In situ evaluation of the low-temperature aging response of Ti–5Al–5Mo–5V–3Cr alloy as influenced by starting microstructure

The impact of starting microstructure on the isothermal behavior of β-metastable Ti–5Al–5Mo–5V–3Cr alloy during aging at 400 °C for 8 h was studied by means of in situ synchrotron X-ray diffraction. Results show that prior deformation does not accelerate phase precipitation during heating, but has an influence on secondary α precipitation during aging, and that the composition of the β matrix, which is affected by the amount of primary α formed depending on the preliminary processing route applied, plays a major role. At this aging temperature, α" phase tends to form from the β matrix in high quantities without the presence of primary α phase or with a low fraction of this phase, and the conversion of α" into α is sluggish, being faster in the β-heat-treated condition, whereas a continuous β → α transformation occurs, with no reliable evidence of α" precipitation, in the α+β-heat-treated condition. Isothermal precipitation was enough to promote a significant hardness increase in the alloy. Observation of the behavior of α" based on its atomic y-coordinate and orthorhombicity helped to elucidate its transformation pathway.