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.
Highlights
Due to their high specific ratio of strength to density, high temperature properties and fatigue properties, titanium alloys have been attracting increasing emphasis on the application in aerospace and aircraft industries, such as landing gear and springs [1-3]
It can be found that the strength increases in the order of lamellar microstructure (LM), equiaxed microstructure (EM), bimodal microstructure (BM) and fine lamellar microstructure (FLM)
The ductility increases in the order of FLM, LM, EM and EM
Summary
Due to their high specific ratio of strength to density, high temperature properties and fatigue properties, titanium alloys have been attracting increasing emphasis on the application in aerospace and aircraft industries, such as landing gear and springs [1-3]. It has been reported that a fatigue crack is formed due to the localized plastic deformation during fatigue loading [9-14] These strain concentration and gradient are primarily caused through the dislocation motion within the phase or grains of materials. It has been realized that the microstructural parameters, including morphology, size, distribution, volume fraction and orientation et al, have important effect on the cyclic deformation and damage mechanism. These results indicate that microstructure has a domain influence of high cycle fatigue properties. Due to the significant of microstructure in initiation of crack, the effect of size of α phases on cyclic deformation and fatigue crack initiation during fatigue of TC21 titanium alloy was investigated in present work.
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