The twinning structure of the orthorhombic martensite phase in alpha + beta Ti-3.5Al-4.5Mo (wt%) titanium alloy was studied using X-ray diffraction and transmission electron microscopy by water quenching from below transus temperatures. While water quenching from 910 induced the formation of twins, quenching from 840 formed the martensite with type I twins. The effect of the principle strains on the twinning structure was discussed. As compared to the previous studies, the principle strains play an important role in the formation of the twinning type. 1. Introduction Commercial α + β titanium alloys are widely used in structural and aerospace applications where the combination of light weight, strength, and room temperature corrosion resistance is highly desired. With different alloy compositions and thermomechanical processing parameters, a wide range of mechanical properties can be achieved in titanium alloys. Beside the enhanced mechanical properties, metastable martensite microstructure can be obtained with fast cooling from body-centered cubic (bcc) β phase region. While near alpha titanium alloys yield only small fraction of martensite structure, alpha + beta titanium alloys can produce a combination of hcp ( ) and c-orthorhombic ( ) martensite phase [1, 2]. phase is particularly interesting because it can be thermoelastic, and better understanding of this phase transformation can be used to design smart systems. There have been extensive studies on the properties of [3–6] and the phenomenological theory martensite crystallography (PTMC) [7, 8]. Unlike the martensite transformation, the martensite transformation which was first found in a Ti-Nb system [9] can be thermoelastic, and the shape memory effect is present [10–14]. The crystal structure of martensite is intermediate between bcc β and hcp α phases [9], and the lattice parameters vary with alloy compositions significantly [15]. The morphology of martensite depends on the magnitude of the lattice deformation. It was found in Ti-Ta alloy that most of the martensite are not twinned state at a certain Ta content [16], while most literatures concerning the martensite twin structure in β titanium alloys reported that the martensite is in {111} twin structure [17, 18]. However, the morphology and crystallography of martensite twin has not been completely described, especially for the α + β titanium alloys in which the β isomorphic elements are very close to the lower critical concentration of martensite. It is well known that twinning is a deformation process of most engineering materials to
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