Y.B. Chun

Deformation heterogeneity of Ti under cryogenic channel-die compression

Abstract Commercial purity Ti was subjected to channel die compression in liquid nitrogen for the purpose of studying its deformation characteristics from the viewpoint of the grain refinement induced by severe plastic deformation. Deformed specimens showed microstructural heterogeneity in that the initial blocky grains of about 50 μm in size turned into a mixture of the easily deforming soft grains and the hard grains revealing local concentration of dislocations and deformation twins. Using electron back scattered diffraction analysis the in-grain misorientation-axis distribution was studied, from which it was found that the deformation heterogeneity resulted from the anisotropy of individual grains with respect to the compression die: while the soft grains were those oriented favorably for the prism slip, the hard ones were oriented for non-prism slips and deformation twinning. The difference in the response of individual grains, therefore, led to a remarkable effect of the crystal texture of specimens on the maximum strain that can be imposed on specimens without cracking. Based on the present results, a way to achieve expedient grain refinement through the cryogenic plane-strain compression was suggested.

Grain Refinement of Vanadium by Low Temperature Severe Plastic Deformation

Recent advances in the severe plastic deformation technique have shown that effective refinement of the microstructure can be achieved in pure metals as well as in alloys. Among the various methods of severe plastic deformation, equal channel angular pressing has been the subject of numerous research works. Since the grain refining effect of this technique appears to reach a peak at a level of approximately 200 nm further microstructural changes are sought—deformation at a cryogenic temperature being one of the candidate routes. In the present study, we opted to combine equal channel angular pressing and low temperature plastic deformation to refine the microstructure of commercially pure V. The starting microstructure consisted of equiaxed grains with an average size of 100 micrometers. This microstructure was refined to a 200 nm thick lamellar microstructure by 8 passes of equal channel angular pressing at 350°C. The lamellar thickness was further reduced to 140 nm upon subsequent cryogenic rolling, which resulted in room temperature yield strength of 768 MPa. In the specimens, recrystallization annealed at 850°C, the grain size reached 1000 nm or larger, and the yield strength obeyed the Hall-Petch relationship with that grain size. The tensile elongation value, which was low and insensitive to the grain size in the as-deformed state, increased significantly up to 43% with the recrystallization annealing.

Monte-Carlo Modeling of Recrystallization Kinetics of Cold-Rolled Titanium

The recrystallization behavior of cold-rolled, commercial-purity titanium was studied experimentally and with Monte-Carlo (MC) modeling. Utilization of EBSD-OIM as input for MC modeling resulted in realistic predictions of recrystallization kinetics, microstructure and texture, which were in good agreement with experimental results. MC modeling of recrystallization kinetics predicted that the non-uniform stored energy distribution, heterogeneous nucleation of recrystallization and recovery in combination leads to a negative deviation from linear JMAK kinetics. It was found that concurrent recovery that takes place during recrystallization is an important process that controls both the overall recrystallization kinetics and the deviation of linear JMAK kinetics. On the other hand, the non-uniformly distributed stored energy itself has little effect on the negative deviation from JMAK kinetics but intensifies the deviation when heterogeneous nucleation is combined. Modeling results also revealed that heterogeneous nucleation of recrystallized grains and their early impingement in local areas of high deformation are essential for producing a log-normal distribution of grain size and a typical recrystallization texture of rolled titanium.

Twinning-Induced Negative Strain Rate Sensitivity in Wrought Magnesium Alloy AZ31

Measurements of strain rate sensitivity (SRS) provide a key link between dislocation-based interpretations of plastic deformation and macroscopic measurements made in mechanical tests. It is well known that plastic deformation of hexagonal close-packed (hcp) metals is achieved not only by dislocation glide but also by twinning and that the atomic rearrangement underlying the latter mode is different from that of slip. This leads to an expectation that co-activation of twinning may affect SRS of hcp metals. This assumption was tested in the present work where strain rate jump tests in both tension and compression were conducted on highly textured AZ31 plate. It was found that the SRS of the alloy in tension decreased with strain whereas that in compression increased with strain, exhibiting negative values at low strain and positive values at higher strain. Microstructure analyses revealed that the strain regimes where negative SRS or decreasing trend in SRS with strain was observed correspond to extensive twinning, implying a negative SRS of twinning. It is concluded that dislocation model alone cannot explain the strain rate dependence of flow stress in metals whose deformation is assisted by twinning.