Cryogenic Deformation Behaviors Of CrMnFeCoNi High Entropy Alloy

In this dissertation,the deformation behaviors of CrMnFeCoNi high-entropy alloy(HEA)were studied from 298K down to 123K.The microstructure of the HEA at cryogenic temperatures has been systematically characterized.The effect of the testing temperature on its deformation mechanism was explored.Cryogenic tensile results show that the mechanical properties of the HEA increase significantly with decreasing temperatures.The yield strength,tensile strength and elongation of the CrMnFeCoNi HEA increase from 170 MPa,300 MPa and 24%at 298 K to 320 MPa,650 MPa and 34%at 123 K,respectively.The strain hardening rates of the HEA are the highest at 123K while the lowest at 298K during tensile deformation,indicating that the strain hardening capability of the HEA increases with decreasing temperature.Evolution of the lattice strain in the HEA during deformation illustrates that the variation of lattice strain is strongly dependent on the grain orientations,indicative of strong elastic anisotropy.The elastic anisotropy of the HEA increases significantly with decreasing temperatures.Meanwhile,the variations of normalized diffraction peak intensities and full width at half maximum indicate the strong plastic anisotropy of the HEA during tensile deformation at cryogenic temperature.Since the elastic and plastic anisotropy of the HEA increase obviously with decreasing temperatures,the slopes of the lattice parameter-true stress curves increase from 0.24 at 298 K to 0.36at 123 K,aggravating the lattice distortion of the crystal at cryogenic temperatures.Large lattice distortion can increase the Peierls-barrier stress and hinder the nucleation of dislocations in{111}planes,which is a reason for the enhanced yield strength of the HEA at the lower temperatures.In situ synchrotron-based high-energy X-ray diffraction tensile results show that no any new peaks are detected from diffraction curves,indicating that no martensitic phase transformation occurs in the HEA from 298K down to 123K.However,the deformation induced nano-twins are found in samples at 123K by Transmission electron microscopy.Twining can provide an additional deformation mode to accommodate plasticity.Meanwhile,the twin boundary may prevent the motion of dislocations,producing the dynamic Hall-Petch.In addition,the higher dislocation density is calculated during plastic deformation at cryogenic temperatures than at room temperature.The combination of dislocation hardening and mechanical twinning lead to an increase of the cryogenic strain hardening capability of the CrMnFeCoNi HEA,the strength and ductility of the HEA also increase significantly.Upon plastic deformation,the passage of Shockley partial dislocation in current FCC HEA can create stacking fault forming twin nucleuses.Using diffraction line profile analysis,the stacking fault energy(SFE)of the HEA is estimated to be 16mJ/m~2 at 123K and 35.6mJ/m~2 at 298K under the same strain level.This means that the SFE decreases significantly with decreasing temperature,leading to the higher stacking fault probability(SFP)at cryogenic temperature.The SFP of the HEA is 0.04at 123K while is 0.015 at 298K at similar strain levels.The SFE in CrMnFeCoNi HEA decreases significantly at cryogenic temperatures,revealing that the reason of the deformation induced nano-twins is observed at 123K.For this HEA,the increased work hardening prevents the early onset of necking instability during the tensile deformation at the lower temperatures.Meanwhile,the increased slip bands at cryogenic temperatures indicate that the number of dislocations increases during tensile deformation,leading to an enhanced strength of the HEA.The size of fracture dimples of the HEA at cryogenic temperatures is larger and the depth is shallower,which is not conducive to the continued existence of the second phase particles,thus,the dislocations are easily to slip to the grain boundary.The ductility of the HEA also increases significantly with decreasing temperatures.Higher dislocation density at room temperature is observed in this HEA after pre-compression strains of 50%than 20%,but twins are not detected using TEM.Numerous dislocations and stacking faults are found in alloy when the compression deformation reaches 5%.Up to 9%,the dislocations,stacking faults,and nano-twins are observed in samples.As the strain reaches 12%,the number of dislocations and twins have a significant increase.When the strain reaches 50%,there are a large number of dislocations and nano-twins are produced.The CrMnFeCoNi high-entropy alloy studied in this dissertation may produce twins when the compressive strain reached 9%,then,the number of nano-twins increases at progressively higher strain values.