Journal of Clinical Immunology and Allergy

Introduction

In the past 20 years, Ti alloys have become increasinglyimportant structural materials for high-value, weight-sensitiveproducts. The successes of applying Ti alloys have largely beenthe result of pragmatic engineering as opposed to bottom-upscientific discovery and application. Nevertheless, Ti alloys arebeing successfully used in aircraft,aircraft engines and rocketengines, among other products. As product realization cyclesbecome shorter, the benefit of using modeling and simulation toreduce the time and cost of qualifying materials for a givenapplication becomes increasingly important. High-fidelitymodels require sound physical understanding of the phenomenathat govern materials behavior. To this end we have attemptedto identify areas where additional research can enable bettermodels.Titanium alloys derive their properties from combinations ofthe two ductile phases based the hcp and bcc allotropicmodifications of titanium. Several key features of these phasesindividually and in combination determine the plasticity andfracture behavior of these single-phase or two-phase mixtures.The alpha phase is both elastically and plastically anisotropic andstiffer and stronger when oriented with its c-axis parallel to theloading direction. Thus texture, both global and local, plays a keyrole in its plasticity. Its anisotropy extends to dislocationbehavior through the non-planar core spread of screw “a” typedislocations that render this line directionrelatively immobile.Composition, especially the Al and O content of alpha,sensitivelyaffects its plasticity through its twinning responseand slip character, whether planar or homogenous, throughshort-range order effects.The fundamental role of alloying additions on plasticity at theatomistic level remains poorly understood. It is also possiblethat hydrogen at relatively low concentration levels plays largerrole in determining ambient-temperature time-dependentproperties than has been recognized thus far. Creep propertiesof the alpha phase are better than those of beta because of theintrinsically lower diffusivities in its close packed structure.Nevertheless the alpha phase can accumulate relatively largeplastic strains at ambient temperatures under constant loadingconditions below its yield stress. A strong interaction of dynamicstrain ageing phenomena in the alpha phase with the creepprocess exists at intermediate temperatures but has not beenwell documented thus far.The beta phase has a variable modulus and its elastic behaviorincluding anisotropy can be controlled through alloyingadditions. It is also subject to a variety of thermal and stress-induced phonon and shear instabilities, a detailed understandingof which is only now beginning to emerge as the biomedicalcommunity exploits its low toxicity combined with a bone-matched modulus, in both conventional and shape-memory-related applications. In combination, the alpha and beta phasescan be crystallographically oriented in a manner that stronglyaffects the plasticity of the two-phase mixture by enhancinganisotropy of slip through preferential alignment of some slipplanes and directionsin the two phases. Thermo mechanicalprocessing is used to relatively reorient and obtain moreisotropic morphologies of the two phases.Quite recently, however, it has emerged that under manyconventional processing conditions, especially in thick sectionproducts, micro textures reflecting this strong crystallographicalignment continueto persist and affectproperties in unusualways. Therefore, efforts to understand and model thermal andcompositionaleffects on crystallographic variant distribution ofthe alpha phase as it precipitates from the parent beta, and theeffects of variant selection on the pathways of subsequent microstructural evolution during processing, have only just begun toemerge.