Identifying the character and the source of partial dislocations associated phase transformation from a hexagonal close-packed (HCP) structure to a face-centered cubic (FCC) structure is essential for understanding phase transformation mechanisms, but was rarely done using microscopy. Here, we report a stress-induced HCP to FCC phase transformation in pure hafnium during cold rolling. Detailed transmission electron microscopy investigations revealed that transformation-related partials stemmed from the dissociation of -type dislocations. Successive gliding of partials on every other basal plane resulted in the orientation relationship between the two phases of <112¯0>hcp//<110>fcc, <101¯0>hcp//<1¯12>fcc and [0001]hcp//<111>fcc.Besides, a new way to form a twin relationship in the FCC structure was discovered.Download high-res image (475KB)Download full-size image

•Deformation mechanism of a commercial pure Zr during cold-rolling was studied.•Grain refinement of Zr during cold-rolling was systematically investigated.•Two different HCP → FCC phase transitions were observed and discussed.•The influence of phase transitions on the mechanical properties was discussed.The grain refinement process and phase transition phenomena of a cold-rolled zirconium have been systematically investigated. The grain refinement was accomplished via a sequential series of dislocation behaviors, reaching a final grain size of < 100 nm. The refinement process can be described as: (1) dislocation nucleation, multiplication and tangling inside the original coarse-grains, (2) formation of microbands via dislocation rearrangement, (3) further division of microbands into thin laths, (4) split of microbands and thin laths into subgrains and then nanograins. Two different types of HCP-FCC phase transitions were also discovered during the cold rolling process, with the transition mechanisms and their influences on the mechanical properties being discussed.Download high-res image (526KB)Download full-size image

•A bimodal microstructure was formed in a high strain-rate rolled MgZnMn alloy.•Plate-like MgZn2, spherical MgZn2 and spherical Mn phases were observed.•The major facet of the plate-like MgZn2 deviated from the matrix basal plane.The microstructure of a high strain-rate rolled MgZnMn alloy was investigated by transmission electron microscopy to understand the relationship between the microstructure and mechanical properties. The results indicate that: (1) a bimodal microstructure consisting of the fine dynamic recrystallized grains and the largely deformed grains was formed; (2) a large number of dynamic precipitates including plate-like MgZn2 phase, spherical MgZn2 phase and spherical Mn particles distribute uniformly in the grains; (3) the major facets of many plate-like MgZn2 precipitates deviated several to tens of degrees (3°–30°) from the matrix basal plane. It has been shown that the high strength of the alloy is attributed to the formation of the bimodal microstructure, dynamic precipitation, and the interaction between the dislocations and the dynamic precipitates.

The microstructural evolution and mechanical properties of pure hafnium subjected to cold rolling were investigated in this paper. Structural characterization indicates that slip dominated the plastic deformation and the microstructural evolution follows the sequence: original coarse grains – formation of microbands – formation of thin laths in microbands and formation of elongated grains in locally formed shear bands – the longitudinal splitting and transverse breakdown of laths to elongated segments – formation of nano-grains through further transverse breakdown of elongated segments. Hardness results indicate that strain hardening occurred during the whole rolling process, which can be attributed to dislocation activities and grain refinement effect.