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

Abstract

Abstract. 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.

Strain controlled fatigue behaviour of a wire + arc additive manufactured Ti-6Al-4V

“…When comparing both the sample orientations in stage 1, in all the cases, horizontal samples showed 2-4% higher peak stress compared to vertical samples. It is well documented that the mechanical properties and the rate of plastic deformation in titanium and its alloys depend on the size, morphology, and volume fraction of the α and β phases and their crystallographic orientation with respect to the loading axis [46][47][48][49][50]. The higher peak stress in the horizontal samples is associated with the columnar primary β grains that are aligned perpendicular to the loading axis and strong crystallographic texture along the α GB .…”