Abstract
The influence of slow heating rates: 2, 5, 10 and 30?C/min (0.033, 0.083, 0.166 and 0.50℃/s) on the β phase decomposition of Ti-5Al-5Mo-5V-3Cr (Ti-5553) during continuous heating were characterized by differential scanning calorimetry (DSC) analysis, light microscopy, scanning electron microscopy, X-ray diffraction and hardness testing. Starting microstructure was the β phase obtained by heating the Ti-5553 above the Tβ temperature and a water quench. Results show that heating rate has a significant impact on the precipitation mechanisms and on the β→α transformation in this range of heating rates. The main formation of α precipitates occurs between 500 and 600℃at all heating rates tested.A heating at 2℃/min produces very fine and homogeneously distributed α plate precipitates which have nucleated on the nanometer size ωiso precipitates. The ωiso precipitates between 350 and 400℃. At higher heating rates 10, 15 or 30℃/min, the amount of precipitation of ωiso is lower so an additional formation of nanometer size precipitates occurs between 450 and 500℃ It is supposed that both precipitates act as nucleation sites for α phase precipitation. The resultant microstructure consists in a fine intragranular distribution of α precipitates and a coarser precipitation of α at the grain boundaries. It is shown that the precipitation of ωiso phase retards or prevents the precipitation of nanometer size precipitates occurring between 450 and 500℃. This cannot be generalized to all the β-metastable titanium alloys since Ti-LCB does not exhibit the same heating rate dependence on DSC curves.
Highlights
Ti-5Al-5Mo-5V-3Cr, or Ti-5553, is a high-strength βmetastable titanium alloy comparable to the old Russian β-metastable alloy VT-22
This paper investigates Ti-5553 and to a lesser extent Ti-LCB solid β phase decomposition and precipitations using differential scanning calorimetry (DSC) during various slow heating rates
The thermal cycles consisted of heating at constant rates from room temperature to 930 ̊C to achieve a final temperature above the Tβ transus
Summary
Ti-5Al-5Mo-5V-3Cr, or Ti-5553, is a high-strength βmetastable (or β) titanium alloy comparable to the old Russian β-metastable alloy VT-22. Ti-5553 was primarily designed for high-strength forging applications on account of improved properties and deep hardenability over large thickness; it exhibits less sensitivity to forming variables compared to Ti-10Al-2Fe-3Al and offers processing benefits [1]. Ti-LCB (Low Cost Beta Titanium) is another β metastable alloy used in mechanical applications; it is very attractive since the more costly β stabilizing elements are replaced. The mechanical properties of titanium alloys are controlled by their microstructure, in particular the size, shape and distribution of the α phase in the β-matrix [3]. A good understanding of the alloy behavior during thermomechanical treatments is closely related to the knowledge of phase transformations or precipitations and the effect of temperature on the microstructure evolution
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