Microstructural Origins Of The Anneal Hardening Effect In The Typical Multi-principal Element Alloys

The concept of multi-principal element alloy(MPEA)extends the scope of alloy design to the central region of the phase diagram,enlarges alloy design space,and is expected to develop high-performance engineering materials with high strength.In contrast to the traditional dilute solid solution alloys,the MPEAs show excellent performance over a wide range of temperatures(from-269℃to 1600℃ultra-high temperature).However,the strength of the eutectic and FCC-structured MPEAs are generally low,and the ductility of BCC-structured MPEAs is poor although they show a high strength.Cold deforamton is an effective way to strengthen metallic materials,and the strength of some cold-deforamed materials can be further improved by recovery annealing,which is referred as to anneal hardening.Anneal hardening phenomenon has been observed in many metallic materials with the FCC structure,but its microstructural mechanism is still controversial,the strength increment is small,and the ductility is greatly sacrified at the same time.Therefore,how to improve the anneal hardening effect,break the strength-ductility trade-off,and achieve a good balance of strength and ductility in metallic materials has always been a difficult problem for researchers to overcome.In this paper,we investigated the effect of cold deformation reduction and“solute”atoms on the anneal hardening in CoCrNi-based MPEAs.The microstructure evolution of cold deformed CoCrNi-based MPEAs before and after annealing was analyzed,and discussed the relationship between the microstructure and anneal hardening effect.Furthermore,Cr segregation was successfully introduced into the dislocation core by strain aging at low temperature,which realized the concurrent improvement of strength and ductility.Cold deformation is a prerequisite for anneal hardening.However,we observed an anneal-induced hardening effect in a cast AlCoCrFeNi 2.1 EMPEAs,where the strength is improved without sacrificing the ductility.The microstructural mechanism is discussed in detail in this paper.Th e main conclusions are as follows:(1)The mechanism of anneal hardening in MPEAs ramins controversial,and there are no well-defined“solute”and“solvent”atoms in MPEAs.It would be intriguing to investigate the solute effect on annealed hardening in MPEAs.The effects of Al addition(2 at.%)on the structure,mechanical properties and anneal hardening effect of CoCrFeMnNi MPEA were investigated.It is found that the micro addition of Al does not change the single-phase FCC structure of CoCrFeMnNi alloy but softens its hardness and strength.The deformation structures of the two alloys at a smaller strain show that the dislocation cross-slip behavior of CoCrFeMnNi MPEA is inhibited by the micro addition of Al,which may be cause the softening behavior.The yield strength of the cold-deformed alloys increases first and then decreases with the increase of annealing temperature,and the optimal annealing temperature is around 500℃,indicating that annealing hardening takes place in both alloys.Microstructural results show that no recrystallization occurs upon annealing before 500℃,the FCC structure of the alloys did not change,and the composition distribution is uniform in the nanoscale,indicating no phase decomposition and element segregation are observed.However,the structure of dislocation cells in the alloys is obviously refined after annealing,so the anneal hardening mechanism in both alloys is supposed to result from the strengthening of dislocation cells.(2)To further understand the relationship between anneal hardening and microstructure,the effect of deformation structure in different height reduction s on the anneal hardening of a CoCrNi MPEA was investigated.Firstly,the microhardness and strength evolution of the cold-deformed CoCrNi MPEA with increasing temperature were studied.Then,the multi-scale microstructure before and after annealing was analyzed by conventional characterization technique.It is found that the mechanical properties display a trend of increasing first and then decreasing with the increase of annealing temperature,and the optimal temperature is 500℃.Additionally,the strength and hardness increment upon annealing increases with the increase of deformation reduction,and then saturates at around 60%reduction.At a small strain level of 20%,the anneal hardening is primarily considered to be caused by the Suzuki hardening resulting from weak chemical segregation of Cr at the core of SFs.At intermediate and high strain levels of 60%and 90%,the formation of subgrains and nanograins after annealing mainly contributes to the anneal hardening in addition to the Suzuki hardening.A high density of defects remains within the grains,which deteriorates the tensile ductility.(3)To overcome the problem that anneal hardening effect is not significant and ductility is reduced,a method of multi-step strain and annealing was used to improve the anneal hardening effect,and increase the ductility.It is found that the strength increment accumulates gradually with increasing the number of cyclic deformations and subsequent annealing.In constrat to the single-pass treated samples,the yield strength increament of a multi-steps treated sample is five times higher than that of it,and the plasticity is not sacrificed.Microstructural characterizations show that the dislocation core is selectively partitioned with Cr atoms by carefully controlling the repetitive straining and annealing conditions.Meanwhile,it is observed that the dislocation core of the annealed samples shows a large dissociation distance,indicating that annealing treatment induces the segregation of Cr and reduces the stacking fault energy(SFE)of the alloy.By imaging the typical 60°mixed dislocation using HRTEM,the SFE was calculated to be 34.3m J/m~2 and 16.9m J/m~2 respectively for without and with annealing treated samples.The segregated Cr atoms can be effective obstacles for dislocation movement to strengthen the alloy,known as Suzuki strengthening.Meanwhile,the dislocations with Cr segregation exhibit high stability against thermal.Therefore,the dislocation with Cr segregation can be remained and accumulated continuously upon annealing,which leads to the accumulation of the anneal hardening effect.The reduction of the SFE is expected to increase the density of stacking faults and the formation of dislocation locks,which improve the work hardening rate,thus improve the ductility.(4)Cold working is a prerequisite for the anneal hardening,however,a n abnormal anneal-induced hardening phenomenon is observed in a eutectic AlCoCrFeNi 2.1 MPEA.The microstructural evolution,mechanical properties,and deformation mechanism of the eutectic AlCoCrFeNi2.1 MPEA associated with annealing temperature were systematically investigated using several different techniques,such as nanoindentation,XRD,SEM,and S/TEM.The main results are summarized as follows:An abnormal hardening is observed in the dual-phase(FCC+B2)AlCoCrFeNi2.1 EMPEA when the as-cast alloy is subjected to isothermal annealing.Nanoindentation experiments show that the FCC and B2 phases in the eutectic alloy harden separately with increasing annealing temperatures.The trend of the nanohardness value of the FCC phase as a function of annealing temperature remarkably resembles the yield strength and microhardness of the alloy itself,suggesting that the anomalous hardening peak at 650℃is attributed to order strengthening from the L12 phase.By contrast,the B2 phase hardens monotonically with increasing temperature.Based upon dislocation structure evolution and mechanical analyses,we conclude that a Friedel shear of the L1 2 phase in the FCC phase and an Orowan looping in the BCC phase are strengthening mechanisms.Both phases are strong and effective for strengthening,in addition to the strong interfacial strengthening owing to the high thermal stability of the eutectic interface.