## 4 Discussion
[71] affirmed that the critical temperature is around 870 \({}^{\circ}\)C. As previously reported, [PERSON] et al. [29] demonstrated that the \(\alpha^{\prime}\)-martsine starts to decompose at 400 \({}^{\circ}\)C and finishes at around 700 \({}^{\circ}\)C. Other authors, however, reported temperatures higher than 750 \({}^{\circ}\)C for \(\alpha^{\prime}\rightarrow\alpha+\beta\) decomposition [28; 72; 73]. Finally, the \(\alpha\)-phase surely decomposes into \(\beta\)-phase following an exponential trend from \(\sim\)800 \({}^{\circ}\)C to the \(\beta\)-transus temperature [29]. Focusing on the SHT treatment, the same cooling in Argon gas induces different microstructural morphologies (Figure 6) in relation to the sample thickness due to different cooling rates. Bars are characterized by \(\alpha\)-colons arranged in a basketweave (Widmanstatten) microstructure and globular \(\alpha\)-phase (Figure 6c,d), while the tensile samples show \(\alpha\)-laths in a basketweave structure within the equiaxed \(\beta\)-grain. At the same time, there are equiaxed grains formed by a mixture of \(\alpha\)-laths at their center and \(\alpha\)-colons that nucleate and grow from the \(\alpha_{\rm GB}\) (Figure 6b). As previously reported, [PERSON] et al. [51] confirmed that the Widmanstatten microstructure of \(\alpha\)-laths and Widmanstatten microstructure of \(\alpha\)-colons form with cooling rates of 15 and 1.5 \({}^{\circ}\)C s\({}^{-1}\), respectively. [PERSON] et al. [74] showed that a rate of 9 \({}^{\circ}\)C s\({}^{-1}\) forms a mixture of \(\alpha\)-colons and \(\alpha\)-laths. Generally, different cooling pathways from the \(\beta\)-region can induce a diffusionless \(\beta\rightarrow\alpha^{\prime}\)-martsine transformation with a rate \(>\) 20 \({}^{\circ}\)C s\({}^{-1}\), or a \(\beta\rightarrow\alpha+\beta\) diffusional transformation whereby the microstructure can assume different morphology with a rate \(<\) 20 \({}^{\circ}\)C s\({}^{-1}\)[12]. [PERSON] et al. [75] affirmed that the air cooling, which is characterized by a rate around 20 \({}^{\circ}\)C s\({}^{-1}\) from 1000 to 500 \({}^{\circ}\)C, can induce the nucleation of the \(\alpha^{\prime}\)-martsine given that its Ms (Martensite start) temperature is 575 \(\leq\) T\({}_{\rm M_{\rm S}}\leq\) 800 \({}^{\circ}\)C [19]. Figure 14: Average ultimate tensile strength, yield strength and elongation of the samples heat-treated at 704 \({}^{\circ}\)C/120\({}^{\circ}\), 740 \({}^{\circ}\)C/130\({}^{\prime}\) and 1050 \({}^{\circ}\)C/60\({}^{\prime}\). Another important factor induced by the SHT at 1050 \({}^{\circ}\)C is the formation of the \(\alpha\)-case layer as shown in Figure 7, both on tensile samples and bars. [PERSON] et al. [49] affirmed that the \(\alpha\)-case formation depends on the oxidizing capacity of the environment during heat-treating at high temperatures. As a matter of fact, the diffusion of oxygen atoms within the sample surface increases the \(\beta\)-transus temperature, as exhibited by the following equation:  
\[\mathrm{T}_{\beta-\mathrm{Tr}}=882+21.1[\mathrm{Al}]-9.5[\mathrm{Mo}]+4.2[ \mathrm{Sn}]-6.9[\mathrm{Zr}]-11.8[\mathrm{V}]-12.1[\mathrm{Cr}]-15.4[ \mathrm{Fe}]+23.3[\mathrm{Si}]+123.0[\mathrm{O}] \tag{2}\]  
where [element] represents the weight percentage of each element [76]. [PERSON] et al. [77] proposed a similar linear relationship between the \(\beta\)-transus temperature and the chemical composition. Firstly, oxygen, as \(\alpha\)-stabilizer alloying element [19], maintains a fraction of the \(\alpha\) phase which does not undergo the \(\alpha\rightarrow\beta\) transformation during the heat treatment. Secondly, it increases the HV\({}_{0.1}\) hardness, as shown in Figure 15, where the globular \(\alpha\)-phase within the samples is characterized by 360 HV\({}_{0.1}\), while the \(\alpha\)-case has a hardness of 403 HV\({}_{0.1}\). The XRD analysis in Figure 8 confirms the presence of different titanium oxides on the sample surface and, therefore, the possible diffusion of oxygen atoms within the sample.