Papers by Author: A. Vyas

Abstract: Addition of silicon promotes the development of a stable phase Ti5Si3 through thermomechanical treatment in TiAl-based alloys develop, while carbon seems to be a promising element for generating both solid solution effect and dispersion strengthening by precipitation of TiC or Ti2AlC during subsequent processing of such alloys. In this study, elemental powder mixture of 58Ti-30Al-6Si-6C (at%) was selected to find the structural changes during mechanical alloying (MA) process in order to understand the mechanism of alloying as well as study the subsequent thermal stability and evolution of phases. The results obtained would be useful towards in situ synthesis of titanium aluminide composites that are targeted to serve applications involving high tempearture. Such route will enable precipitation of harder Ti5Si3 and TiC particles within a matrix of TiAl through hot isostatic pressing (HIP). Firstly, MA of the chosen powder mix was performed up to 40 hours. The structural evolution of MA powders and/or annealed powders was characterized by X-ray diffraction (XRD). The stability of milled powders was investigated by differential scanning calorimetry (DSC) to study the phase transformations. Then, 10% of MA powder or the so called ‘precursor’ was incorporated into Ti-45Al-2Cr-4Nb-1.5Mn (at%) baseline matrix or baseline elemental powder, and was packed into stainless steel tubes after thorough blending. The sealed tubes were subjected to HIPing under a pressure of 150 MPa at 1100°C for 4 hours, followed by furnace cooling to room temperature. After HIPing, the resulted composites were annealed at 1150°C for 4 hours. The composites were evaluated for their mechanical properties and deformability at room and elevated temperatures. The results indicate that the synthesized composites have indicated extensive workability at a temperature of 800 oC as well as good mechanical proporties up to this temperature.

Abstract: CrN/CNx nano-scale multilayered films were deposited on Si (100) substrate by closed-field unbalanced magnetron sputtering. Designed experimental parameters enabled an evaluation of the effects of negative substrate bias voltage (Vb), and bi-layer thickness λ (by changing substrate rotation rate) during deposition on the structural and mechanical properties of multilayer films. These multilayers were characterized and analyzed by transmission electron microscope (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation measurements. In all cases, the CNx layers were amorphous and independent of Vb, while the microstructures of the CrN layers were dependent primarily on Vb. The CrN layers showed a mixed structure phase consisting of CrN, Cr2N, and Cr at Vb = -(40-120) V. At higher Vb values (-140 V or above), the Cr2N phase was dominant along with low CrN phase content. AFM measurements revealed that the root-mean-square (rms) surface roughness of the CrN/CNx film was 2 nm at Vb= -200 V whereas the rms values were about 9.5-3.3 nm for lower Vb values of -(40-180 V). By nanoindentation measurements, a maximum hardness of about 36 GPa was observed at Vb= -140 V. The improved mechanical properties of the films are correlated to the phase formation during deposition.