron concentration range of phase formation and crack control criterion were proposed.Cold cracks were initiated and propagated in the B2/bcc basket-weave microstructure of Ni1.0 and hypoeutectic alloys（Ni2.0 and Ni2.5）.The thermal strain accumulated by multiple rapid thermal cycles during SLM generates cold cracks in the hard and brittle B2/bcc basket-weave microstructure.The synergy of the lamellar eutectic structure and the small near-equiaxed grains effectively prevent the initiation and propagation of cracks,thus the cracks disappear in Ni3.0 alloy.The reduced eutectic liquid phase at the solidification end fails to fill the intergranular gap caused by solidification shrinkage,generating hot cracks at the grain boundaries in the as-built Ni3.5 hyper-eutectic alloy.When the valence electron concentration is between 7.82 and 8.40,fcc and bcc phases coexist.When the valence electron concentration falls in the range of 8.15～8.40,the cracking behavior can be controlled for the SLM-built AlCrCuFeNix HEAs.Based on the rapid solidification dendrite growth and eutectic growth models,the lamellar eutectic coupled zone of the AlCrCuFeNix HEAs was predicted.This criterion can be used to control the cracking behavior of the rapidly solidified AlCrCuFeNix HEAs.The undercooling degree to lamellar eutectic microstructure and avoid cracking for the SLM-built Ni3.0 alloy is56～185 K.（4）The mechanical properties of SLM-built AlCrCuFeNix HEAs were studied,and the effect of subsequent heat treatment on microstructure and mechanical properties of the as-built Ni3.0 HEA was investigated.The as-built Ni3.0 HEA possesses the best room temperature compression and tensile properties.The deformation mechanism showed that the fcc phase and B2 phase are plastically deformed by dislocation slipping and dislocation shearing,respectively.The eutectic microstructure composed of soft FCC phase and hard B2 lamellar produces intragranular back stress during plastic deformation,the bcc nano-precipitates complement the intragranular back stress.The mechanical properties of Ni3.0 eutectic dual-phase HEA are obviously improved by dislocation strengthening,precipitation strengthening,intragranular and intergranular back stresses strengthening.The abnormal phase transition of FCC→B2 occurs in the as-built Ni_3.0alloy between 700℃and 900℃,which increases the room temperature compressive yield strength but reduces the fracture strain.This alloy was soluted at 900℃for 1 hour followed by aging at 500℃for 6 hours,the room temperature compressive strength（～2952.5 MPa）and fracture strain（～45.2%）reach a good combination.The developed heat treatment regime is 900℃solution for 1 hour and air cooling,followed by aging at500℃for 6 hours and air cooling.This research is of guiding significance to the composition design of non-equilibrium rapid solidification alloys and to solve the cracking and strength-ductility trade-off problems of SLM-built HEAs.