New mechanism for the alpha to omega martensitic transformation in pure titanium

We propose a new direct mechanism for the pressure driven alpha-->omega martensitic transformation in pure titanium. A systematic algorithm enumerates all possible pathways whose energy barriers are evaluated. A new, homogeneous pathway emerges with a barrier at least 4 times lower than other pathways. The pathway is shown to be favorable in any nucleation model.

Nucleation in systems with elastic forces

Systems with long-range interactions when quenched into a metastable state near the pseudospinodal exhibit nucleation processes that differ from classical nucleation seen near the coexistence curve. In systems with long-range elastic forces the description of the nucleation process can be quite subtle due to the presence of bulk and surface elastic compatibility constraints. We analyze the nucleation process in a simple 2D model with elastic forces and show that the nucleation process generates critical droplets with a different structure than the stable phase. This has implications for nucleation in many crystal-crystal transitions, specifically martensites and shape memory alloys, and for the structure of the final state.

Raman spectroscopic analysis of mo/si multilayers

Raman spectra are reported from MoSi2 polycrystalline powder and soft x-ray Mo/Si multilayers. The sharp lines at 323 and 438 cm-1 are all due to crystalline MoSi2. These lines in the powder sample intensify with annealing. The Raman spectra of as-deposited multilayers shows a broad asymmetric peak, highest at about 480 cm-1. We attribute this to α-Si which is highly disordered. In contrast to α-Si in semiconductor/semiconductor and semiconductor/dielectric multilayers, in the Mo/Si samples the Raman signal can vanish after modest heating. This provides evidence that the composition of the silicon component of the multilayer changes even with 200°C annealing. Further annealing also produces the signature for crystalline MoSi2 in the multilayer samples. This is the first report of the characterization of Mo/Si soft x-ray multilayers by Raman spectroscopy, and it indicates that Raman spectroscopy may be an effective technique for characterizing these soft x-ray multilayers and may be useful in studying their interfaces.