Atomistic analyses of HCP-FCC transformation and reorientation of Ti in Al-Ti multilayers

SK Maurya and JF Nie and A Alankar, COMPUTATIONAL MATERIALS SCIENCE, 192, 110329 (2021).

DOI: 10.1016/j.commatsci.2021.110329

Atomistic simulations have been performed in this study to examine the roles of interface characteristics, layer thickness, and heating rate on microstructural transformation of Al-Ti multilayers. The individual layer thickness varies from 4 nm to 55 nm, and eight different configurations that have different crystallographic orientation relationships are examined. All the multilayers are heated to 300 K with a heating rate varying between 1.5 ? 1013 K?s- 1 and 3.0 ? 1010 K?s- 1. No microstructural changes occur for the multilayers having the traditional orientation relationship (OR) (configuration A) observed experimentally. Similarly, the multilayers having other configurations that are defined by different degrees of misorientations (configurations 1?6) from the traditional OR also do not show any microstructural change. The transformation in multilayers having incoherent and high energy interface (configuration B) is heavily influenced by layer thickness and heating rate. The Al layer does not show any transformation for all the considered configurations, layer thicknesses, and heating rates. However, the Ti layer has shown the formation of FCC Ti and reorientation of parent HCP Ti in multilayers with configuration B. The HCP to FCC structural transformation of Ti occurs by creating a series of successive prismatic stacking faults in the Ti layer, and results in a decrease of energy by 4?5 meV/atom for complete transformation and 1?3 meV/atom for partial transformation. The HCP-FCC transformation of Ti results in a volume change in the range of 0.06?0.08 ?3/atom. At larger layer thicknesses, reorientation in the Ti layer, which is equivalent to an effective rotation of parent Ti about 1210 direction by 90?, occurs. The reorientation occurs via the formation of an intermediate BCC Ti phase resulting from the coordinated displacement of atoms on alternating (0001) planes in 1010 direction, accompanied by almost 10% compression along 1010 direction. The intermediate BCC Ti transforms to reoriented HCP Ti by displacements of atoms on alternating (1 1 0) planes in 1 1 0 direction, accompanied by almost 2% compression along 110 direction. Reoriented Ti is less strained as compared to parent HCP Ti because of the change in the c/a ratio.

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