No abstract

The microstructure and texture evolution of a commercialized twinning‐induced plasticity automotive steel during friction stir spot welding/processing are addressed. The welding is conducted at various rotational speeds of 1000, 1600, and 2500 rpm, considering effects of strain rate and heat input on microstructural evolution. Sub grains with low angle grain boundaries are mainly developed at the thermomechanically‐affected and stirred zones. At low rotational speed of 1000 rpm, continuous dynamic recrystallization occurred at the stirred zone, which eventually led to considerable grain refinement. At higher rotational speeds, dynamic recrystallization is also observed near the stirred zone. The static/dynamic microstructural evolution increased the extent of bimodality of the grain size distribution, and enhanced mechanical properties. The hook formation is hindered owning to the influence of the dynamic recrystallization on the material's flow during welding. Some fiber textures belonging to shear components are detected under low rotational speed. As deformation increased with rotational speed, shear texture further developed. Different types of shear textures of A‐Fiber and B/B¯ are observed at 1600 and 2500 rpm, respectively. The mechanical properties are explored in connection with macrostructure, microstructure, and texture evolutions.

The present work deals with the room and high temperature microstructure evolutions of a near alpha titanium alloy (Ti-6242S) during isothermal low cycle fatigue. The initial fully lamellar microstructure has been purposefully annealed to form α/β interface phase layers. The shearing and displacement of the β layers is identified as the main governing internal damage mechanism. This is phenomenal owning to the very low imposed strain; and is justified considering: (i) the large colony size and thin thickness of the β layers within the initial lamellar microstructure, (ii) the presence of the interface phase, and (iii) the intensified substructure development within the α and β phases. The shearing and displacement of the β layers lead to the segmentation and this is followed by initial stage of dynamic globularization even at room temperature condition. The globularization kinetic is intensified at higher temperature cyclic loading. In this regard, the sub-boundary induced boundary splitting/grooving is characterized as an involved mechanism. Interestingly, the early dynamic globularization of the α lamellae is also traced through the high temperature fatigued microstructures. The β phase penetration along the α phase sub-boundary is realized as the main globularization mechanisms. The interface phase layer stimulates the possibility of globularization via providing the high-energy non-coherent interfaces in structure. In addition, the deformation twinning within the interface phase, so called as "twinning induced micro-plasticity effect", appears to have a significant effect on the fatigue damage behavior via compensation of incompatibility between α and β phases and deformation twin formation.

No abstract

No abstract