High-pressure Synchrotron Radiation X-ray Studies On The Polymorphic Transitions Of High-entropy Alloys

Polymorphic transition(PT)refers to the change from one crystal structure to another structure of the same composition in a crystalline material.Research on the PTs of alloys can not only gain in-depth understanding of phase structures of alloys,but also open up new avenues towards tuning their microstructures and properties.Pressure,as one of the main influencing factors,has been employed to understand the PTs of alloys.However,most studies of pressure-induced PTs of alloys focused on the binary and ternary solid solutions based on one principal element.High-entropy alloys(HEAs),which contain multiple principal elements in an equal molar ratio,maximizes the mixing entropic contributions to the thermodynamic landscape and promotes the formation of multicomponent solid solution.Investigating PTs of HEAs under high pressure is not only important for understanding PTs itself,but also shed new light into the phase stability of HEAs.In this thesis,pressure-induced PTs in the HEAs with different lattice structures and the related mechanisms were studied by employing variously advanced in situ high-pressure synchrotron radiation X-ray techniques.Effects of non-hydrostaticity of the pressure media and grain size of the initial sample on the onset pressure of phase transition were determined.Also,the effects of types and numbers of constituent elements on the PTs of face-centered-cubic(FCC)HEAs were systematically investigated,and the relationship between various structural parameters and PT was established via analyzing the changes of crystal and electronic structures.The main findings of this thesis are as follows:(1)The pressure-induced PTs of the prototype FCC CoCrFeMnNi HEA was investigated in detail.A PT from FCC to HCP(hexagonal-close-packing)in this HEA was observed at room temperature.The transition starts at?22 GPa and almost completes at?41 GPa,and the transformed HCP phase could be retained when the pressure was released.The synchrotron radiation XRD and HRTEM results revealed that the orientation relationship between the FCC and the HCP phases is consistent with the classic Shoji-Nishiyama relation for the typical ?-martensitic phase transitions,i.e.,(111)FCC//(0002)HCP and[110]FCC//[1120]HCP.In situ high-temperature high-pressure X-ray diffraction experiments show that the FCC phase is stable at high temperatures while the HCP structure is more thermodynamically favorable at relatively lower temperatures.Formation of the FCC phase follows the Ostwald's rule,i.e.,high-temperature polymorphs often form first and retain during the solidification process.(2)The structure evolution behaviors of body-centered cubic(BCC)AlxCoCrFeNi(x=0.6,1,1.5,and 2)HEAs under high pressure were studied.Partial reversible pressure-induced PT was observed at 17.6 GPa in the ordered BCC(i.e.,B2)AlCoCrFeNi HEA.The PT in the BCC AlxCoCrFeNi HEAs becomes more difficult as the Al content is increased.Comparative analysis shows that,with more Al added,the enlarged difference of Pauling electronegativity in HEAs enhances the interaction among the components,leading to the formation of more valence like bonding(e.g.,Ni-Al and Fe-Al)and thus hindering the PTs under high pressure.(3)The pressure-induced PTs of refractory HEAs containing Ti,Zr,and Hf elements were systematically investigated.It was found that the B2 lattice of the TiZrHfCoNiCu HEA becomes distorted at 10.2 GPa and transforms to B19'structure at 26.7 GPa.This transition is fully reversible after releasing the pressure.However,no phase transition in the BCC TaTiZrHf HEA was observed up to 40.1 GPa.The analysis shows that the B2 TiZrHfCoNiCu HEA seems to inherit B2-B19'PT of binary TiNi and CuZr alloys,whilst the BCC TaTiZrHf HEA represents the phase stability of Ta metal under high pressure.(4)Effects of non-hydrostaticity of the pressure medium and grain size of the HEA samples on the onset pressure of PTs were investigated.The non-hydrostaticity of pressure medium can prompt pressure-induced PTs in the CoCrFeMnNi and TiZrHfCoNiCu HEAs,which revealed that the significant role of lattice distortion caused by non-hydrostaticity and the dominant shear mechanism in the PTs under high pressure.The smaller grain size of the initial sample can postpone pressure-induced PTs in the CoCrFeMnNi and AlCoCrFeNi HEAs,which is associated with the significant increment of the grain interface energy of the high-pressure phase nuclei once the grain size of the starting material decreases.(5)PTs under pressure,along with the crystal and electronic structure,of the CoCrFeMnNi HEA and its FCC subsets were studied.Diverse PT behaviors were observed in these 8 equiatomic alloys during compression and decompression,which includes irreversible FCC to HCP in CoCrNi,CoCrFeNi,and CoCrMnNi and reversible FCC to HCP PT in CoFeMnNi and FeMnNi.However,no phase transition up to?40 GPa was detected in CoFeNi,CoMnNi,and CrFeNi.With the increase of applied pressure,high spin of the Fe and Co elements in the HEAs investigated changes to low spin state,and local magnetic moment of their 3d electrons decreases.Once the pressure is released,the electron will back to its initial state.Therefore,it is concluded that the reduction of local magnetic moment is not a controlling factor for the pressure-induced PTs in the FCC multicomponent HEAs.(6)Effects of constituent types and structural parameters on the pressure-induced PTs of FCC multicomponent HEAs were analyzed.It was found that the pressure-induced PTs of HEAs have no relation with their particular constituents.In addition,PTs of HEAs are not dependent on the atomic size difference,configuration entropy,and valence electron concentration,but stacking fault energy(SFE)indeed shows a strong correlation with the PTs,namely,the PTs occurred much easily in the alloys with lower SFE.In summary,these results not only deepen the understanding of phase stability of HEAs,but also lay the foundation for manipulating of microstructures and properties of HEAs via adjusting pressure.