TRIP Titanium Alloy Design

The potential of lower-cost Ti production processes has motivated new directions of Ti alloy design for naval structural applications. The objective alloy should have the same high stress-corrosion-cracking resistance, weldability and low cost with the near-alpha Ti-5111 alloy, and an increased strength and fracture toughness. A systems design approach is used to link processing, structure, properties and performance with mechanistic quantitative models, which enables a computational Ti alloy design.;The primary method to improve both yield strength and fracture toughness is transformation-induced plasticity (TRIP) and toughening caused by the beta-to-alpha'/alpha" martensitic transformation in Ti alloys. Research on TRIP steels has shown that the maximal transformation toughening is obtained at the MS sigma temperature for the crack-tip (ct) stress state, and a large volume dilatation of martensitic transformation, which are accurately modeled for Ti alloys. We have created a new thermodynamic database tailored for martensitic transformations of Ti alloys near room temperature, which also includes athermal beta-to-o transformation to model its competitive relationship with martensite.;A molar volume database for beta, alpha', and alpha" phases is established. The stable martensitic structure is predicted by the criterion of lower molar volume, which reflects the effect of alpha" orthorhombicity on transformation dilatation. To explain the physical origin of alpha" orthorhombicity, we have verified the Cahn-Rosenberg model by beta phase short-range ordering using first-principles calculations. The invariant-plane shear magnitude and dilatation are input to mechanical driving force calculations. The critical driving force for martensitic nucleation is also modeled based on interfacial solution-hardening friction. These models enable the calculation of M Ssigma, which is calibrated for Ti-1023 alloy with tensile tests at temperature to account for the effect of alpha" orthorhombicity and/or athermal o. Fracture toughness measurements on the commercial Ti-1023 alloy demonstrate the transformation toughening effect at MS o(ct).;The final design of a near-alpha TRIP Ti alloy is Ti-8Al-1V-1Sn-1Zr-0.9Fe-0.6Mo-0.1Si-0.1O(wt%). By annealing at 865°C the design alloy is predicted to have an M Ssigma(ct) of room temperature and a transformation dilatation of +0.27%, compared to -0.55% of Ti-5111. In addition to transformation toughening, the design composition should provide more strength without forming detrimental phases, meeting the design objectives.