Papers by Author: Yuntian T. Zhu

Abstract: This paper studies the effect of combined SPD treatment on microstructure and mechanical properties of semi-products out of CP Ti. The combined processing, consisting of equal-channel angular pressing and further thermomechanical treatment, produced ultrafine-grained rods out of Grade 2 CP Ti with a diameter of 6.5 mm and a length of up to 1 m. It was established that the formation of homogeneous ultrafine-grained structure in Ti rod with α-grain size of about 100 nm allowed to enhance yield stress by 200% in comparison with initial annealed state.

Abstract: This paper investigates microstructures and mechanical properties of the TI-6AL-4V ELI alloy processed by ECAP and extrusion with various morphology of α and β-phase. Preliminary thermal treatment consisted of quenching and further high-temperature ageing. The present work reveals that the decrease of volume fraction of α-phase globular component in the initial billet results in a more homogeneous structure refinement during SPD, lower internal stress, enhancement of microstructure stability and mechanical properties. An ultimate strength of UTS ≥1350 MPa was obtained in the Ti-6Al-4V ELI alloy while maintaining a ductility of δ≥11%.

Abstract: The nanostructured TiNi-based shape-memory alloys were synthesized by multi-step SPD deformations – ECAP plus cold rolling or drawing. It is found that the SPD processing changed the morphology of the martensite and temperature of martensite transformation. Also, we found that the mechanical and shape memory properties can be enhanced by forming nanostructures in these alloys. SPD processing renders higher strength, higher yield dislocation strength and in results - higher recovery stress and maximum reverse strain of shape memory.

Abstract: Deformation twins have been oberved in nanocrystalline (NC) Al synthsized by cryogenic ball-milling and in NC Cu processed by high-pressure torsion under room temperature and at a very low strain rate. They were found formed by partial dislocations emitted from grain boundaries. This paper first reviews experimental evidences on deformation twinning and partial dislocation emissions from grain boundaries, and then discusses recent analytical models on the nucleation and growth of deformation twins. These models are compared with experimental results to establish their validity and limitations.