Effect of size of alpha phases on cyclic deformation and fatigue crack initiation during fatigue of an alpha-beta titanium alloy

Alpha phase exhibits equiaxed or lamellar morphologies with size from submicron to microns in an alpha-beta titanium alloy. Cyclic deformation, slip characteristics and crack nucleation during fatigue in different microstructures of TC21 alloy (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.1Si) were systematically investigated and analyzed. During low-cycle fatigue, equiaxed microstructure (EM) in TC21 alloy exhibits higher strength, ductility and longer low-cycle fatigue life than those of the lamellar microstructure (LM). There are more voids in the single lamellar alpha than the equiaxed alpha grains. As a result, voids more easily link up to form crack in the lamellar alpha phase than the equiaxed alpha phase. However, during high-cycle fatigue, the fine lamellar microstructure (FLM) shows higher fatigue limit than bimodal microstructure (BM). The localized plastic deformation can be induced during high-cycle fatigue. The slip bands or twins are observed in the equiaxed and lamellar alpha phases(>1micron), which tends to form strain concentration and initiate fatigue crack. The localized slip within nanoscale alpha plates is seldom observed and extrusion/intrusion dispersedly distributed on the sample surface in FLM. This indicates that FLM show super resistance to fatigue crack which bring about higher fatigue limit than BM.

Submicron Grain Size Formation in Thermohydrogenated and Deformed Ti-6Al-4V: The Effect of Processing Route on the Degree of Grain Refinement

The aim of this work was to refine the as-cast Ti-6Al-4V grain size and its Widmanstätten morphology to optimise the mechanical performance of Ti-6Al-4V castings. Hydrogenation and deformation were used as variables in processing routes aimed at assessing the degree of refinement in the as-cast Ti-6Al-4V microstructure. Thermohydrogen processing (THP) refined the Widmanstätten morphology and not the prior beta grain boundary network. Therefore, the degree of refinement in THP processing is limited to morphology refinement within pre-existing prior beta grains. Deformation processing and recrystallisation is necessary to eliminate the indelible influence of the prior beta grain boundary network on the extent of THP refinement. In this context, a substantial degree of refinement is achieved from thermohydrogen and deformation processing (THDP). The as-cast Ti-6Al-4V grain boundary network was refined from an average diameter of 2000μm to 20μm. In addition, the Widmanstätten morphology was refined to submicron equiaxed alpha and beta grains.