PSI - Issue 14

2

Afroz Shaikh / Structural Integrity Procedia 00 (2018) 000–000

Afroz Shaikh et al. / Procedia Structural Integrity 14 (2019) 782–789

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1. Introduction Titanium alloys are used for a variety of applications because of their superior properties such as high strength, low density, excellent corrosion resistance, etc. Applications of this alloy include aerospace components (high strength in combination with low density), biomedical devices (bio compatibility, corrosion resistance & high strength) and components in chemical processing equipment (corrosion resistance) [Dawari and Kashyap (2015)]. Titanium alloys are categorized into three groups namely, α alloys, β alloys and α-β alloys depending on the type and amount of alloying elements which decide the phases which will be present at room temperature [Shaikh et al. (2016)]. The mechanical properties of titanium alloys are more dependent on the phases present than on the actual composition of the alloy. Substitutional elements partially replace the titanium atoms in the lattice and thus, alter the properties. The phases present in these alloys are dependent on the heat treatment process. Most alloying elements stabilize the body centered cubic (BCC) β phase and lower the temperature of transformation (called β transus) to such an extent that at room temperature the alloys have a mixture of both α and β phase. A work by Jadhav et al. (2017) shows that in heat-treated specimens, the tensile strength and hardness decrease with the increase in the volume fraction of α. So, depending upon volume fraction of these phases, mechanical properties can be varied. Three different types of microstructures can be obtained by changing thermo mechanical processing route: equiaxed, lamellar (widmanstatten) and bi-modal (duplex) containing equiaxed primary α (α p ) in a lamellar α+β matrix. Previous work by Loier et al. (1985) has indicated that lamellar microstructure exhibits better fatigue propagation resistance as compared to that of equiaxed microstructure. Equiaxed microstructure provides better fatigue crack initiation resistance but poorer propagation resistance than lamellar microstructure [Peter et al. (2003)]. Bimodal microstructures exhibit well-balanced fatigue properties [Fan et al. (2016)], since they combine the advantages of both lamellar microstructure (i.e., higher fatigue crack propagation resistance) and equiaxed microstructure (i.e., higher fatigue crack initiation resistance). Initial microstructural parameters such as the fraction of α and β phases, the morphology & thickness of α-laths, the size of α colony (i.e. the geometrical arrangement of α and β-phases) also have major effect on the mechanical properties of the Ti-6Al-4V alloy. A work by Lütjering et al. (1995) has shown that among all the microstructural characteristics, the size of α colony has the most significant effect on the mechanical properties. He has furthermore shown that parameters such as the cooling rate from the β phase field, the initial β grain size and the presence of interstitial impurities (oxygen and carbon), can affect the geometrical arrangement within the microstructure of the Ti-6Al-4V alloy. The cooling rate is most important parameter affecting microstructural development. Slow and intermediate cooling rates lead to nucleation and growth of α-lamellae into the β grains through a diffusion controlled process, whereas, higher cooling rates lead to formation of martensitic α (ά ) through diffusion less transformation. In the present work, different microstructures obtained through different heat treatment processes is studied and presented. Furthermore, the effect of different microstructures on hardness is also presented. 2. Experimental work The material used in this study is Ti-6Al-4V alloy rod of size Ø85 mm x 210 mm. Material was supplied by the TIMET Metal Corporation. The material (VAR - Vacuum Arc Remelted) was in rolled condition. The β transition temperature for the material was 995 °C as given by supplier considering top, bottom and middle portion of the ingot. The chemical composition of the material is listed in Table 1.