| Titanium and its alloys have been extensively used in aviation,aerospace,chemical industry,and other fields,owing to their advantages of high strength-to-density ratios and good corrosion resistance.Deformation processing is the main forming process of titanium alloys,and the research on the relationship between their microstructure and deformation processing conditions has been a research hotspot in this field.Commercial-purity titanium TA2 was one of the most widely used ?-type deformed titanium alloys.In this dissertation,the TA2 plate was taken as the research object.The plate was pre-deformed by cryorolling and adjusting the initial texture,and thus different types and proportions of twins were introduced into the sheet to improve the properties of the sheet.On this basis,the present study focused on the cryorolled A plate(initial texture ND//c-axis)which had the best mechanical properties and most abundant twins,by using the electron back scattering diffraction(EBSD)technology.And the deformation mechanism and twinning behavior of titanium subjected to cryorolling were systematically expounded:the variant selections of the dominant {1122} and {1124} twinning type in cryorolled A plate were studied by the generalized Schmid law;the interaction of {1122}and{1124} twins in the grains and the related sequential twinning phenomenon were also studied;the strain accommodation and propagation of {1122} and {1124}twins at the grain boundary were investigated.The reason of the cryorolled A plate with abundant twins was established.The above studies were expected to provide theoretical support for the development and preparation of high-performance titanium plate.The main conclusions were as follows:(1)Low-temperature deformation conditions and the initial texture of ND//c-axis can introduce a large number of {1122} and {1124} compression twins in the sheet with a small reduction,making the A plate own high strength and good ductility.(2)The variant selections of twins in cryorolled A plate are systematically expounded.The results revealed that most of the {1122} and {1124} primary twinning variants obeyed the generalized Schmid law,and the maximum generalized Schmid factor maps showed that the basal texture was conducive to the formation of these two twinning types.Three types of {1124}-{1012} double twins were characterized.The variants in GA group(with a misorientation of 39.5°<2243>with respect to the matrix grain)had the best accommodation capacity of self strain,and the GA variant with a high generalized Schmid factor was preferred.(3)When a {1122} twin interacted a {1124} twin with a misorientation of 12.4°<1100>within one grain in the cryorolled A plate,a {1012} secondary twins can be formed.This {1012} secondary twin can accommodate the strain induced by the incoming {1122} twin,and allowed the {1122} twin to propagate through the {1124} twinning.(4)The phenomenon of sequential {1122} twinning was characterized.This sequential {1122} twinning is found to be generated in the matrix grain at the primary {1124} twinning boundary or at the intersection of primary {1122} and{1124} twins.The generalized Schmid law could not explain the preference of this sequential twinning.The analysis showed that the sequential {1122} twinning can accommodate the strain induced by the neighboring twins,and its nucleation can be promoted by the dissociation of basal dislocations at the twinning boundaries of neighboring twins.The sequential mechanism was conducive to the formation of more {1122} twins and the ease of the local stress concentration caused by the interaction of basal dislocations and twinning boundaries.(5)A concept of the m' factor-grain boundary misorientation map(denoted as m'-GBM map)and its calculation method were proposed.The m'-GBM maps could reveal the favorable and unfavorable GBMs for different twinning/slip transmissions,and provide a direction for the selection of initial material and design of grain boundary in hexagonal metals as well as other structural metals.Based on the corresponding m'-GBM maps,the phenomena of {1122} and {1124} twinning transmission over grain boundaries were studied,and favorable GBM for the formation of {1122}-{1122},{1122}-{1124} and {1124}-{1124} twin pairs were predicted.Specifically,the map showed that low GBM angles(<15°)were conducive to the formation of {1122}-{1122} and {1124}-{1124} twin pairs,and both the {1122}-{1122} and {1122}-{1124} twin pairs tended to occur at grain boundaries with media angles(15°-40°).These predictions were then verified by EBSD experimental analysis,revealing that the local strain accommodation played a key role in the formation of these three types of twin pairs.{1122} and {1124}twins in the cryorolled A plate could cooperate and propagate through grain boundaries with larger grain misorientation angles.This resulted in the formation of more twins and thus raised grain refinement.The grain boundary design of the initial material based on the m'-GBM map is a significant approach to adjust the types and proportions of twinning and improve the performance of the material.The research results in this dissertation are the systematic exposition of the deformation mechanism and twinning behavior of titanium deformed at cryogenic temperature,which is of great significance for the prediction and adjustment of deformation microstructure and texture.The proposed method of pre-deformation at deep and low temperatures also has guiding significance for the design of the toughening process during the deformation processing of titanium alloys. |
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