Effect of Pre-Strain on Microstructure and Tensile Properties of Ti-6Al-4V at Elevated Temperature

Research on pre-deformation influences on material properties in multistep hot forming is of important scientific interest. In this paper, hot tensile tests at 850 °C and a strain rate of 0.001 s−1 were performed to study the microstructural evolution and mechanical properties of Ti-6Al-4V with pre-strains at 0.05, 0.1 and 0.15. The tensile test results showed that the specimen with 0.05 pre-strain exhibited higher flow stress and larger elongation. Additionally, increasing the pre-strain resulted in a decrease in ultimate tensile strength (UTS) and elongation (EL). The EBSD results showed that the main deformation mechanism of Ti-6Al-4V was high-angle grain boundary sliding. Pre-strain promoted dynamic recrystallization (DRX) by increasing the deformation substructure. The refinement of grains and the eradication of dislocations enhanced the deformability, resulting in an increase in flow stress.

Microstructure Evolution and Softening Processes in Hot Deformed Austenitic and Duplex Stainless Steels

The microstructure evolution and softening processes occurring in 22Cr-19Ni-3Mo austenitic and 21Cr-10Ni-3Mo duplex stainless steels deformed in torsion at 900 and 1200 °C were studied in the present work. Austenite was observed to soften in both steels via dynamic recovery (DRV) and dynamic recrystallisation (DRX) for the low and high deformation temperatures, respectively. At 900 °C, an “organised”, self-screening austenite deformation substructure largely comprising microbands, locally accompanied by micro-shear bands, was formed. By contrast, a “random”, accommodating austenite deformation substructure composed of equiaxed subgrains formed at 1200 °C. In the single-phase steel, DRX of austenite largely occurred through strain-induced grain boundary migration accompanied by (multiple) twinning. In the duplex steel, this softening mechanism was complemented by the formation of DRX grains through subgrain growth in the austenite/ferrite interface regions and by large-scale subgrain coalescence. At 900 °C, the duplex steel displayed limited stress-assisted phase transformations between austenite and ferrite, characterised by the dissolution of the primary austenite, formation of Widmanstätten secondary austenite and gradual globularisation of the transformed regions with strain. The softening process within ferrite was classified as “extended DRV”, characterised by a continuous increase in misorientations across the sub-boundaries with strain, for both deformation temperatures.

Direct Observations of Cube Nucleation from Intergranular Cube Segments in Cold Rolled Al-Mn

Two major types of Cube bands/segments have been observed in heavily (90%) cold rolled Al-0.1wt %Mn using the EBSD technique with a FEG SEM: i) intergranular transition bands as thin cube segments aligned along RD between S and Cu oriented grains and ii) as transgranular strain localized bands situated in some particular grains. Their evolution is studied by light annealing at 275°C and 300°C and EBSD observations of exactly the same areas to directly correlate local deformation substructure with recrystallization. Only the intergranular cube transition bands give rapid recrystallization nucleation to cube grains of dimension >10µm. In particular the fastest growing cube grains have a near 40°<111> relation with part of their surroundings.