Deformation Mechanism Of High-entropy Alloys

In this thesis,three typical high-entropy alloys,FeCoNiCrAl0.1,NbHfZrTi and Ta0.5HfZrTi,were studied by using in-situ TEM tension method.The behaviors of dislocation slipping,twinning and phase transforming were observed for probing into microcosmic deformation mechanism.The conclusions were got as follows:There generally were dislocation arrays in FCC high-entropy alloys,typified by FeCoNiCrAl0.1.They often glided on one plane and cracks often existed on these planes.The dislocations were in chaos in BCC high-entropy alloys,typified by NbHfZrTi.They often glided on different planes in the same direction.Moreover,the other microstructures in high-entropy alloys would have some effects on slip.Grain boundaries would block dislocations.Conjugate planes would also block dislocations but supply the new glide paths.There are different deformation mechanisms at different deformation stages in FeCoNiCrAl0.1,At low stain the main deformation mechanism was dislocation slip.At medium stain stacking faults began to appear and gradually played an important role.At high stain a large number of twins could be found.Twins in high-entropy alloys were usually on conjugate planes and twin boundaries often had atomic steps.The twinning mechanism was suggested to be a dislocation cross-slip model.Perfect dislocation on primary plane would cross-slip onto conjugate plane and then dissociated to a Frank partial and a Shockley partial.The Shockley partials could be generated layer by layer through the above successive cross-slip.Twins would grow on conjugate planes by gliding of Shockley partials.There were two types of phase transformation,BCC?FCC and BCC?HCP,induced by stress-strain in metastable Ta0.5HfZrTi.Besides BCC ? HCP transformation could also be induced by thickness.The FCC lamellar would penetrate the sample.There were multiple orientation relationships between the three phases in Ta0.5HfZrTi.The common orientation relationship between BCC and FCC was Nishiyama-Wasserman relationship,(110)BCC/(11(?))FCC,[001]BCC//[011]FCC.While the common orientation relationship between BCC and HCP was Pistch-Schrader relationship,(110)BCC//(0002)HCP[001]BCC//[(?)]HCP.The interface between BCC and HCP was usually shown a transition region with 5?8 atomic thickness.It's helpful to deepen the understanding of high-entropy alloys by studying the microcosmic deformation mechanism.New understanding of deformation mechanism of high-entropy alloys is acquired through new material,new technology and new method.