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The ultra-fine grain Ti–6Al–4V alloy sheet was successfully achieved by friction stir processing (FSP). The low-temperature superplastic deformation mechanism of Ti–6Al–4V alloy under the strain rate of 3 × 10−4 s−1 at 600 °C was studied by scanning electron microscope, electron backscatter diffraction and superplastic tensile test. It is found that the grain size of Ti–6Al–4V alloy by FSP is refined from 7.48 μm to 0.82 μm, and there is no preferential crystallographic orientation of the uniform grain. After FSP, the β phase of the base material (BM) in the grain boundary is refined and homogenized. During the superplastic tensile processing, with the increase of strain, the grain is refined continuously, and the α→β phase transition is induced by the dislocations pile-up at the β grain boundary, and the content of β phase increases. The grain near the fracture was refined to 0.35 μm, and the stress concentration led to the formation of cavities near the β phase, which eventually caused the fracture. The dynamic recrystallization in superplastic tensile processing is mainly geometric dynamic recrystallization and discontinuous dynamic recrystallization. The elongation of superplastic tensile (1033%) is 19.7 times higher than that of BM (50%). The order of superplastic deformation mechanism in the low-temperature superplastic deformation of the ultra-fine grain Ti–6Al–4V alloy sheet is as follows: intragranular dislocation slip and diffusion, grain boundary sliding, and phase boundary sliding. Phase boundary sliding is the main deformation mechanism for low-temperature superplastic deformation.

The microstructural variations in different sub-regions of the Fe–1.2Mn–0.3Cr–1.4Ni–0.4Mo-C steel welded joint were investigated by means of optical microscope and scanning electron microscope equipped with electron backscattered diffraction. This study focused on the correlation between microstructural characteristics and mechanical properties of the welded joint which was carried out by gas metal arc welding. The microhardness of the fusion line (FL) and fine-grained heat affected zone was inferior to other locations of welded joint due to the diffusion of alloy elements and granular bainite phase transformation, respectively. The tensile strength of the welded joint was about 980 MPa, which reached the required strength of the 1000 MPa grade steel plate for hydropower station. The final fracture was located at the weld metal (WM) because that the crack easily nucleated and propagated in proeutectoid ferrite. The impact toughness at −60 °C of various sampling locations for welded joint was higher than 47 J. The low temperature impact toughness improved gradually as the distance between the V-notch and weld center line increased. The inclusions in the WM and non-uniformity of microstructure and composition near the FL could increase the deterioration on impact toughness at −60 °C.