Tailoring The Mechanical Properties Of Titanium Alloys Via Stress-induced Martensitic Transformation

Titanium and its alloys start to gain a unique position amongst other structuralmaterials, like Fe, Al-based alloy. Titanium alloys are attractive for a variety ofapplications such as aerospace, marine, petroleum, chemical, metallurgy, machinery,energy and health care due to their excellent properties. Titanium alloys offer manyopportunities to tailor the microstructure and the resulting deformation behavior andresulting application fields, because of their wide range of complex microstructuresand phase transformations. Phase transformation resulting from the deformation isintentiona lly used to improve properties. Previous studies sho wed that thestress-induced martensite （SIM） transformation starting from a metastable β phase canindeed result in an improved balance of strength and ductility and has a good potentialapplication.An initia l investigation on an existing commercial titanium alloy Ti-10V-2Fe-3Alwas conducted. A variety of different heat treatments and mechanical tests wereperformed. A series of phase transformations including stress-induced martensitictransformation were studied by X-ray diffraction, optical microscopy, scanningelectron microscopy, transmission electron microscopy, electron probe microanalysis,combined with the Thermo-calc software. Based on the theoretical foundation, twocertain kinds of new alloys were designed and manufactured; characterization andevaluation were also carried out. The obtained results are summarized as follows:（1） In Ti-10V-2Fe-3Al alloy, the bi-modal microstructures possess a betterplasticity. After α+β phase field solution treatment, the failure strain was largelydecreased due to the disappearance of α laths in bi-modal microstructures. Comparewith β phase field solution treatment, samples under β+（α+β） conditions have higherfailure strain. Globular α phase has a great influence on triggering stress for SIMeffect, but does not show too much effect on mechanical properties for sampleswithout SIM. Acicular α phase has a weak influence on triggering stress for SIMtransformation, but shows some effect on samples without SIM. The deformationmechanism of α phase always contains {1011}<012> type twin. With increase the αphase volume fraction （both globular and acicular）, the deformation mechanism of βphase varies from stress induced martensitic transformation to {112}<111> t ype twins.After single aging or dual aging treatments, alloys exhib it almost identical microstructures and thus similar mechanical properties, and age hardening is quiteeffective. The main deformation mode is slip.（2） In Ti-10V-2Fe-3Al, no SIM was observed for specimens with the α volumefraction higher than a critical value （50%）. The triggering stress of SIM in this alloy ismainly independently affected by these two factors: the β grain （or domain） size andthe stability （Moeq） of the β phase. For globular α samples, the β matrix stability has adominating effect on the triggering stress, but in samples containing acicular α, thesize of the retained β domains is also important. These two factors compete with eachother, and eventually decide the triggering stress. A linear super-position of the twoeffects was found to describe the data very well in the formula below:σ_SIM(Mo_eq=σ_si（d）|_Moeq0+△σ_st(Mo_eq-Mo_eq0)An increase in the β grain size linearly increases the triggering stress for SIM.（3） The proposed relation between the occurrence of SIM formation upon roomtemperature compression of metastable titanium alloys derived for Ti-10V-2Fe-3Alwas found to apply also to the Ti-10V-1Fe-3Al, albeit that the actual triggering stresswas affected by the presence of thermal martensite already present prior to thecompression experiment. The formation of a martensite free zone‘around the α phasein the Ti-10V-2Cr-3Al due to a very limited diffusion distance for Cr atoms upon αformation during intercritical annealing, prevented the wider validation of theproposed relation between the chemical and microstructural β stability. However,based on the results obtained so far SIM martensite transformation is predicted tooccur for β phase fractions with a Mo equivalence between9and16. For Moequiva lences lower than9, thermal martensite may be present prior to deformation.For that higher than16, the β stability is such that even upon deformation the β phasefraction does not transform martensitically.（4） α’ martensite was observed in Ti-10V-2Cr-3Al alloy and Ti-10V-1Fe-3Alalloy after quenching from β phase fie ld. All heat treated samples researched showedstress-induced martensitic transformation for all strain rates (10^-4s^-1<ε<10^-1s^-1)explored. The SIM triggering stress increases linearly with the logarithm of the strainrate. The rate of increase is the same as that for the yield stress of the grade notshowing SIM transformation. The failure strain increases linearly with the logarithmof the strain rate, irrespective of the occurrence of SIM transformation.（5） After phase field solution treatment, the phase precipitation rateof Ti-10V-2Cr-3Al alloy is lower than Ti-10V-1Fe-3Al alloy. With increasing phase content, the compressive strength of both alloys decreased, but the fracture strainremained stable. After single aging, the microstructure of Ti-10V-2Cr-3Al alloy isfiner than Ti-10V-1Fe-3Al alloy, and results in a higher yield stress, a highercompressive strength and a lower fracture strain. After dual aging, the microstructuresand mechanical properties of Ti-10V-2Cr-3Al alloy change little, but forTi-10V-1Fe-3Al alloy, microstructures was slightly refined and thus increases in yie ldstrength and compressive strength, and reduces fracture strain.