Abstract
TC21 is considered a new titanium alloy that is used in aircraft applications as a replacement for the famous Ti-6Al-4V alloy due to its high strength. The effect of single and duplex stage heat treatments on fatigue behavior of TC21 Ti-alloy (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.09Si, wt.%) was investigated. Two heat treatment cycles were applied on as-received TC21 Ti-alloy. The first cycle was called single stage heat treatment (SSHT). The other cycle was named duplex stage heat treatment (DSHT). Typical microstructures of SSHT & DSHT composed of primary equiaxed α phase, residual β phase and secondary α phase (αs). Secondary α phase was precipitated in the residual β phase due to low cooling rate using air cooling and aging treatment. Morphology of α phase does not change after solution treatments, while their volume fraction and grain size were changed. SSHT showed the highest fatigue strength of 868 MPa due to high tensile strength, hardness and existing of high percentages of residual β phase in the microstructure. However, DSHT reported lower fatigue strength of 743 MPa due to increasing grain size of α phase. The fracture surface of fatigue samples showed cleavage ductile fracture mode for both heat treatment cycles.
Highlights
Titanium alloys, especially α + β titanium alloys, are widely used in advanced aerospace applications, aero-engines and chemical industries
Solution Treated Condition The typical microstructure of single stage heat treatment (SSHT) & duplex stage heat treatment (DSHT) samples showed an equiaxed shape of primary α phase that distributed homogenously in the β phase, Figure 3
DSHT, αs was precipitated in the residual β phase due to step cooling in furnace and low cooling rate using air cooling as well as aging treatment
Summary
Especially α + β titanium alloys, are widely used in advanced aerospace applications, aero-engines and chemical industries. TC21 (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-Si, wt.%) alloy is a new category of α + β titanium alloys with high strength, toughness, damage-tolerance properties and low crack propagation rate and provides weight reduction, long service life, and high reliability in fabricated aircraft structural components such as frames and beams [2] [5]. By applying a heat treatment technique, TC21 alloy can obtain a better combination of tensile properties, fracture toughness, and low fatigue crack growth rate. In such a case, the performance and engineering application value of TC21 Ti-alloy will be better than the widely used conventional Ti6Al4V alloy [6]. Fatigue strength depends on microstructure parameters such as grain size of the primary α in bimodal microstructures, α grain size in equiaxed microstructures, and lamellar α width in lamellar microstructures [7] [8]
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