放电等离子烧结Fe_(50)Mn_(30)Co_(10)Cr_(10)高熵合金的显微组织演化

Microstructure Evolution of Fe_(50)Mn_(30)Co_(10)Cr_(10) High-Entropy Alloy Fabricated by Spark Plasma Sintering

以金属元素粉末为原料,采用机械合金化(MA)+放电等离子烧结(SPS)法制备Fe_(50)Mn_(30)Co_(10)Cr_(10)高熵合金,研究了烧结温度、烧结压力对Fe_(50)Mn_(30)Co_(10)Cr_(10)高熵合金显微组织和力学性能的影响。结果表明,不同温度下烧结获得的Fe_(50)Mn_(30)Co_(10)Cr_(10)高熵合金均为包含fcc基体相和层片状hcp相的双相高熵合金。随着烧结温度升高,合金的晶粒尺寸不断增大,合金中hcp相和孪晶界的比例也在不断增大,同时硬度和屈服强度在不断降低,塑性持续增加。在1000℃烧结时合金达到最优的综合力学性能,其压缩屈服强度(σ_(0.2))、抗压强度σ_(max)、断裂时的塑性应变(ε_(p))和维氏硬度分别达到878 MPa,2013 MPa,25.0%和HV 315。随着烧结压力的增加,合金的晶粒尺寸增大,孪晶界的比例也不断增多,屈服强度差距不大,塑性却有着明显的提高。烧结压力从50 MPa升高到80 MPa时,屈服强度由791 MPa降低到762 MPa,而断裂时的塑性应变由19.2%升高至26.8%。

In recent years,with the continuous development of human society,the existing traditional metals and alloys could not fully meet the needs of human beings,thus promoted the invention and research of various new materials.In the field of metal materials,three new metal materials:bulk amorphous alloys,high-entropy alloys and composite materials have attracted people’s attention because of their excellent properties,and were considered to be the three most potential research directions of metal materials.High-entropy alloys are a new type of multi-principal element alloys,which had better comprehensive properties than traditional alloys,such as high strength,hardness and ductility.With the deepening of researches on high-entropy alloys,a Fe_(50)Mn_(30)Co_(10)Cr_(10)dual-phase highentropy alloy with TRIP(transformation induced plasticity)phenomenon has been developed.Deformation-induced martensitic transformation occured during the deformation,which could remarkably enhance the strength and ductility of the alloy.Melting and casting,powder metallurgy and deposition were three commonly used methods for preparing high-entropy alloys.The most common method used for preparing Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy was the casting method.During casting,multiple remelting and post-treatments were required to make the composition uniform and refine the crystal grains.By spark plasma sintering(SPS),a high-performance bulk material with fine grains and homogeneous structure could be obtained.A Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy with ultrafinegrained microstructure was prepared by mechanically alloying(MA)and spark plasma sintering using elemental metal powders as raw material.The uniformly mixed elemental powders were high-energy ball milled for 40 h to obtain Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy powders,and then the powders were densified on SPS-3.20 MKII spark plasma sintering system.The phase composition,microstructure(grain size,grain orientation and grain boundary characteristics)and mechanical properties of sintered Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy bulk samples were systematically analyzed by X-ray diffraction(XRD)analysis,electron backscatter diffraction(EBSD)analysis,micro Vickers hardness test and room temperature compression test.In addition,effects of processing parameters including sintering temperature and sintering pressure on the microstructure and mechanical properties of Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloys were studied.Based on the research results,the optimal process parameters of the Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy were obtained.The results showed that Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy powders with single fcc structure were obtained after ball milling for 40 h and the sintered Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy bulks exhibited dual phase microstructure containing the fcc matrix and hcp laminate layers.The results of EBSD analysis showed that the{111}crystal plane group of fcc phase was parallel to the{0001}crystal plane group of hcp phase,and the two phases satisfied the Shoji-Nishiyama(SN)orientation relationship in crystal structure.There were a lot of twins in the fcc matrix phase of the Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy obtained at all sintering temperatures,and with the increase of sintering temperature,the proportion of small angle grain boundaries gradually decreased,and the twin boundaries gradually increased.The average grain size of fcc matrix phase increased from 226 nm at 900℃to 660 nm at 1050℃,and that of hcp phase also increased from 76 to 220 nm.Meanwhile,the volume fraction of hcp phase increased gradually.The increase of the average grain size led to the weakening of the fine grain strengthening effect,which made the compressive strength and microhardness of the sintered Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy bulk samples gradually decreased,and the increase of the relative density of the sintered bulk led to the increase of the compressive plasticity.The sintered Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy bulk sample achieved the best comprehensive mechanical properties at the sintering temperature of 1000℃,with compressive yield strength(σ_(0.2))of 878 MPa,compressive strength(σ_(b))of 2013 MPa,plastic strain(εP)of 25.0%and Vickers hardness of HV 315.With the increase of sintering pressure,the diffusion driving force increased,which was beneficial to the sintering of powder,element diffusion and grain boundary migration,therefore resulted in the grain growth from 569 nm at 50 MPa to 662 nm at 80 MPa.With the increase of sintering pressure,the twin boundary fractions of Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloys increased significantly.Additionally,due to the increase of bulk density and average grain size,the yield strength of Fe_(50)Mn_(30)Co_(10)Cr_(10)high-entropy alloy bulk specimens lightly declined,but the plastic strain obviously increased.When the sintering pressure rose from 50 to 80 MPa,the compressive yield strength decreased from 791 to 762 MPa,but plastic strain sharply increased from 19.2%to 26.8%.