PSI - Issue 14

Afroz Shaikh et al. / Procedia Structural Integrity 14 (2019) 782–789 Afroz Shaikh / Structural Integrity Procedia 00 (2018) 000–000

788

7

4. Discussions Heating of titanium alloys above the β transus temperature changes its crystal structure from HCP to BCC. Quenching from this region leads to formation of martensitic α' which is thermodynamically unstable phase [Peter et al. (1982)]. As the transformation does not involve diffusion, α' and β regions have the same chemical composition. Slow cooling from ‘above the β transus’ into the two-phase region results in the formation of Widmanstatten colonies of α+β. The mechanism of transformation from β to α phase with cooling is shown in Fig. 7. Upon cooling, α phase precipitates from the β phase where it nucleates at the incoherent interfaces such as prior β grain boundaries [Peter et al. (1982)]. α phase grows in a lamellar fashion into the interior of the β grains where its final length is restricted by the prior β grain size. The widths of the lamellar platelets are controlled by diffusion hence, coarsening occurs when the cooling rates are slower. Heating below the β transus temperature followed by cooling results into a duplex microstructure consisting of primary α and transformed β. The volume fraction of primary α and transformed β is governed by heat treatment temperature. With increase in temperature, the volume fraction of transformed β increases. Cooling rate influences the primary α grain size and width of α lamellae. From the microstructure (Fig. 4.) it is also observed that faster cooling rate (WQ) have produced finer grain structure. Slower cooling (FC) has produced coarse α lamella whereas air cooling has produced fine α lamella.

Fig. 7. Widmanstatten microstructure development in Ti-6Al-4V [Donachie (1988)].

In general, α phase is harder compared to β phase. The β to α martensitic transformation induced by rapid cooling leads to the formation of α' phase. The lattice parameter of α' can be different from “normal” α (diffusion controlled β to α transformation). α' is supersaturated with β stabilizing elements. As, there is no severe distortion of the lattice in titanium by interstitials (like in steels), so α' normally has a similar hardness as that of α. Sometimes β phase can be harder as compared to α'. This characteristic is governed by several factors such as the amount of solvent element, secondary phases and their distribution, grain size and boundaries etc.