Quasi in-situ investigation on the twin pair in cryogenic rolled Ti
Introduction
Twinning is important in HCP materials to achieve uniform deformation, because of inadequate slip systems [1]. Twin embryo is preferred to generate from grain boundary under the effect of deformation strain, and induces stain localization on the boundary [2,3]. Similarly, the twin tip would result in the increment of local stress when it reaches the other side of the grain boundary [4]. Previous investigations reported that twinning in the neighbor grain was stimulated to release the strain localization [[5], [6], [7]], which is known as twin pair (TP) in HCP metals [[8], [9], [10], [11]].
The variant selection criterion of TP is widely investigated by using Schmid law, geometrical compatibility factor (GCF) and parent grain misorientation [[12], [13], [14], [15]]. Twin variant with the highest Schmid factor is the most possible to generate. GCF is found effective in predicting the variant selection of TP [16]. It is reported that paired twin with GCF close to 1 represents good strain accommodation ability [17]. Misorientation between neighbor grains is also important in determining the generation of TP, and a lower misorientation is more beneficial to stimulate TP [18,19]. Among the three factors, GCF was considered as the dominant criterion in predicting the variant selection [12,13]. It is known that slip is active when deformed at ambient temperature, which may influence the variant selection of TP [20]. However, the effect of slip is underappreciated during the generation of TP in previous investigations. The details of formation process and intrinsic variant selection criterion still lacks of comprehensive understanding.
The aim of this work is to investigate the formation and the variant selection criterion of TP, by using quasi in-situ electron backscattering diffraction (EBSD) technique. The rolling treatment is carried out at liquid nitrogen temperature (LNT) to reduce the influence of slip on the formation of TP. More specifically, TP, a TP type that was rarely reported in previous investigations. All the aforementioned influence factors are taken into consideration to ensure reliability.
Section snippets
Material and methods
Materials used in the study were commercially pure titanium with impurities of (wt%): 0.19% Al, 0.13% V, 0.15% Fe, 0.10% Zr and 0.03% Cr. Samples were annealed at 750°C for 1 h with nitrogen protective atmosphere and a fully recrystallized structure was obtained. Prior to rolling process, annealed Ti was mechanical polished to remove oxides and contaminants on surface. Before EBSD observations, cross-sectional samples were grinded with abrasive papers, following by electrolytic polishing until
Results and discussion
Fig. 1 gives cross-sectional EBSD morphologies and local misorientation images of annealed and LNT-rolled Ti. Grain boundaries with misorientation range from 2° to 15° are identified as low angle grain boundaries (LAGBs) and colored sliver, while those with misorientation larger than 15° are identified as high angle grain boundaries (HAGBs) and colored black in the EBSD images. Areas with local misorientations close to 0° (2°) are colored blue (green) on the local misorientation distribution
Conclusions
In summary, the formation process and variant selection criterion of TP in LNT-rolled Ti are systematically investigated. The variants of TP could generate successively along the same direction or simultaneously along reverse directions in neighbor grains. Schmid law is found more convincing than that of GCF in predicting the variant selection of TP. TP becomes harder to generate with increasing parent grain misorientation because of lower GCF.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Development Program of China (2017YFA0204403), the National Natural Science Foundation of China (51741106), Natural Science Foundation of Jiangsu Province (BK20191292), the Fundamental Research Funds for the Central Universities (30919011256), the Jiangsu Key Laboratory of Advanced Micro & Nano Materials and Technology. EBSD was performed in the Materials Characterization and Research Center of the Nanjing University of Science and
Data availability
The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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