In-situ Synchrotron X-ray Diffraction Studies On Micromechanical Behavior Of Metals With Cubic Crystalline Structure

The in-depth research of the relationship between mechanical behaviors and microstructure is the fundamental requirement for improving the mechanical performances in metallic alloys.Generally,macroscopic mechanical properties are dependent on the microstructure of materials,including chemical composition,crystalline structure,phase constituent,phase distribution,grain size and grain orientation.Through the characterization of microstructure,various micro-mechanism,such as dislocation slide,deformation twinning,phase transformation and crack growth could be revealed to explain some peculiar macroscopic mechanical behaviors,such as non-linear elasticity and superelasticity.Then,it can provide new insight of designing alloys and enhancing mechanical properties.Metals with cubic crystalline structure,such as Ti,Fe and Al,are widely used in the engineering application.Especially for titanium alloys and steels,are used as key engineering parts due to their excellent mechanical properties.In this thesis,we chose two kinds of metals with cubic crystalline structure,i.e.,metastable?-type titanium alloy(Ti2448)and stable austenite stainless steel(AL6XN).Based on metastable microstructural characteristics in Ti2448 alloy,we focused on the research of the relationship between phase transformation and macro-mechanical behaviors,theoretically providing new idea of designing advanced structural materials with high strength and plasticity.Through the study of stress distribution and failure behavior during the crack growth in stable AL6XN steel,we can figure out the evolution of defect and micro-stress in grains with different orientation,which can contribute to the increase of service life in structural materials.Synchrotron radiation experimental techniques(XRD and XRI)can not only achieve a deep penetration in samples without damage,but also possess many advantages like high spatial resolution and high intensity.Therefore,they are suitable to be used in the investigation of the evolution of microstructure during phase transformation.The in-situ observation techniques have many obvious advantages of characterizing phase transformation(crystallography and kinetics)in different temperature and stress conditions.Through the experimental data collected from these in-situ techniques,we can accomplish the microstructural measurements from micro-scale to nano-scale and establish the model of deformation mechanism which can contribute to explain macro-mechanical behaviors and design new multifunctional alloys.On the other hand,based on the characteristic of deep penetration in samples,we can combine XRD and XRI techniques together to directly observe the crack growth and make some measurements of the stress field.The research of crack growth and stress distribution contributes to deeply understand fracture behaviors.Ti2448 alloy is a new kind of metastable?-type titanium alloy,which has an extensive application in the biomedical area due to its relatively low elastic modulus and excellent strength and plasticity.However,it is still controversial to explain the relationship between deformation mechanism and macro-mechanical behaviors.Due to the metastable microstructure in Ti2448 alloy,the stress-induced martensitic transformation is considered to be one of possible deformation mechanisms.The other deformation mechanism introduces the nanodisturbance as a new kind of defects which accommodate the deformation strain.In this thesis,we conducted uniaxial tension and compression tests,as well as dynamic shock compression,on the Ti2448 alloy.Through the change of experimental parameters,such as temperature,deformation direction and stain rate,we used in-situ XRD technique to observe the kinetic evolution of phase transformation.We found that two kinds of reversible nano-scaled stress-induced martensites(??and?)were formed by tensile stress.On the other hand,the irreversible stress-induced?martensite was formed by compressive loading.It revealed the relationship between tension-compression asymmetry and microstructure.The direct evidences on two kinds of stress-induced martensites(??and?)also explained the novel intrinsic physical origin of non-linear elasticity and superelasticity.Meanwhile,the formation of?martensite was closely related to the deformation direction.The?martensite prefer to be formed through<001>_?and<110>_?compressive loading,but not<111>_?compression direction.In the dynamic shock compression tests,we in-situ observed the formation of?martensite in micro-second temporal resolution and proved that the strain rate almost had no influence on the elastic modulus.All these conclusions have deeply revealed the connection between phase transformations and macromechanical behaviors in different deformation modes,and provided theoretical supports to prepare new kinds of multifunctional materialsAL6XN stainless steel is one of candidate materials used for the protection of fourth generation of nuclear reactor.It is important to make some detailed studies on its fatigue properties and fracture failure behaviors so that AL6XN stainless steel could have more practical applications.Due to the existence of stable austenite microstructure in AL6XN,we can avoid the influence of new phases and focus on the measurement of stress distribution during crack growth.First,we conducted cyclic tension-compression fatigue tests on the AL6XN in order to introduce some defects into samples.High density of defects can contribute to the crack initiation and growth during the tension deformation.Second,we applied the XRI technique to in-situ observe the growth of crack during the uniaxial tension and locate the area ahead of crack tip.Third,the XRD technique was employed to directly measure the stress field ahead of crack tip.Compared to the as-received samples,the samples after fatigue cyclic tests featured less tensile stress due to its more defects inside.Also,the crack initiation and growth could lead to the decrease of tensile stress.Through the evaluation of stress field ahead of crack tip,we have known that the lattice strain is considered to have an increase with the approach of crack due to the stress concentration which results in the non-uniform stress distribution in the whole specimen.Meanwhile,we found that the crack tend to propagate through{200}oriented grains.Because{200}oriented grains accumulated a large density of dislocation and formed planar slip bands during the fatigue cyclic tests,which making the{200}oriented grains more likely to be damaged than other oriented grains.On the other hand,{111}oriented grains had less damage during the fatigue cyclic tests so that they could bear more stress during the tensile tests.All these results exhibit a correlation between fracture and microstructural damages from the fatigue cyclic process,and help us to increase the service life of structural materials.