In-Situ Study Of Room Temperature Tensile Deformation Behavior Of CrMnFeCoNi High-Entropy Alloy

In this dissertation,CrMnFeCoNi high-entropy alloy(HEA)was selected as the research subject.The microstructure evolution of the HEA samples during deformation were characterized by using advanced analysis and testing technologies,including in-situ experimental methods,digital image correlation(DIC)technology and three-dimensional(3D)visualization image.The deformation mechanism of CrMnFeCoNi HEA during tensile tests were systematically clarified.The feature of slip bands in CrMnFeCoNi HEA during tensile deformation was examined.The results show that only one slip system activated in the initial deformation stage,and slip bands increased,accompanied by the appearance of new slip planes.With ongoing deformation,the second slip system is activated and more interactions occurs between dislocations.The increase in the dislocation density improves the deformation resistance.In the final deformation stage,the loading stress is high enough.The dislocations reached saturation,and slip bands no longer increases.A higher dislocation density promotes the meet of the opposite sign dislocations,and thus increases the probability of annihilation of dislocations.With increasing the applied load,the cracks initiate and propagate at the final stage of tensile deformation.The initial crack firstly forms inside the HEA sample,and starts deflect,which causes the change from a sharp state to a blunt state for the crack tip.When two adjacent grains deflect with different orientations,plastic deformations with different degree occur,and the main crack tends to propagate in the larger deformation side.At the same time,the crack tip opening angle(CTOA)value was measured.It was found that the CTOA value decreased rapidly from 65 ° to 26 ° and finally stabilized at 22 °.In the plastic deformation stage of CrMnFeCoNi HEA,the strain concentration happens at the notch root of the HEA tensile sample.Strain distribution results show that for the transverse strain distribution before crack initiation,with deformation time prolonging,the local strain at the notch increases rapidly,and the local strain nephogram shows a unique shape of strain distribution.After crack initiation,the crack propagation path can be seen according to the transverse strain .The longitudinal strain distribution shows that the strain converges in the middle of the HEA sample,and the shear strain distribution shows a bilaterally symmetric strain distribution.The DIC experiments were carried out for the samples with different notch sizes,and it was found that with the increase of load the deformation of the sample near the notch gradually increased.When the deformation reached a certain extent,the wrinkle of the sample surface could be distinguished by naked eyes.The quantitative analysis shows that the maximum strain increases with the increase of notch size under the same applied load.Along the plane of the notch root the strain distribution has a gradient change.With the distance from the notch root increasing the strain value decreases.Using threedimensional reconstruction scanning to analyze the distribution of two fracture dimples,it is found that the two sections of the corresponding part shows a peak-to-peak,valleyto-valley corresponding characteristics in its height data.