Twin Variant Identification And Austenite Parent Reconstruction Of The Nanoscale Martensite In Negative Thermal Expansion Ti?Ni Alloys And Their Phase Transformation Behavior

Ti?Ni alloys exhibit excellent shape memory effect and unique superelasticity due to the thermoelastic martensitic transformation and self-accommodation of martensite variants.At present,as the most conventional approach applied to characterizing nano-scale B19?martensite variants in Ti?Ni alloys,transmission electron microscopy(TEM)is insufficient to acquire statistical results in the study of martensite variant identification,B19?martensitic microstructure and orientation relationship(OR)between the B2 austenite and monoclinic B19?martensite.Besides,this approach also requires the retained austenite as a reference to identify martensite variants and determine the precise OR between B2 and monoclinic B19?in Ti?Ni alloys.In addition,it is tedious and time-consuming to identify B19?martensite variants by TEM,let alone that the twinning elements cannot be fully acquired in this way.Therefore,there is a need for developing an efficient method to achieve accurate statistical results of martensite variant identification,B19?variant microstructure,OR between B2 and B19?as well as twinning characteristics between B19?variants,which are crucial to the comprehensive understanding of martensitic transformation in view of crystallography in Ti?Ni alloys.Moreover,significant negative thermal expansion(NTE)behavior in Ti?Ni alloys has received increasing interests recently.However,the influence of martensitic transformation on the NTE behavior and the underlying micro-mechanism have not been clarified.Therefore,it is necessary to conduct a systematic and in-depth study on the martensitic transformation related NTE behavior in Ti?Ni alloys.In this thesis study,the off-axis transmission Kikuchi diffraction technology in a scanning electron microscope(Off-axis SEM-TKD)is used to characterize nano-scale B19?martensite variants and their orientations.According to TEM observations,both step-terrace and lamellar structures have been found in the convention cast Ti₅₀Ni₅₀(at.%)alloy,which are composed of4 and 2 martensite variants,respectively.More than 70%of the variants show a width smaller than 50 nm and more than 99%of the variants have a width larger than 5 nm.By using off-axis SEM-TKD with the optimized characterization parameters,orientation maps of nano-scale B19?martensite variants in the two structures have been successfully obtained,with the indexing rates of 88%and 85%,respectively.The characterization parameters are optimized as follows:mounting the thin foil sample with a thickness of 87?180 nm in the thin area on the pre-tilted sample stage(51-H5-550)with a 20°inclination angle to the electron beam,using an acceleration voltage of 30 kV and a current of 3.2 nA for data acquisition,and adopting a working distance of 2?3 mm and a step size of 3?5 nm.Based on the framework of the theoretical groupoid structure method,an improved method for identifying B19?martensite variants of Ti?Ni alloys without retained austenite is proposed in this study.This method improves the efficiency and accuracy of matching the experimental axis/angle pairs of the rotation with the theoretical ones,and the identification results are consistent with that obtained by the theoretical groupoid structure method and the standard stereographic projection method.Furthermore,according to the classic theory of twinning,a method for directly determining the type of twinning and twinning elements in Ti?Ni alloys using off-axis SEM-TKD data is proposed.The method has been applied to studying the twinning characteristics of step-terrace and lamellar structure B19?martensite variants in the convention cast Ti₅₀Ni₅₀ alloy.The results show that the adjacent martensite variants in the step-terrace structure exhibit the(011)B19?type II twin and(100)B19?compound twin relations,whereas the adjacent martensite variants in the lamellar structure have a typical(011)B19?type II twin relation.These results are consistent with the twin relations determined by using TEM in the literatures.In order to further obtain the orientation of the prior parent phase,an algorithm for reconstructing the parent orientation of B19?martensite by using the off-axis SEM-TKD data is proposed based on the classic parent orientation reconstruction method.The numerical fitting process is used to determine the OR firstly,then the adjacent pixels are grouped according to the value of misorientation,and finally the parent orientation is reconstructed pixel by pixel in the unit of cluster.The results of parent reconstruction in Ti₅₀Ni₅₀ alloy show that the local misorientation of the reconstruction zone corresponding to the martensite variant boundary is less than 2.0°.The retained austenite method and in-situ heating in automated crystal orientation mapping(ACOM)with TEM were used to validate the as-proposed algorithm,the validation results show that the B2 orientation has been accurately reconstructed from the orientation map of B19?martensite by using the as-proposed algorithm.Systematic studies have also been carried out to investigate the influence of phase constitution and microstructure characteristics on the martensitic transformation and NTE behavior of Ti?Ni alloys.In the convention cast Ti?Ni alloys with microstructure characteristics of both preferential growth and non-preferential growth,all the NTE characteristic temperatures measured along all directions are very close to the corresponding characteristic temperatures of phase transformation,indicating that the macroscopic NTE behavior of the alloy is closely related to the microscopic martensitic transformation.The preferentially grown Ti?Ni alloys with Ni content of 38.0?46.0 at.%and the homogeneously grown Ti?Ni alloys with Ni content of 38.0?50.0 at.%show reversible isotropic NTE behavior during the heating and cooling stages,and the coefficient of thermal expansion(CTE)in all directions decreases with the increase of Ni content.The preferentially grown Ti?Ni alloys with Ni content of 48.0?50.0 at.%show an anisotropic NTE behavior during the heating and cooling stages.The CTE along the direction of the maximum temperature gradient sharply decreases with the increase of Ni content,while the CTE along the direction perpendicular to the direction of the maximum temperature gradient increases with the increase of Ni content.In addition,an in-depth study has been done to look at the correlation between the NTE performance and the phase transformation behavior of the Ti?Ni alloys during thermal cycling.The convention cast Ti₅₀Ni₅₀ alloy exhibits near-zero thermal expansion behavior after undergoing 25 thermal cycles.Both the thermal expansion and DSC curves show that the two-step phase transformation of B2?R?B19?occurs on cooling.The changes of characteristic parameters of NTE and phase transformation during the thermal cycling of the suction-cast Ti₅₄Ni₄₆ alloy exhibit two obvious stages:The characteristic parameters of NTE and phase transformation change rapidly in the first 30 thermal cycles,then change gradually in the 31^st?100^th thermal cycles and finally tend to be relatively stable.After100 thermal cycles,both the characteristic temperatures of NTE and phase transformation shift to lower temperatures,and both the coefficients of NTE during heating and cooling stages are-23.1×10^-6 K^-1.