Thermomechanical and mechanicothermal treatments of titanium alloys
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In terms of effectiveness in increasing the structural strength of titanium alloys, the methods of treatment can be arranged in the following order: PTMT, HTMT, MTT. Deformation during HTMT of α and β alloys should be conducted at temperatures in the β range, but in the α+β range for α+β alloys. Alloys close to the critical concentration should be subjected to PTMT with deformation in the α+β range.
The effective increase in the strength during TMT with increasing concentrations of β stabilizing elements reaches a peak, the position of which during HTMT corresponds to the alloy containing β-stabilizing elements in amounts equivalent to 5–8% Mo (alloy VT23), but 15% Mo during MTT (alloy VT19). The effect of TMT depends on the phase composition of the alloy before deformation and the character of the transformation in the process of deformation and subsequent heat treatment.
The effect of TMT is due to formation of curvilinear, sawtoothed, or broadened boundaries of primary β grains, substantial refining of the intragranular structure, and the more dispersed and even decomposition of metastable β, α′, or α″ phases during aging. The last factor is of decisive importance.
The most effective methods are those in which the increase in strength from precipitation hardening exceeds that from phase strain hardening.
KeywordsPrecipitation Titanium Heat Treatment Phase Composition Titanium Alloy
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