B. Srinivasarao

Effect of high pressure torsion on the microstructure evolution of a gamma Ti–45Al–2Nb–2Mn–0.8 vol% TiB2 alloy

The technique of high pressure torsion (HPT) has been widely used to refine the microstructure of many metallic materials, especially pure metals and disordered alloys. Comparatively fewer studies have, however, been carried out in intermetallics. γ-TiAl alloys are envisioned as high potential materials to replace Ni superalloys in some turbine components due to their good performance at high temperatures and light weight. Exploring the potential beneficial effects of severe plastic deformation techniques in these materials is now timely. In this work, a γ-TiAl alloy with a lamellar microstructure has been processed by HPT using pressures ranging from 1 to 6 GPa and 0 to 5 anvil turns at room temperature. Significant refinement of the microstructure via twin formation, bending of the lamella and the accumulation of a high dislocation density upon the application of shear give rise to a drastic hardness increase.

Microstructure and mechanical properties of Fe-C alloy produced by mechanical alloying and spark plasma sintering

Nanocrystalline Fe with carbon concentrations of 0.2 wt.% and 0.4 wt.% was prepared by the mechanical alloying technique. The grain size after milling for 100 h was 22 and 17 nm for 0.2 and 0.4 wt.% C, which shows the carbon addition is beneficial in getting smaller nano-crystalline grain size. Consolidation into bulk shape with a relative density above 98% was performed with the spark plasma sintering technique at 650, 675, 700 and 730 °C. A maximum compression strength of approximately 2000 MPa was obtained for 650 °C consolidation sample with an absolute lack of plasticity, whereas a strength to plasticity balance could be obtained by increasing the temperature. Detailed microstructural analysis has been done with scanning electron microscopy and transmission electron microscopy. The microstructure shows a bimodal grain size distribution with a fine dispersion of oxide particles having a size ranging from 20 to 200 nm and cementite particles having a size range from 100 to 500 nm depending on the sintering temperature. The presence of oxide particles along with bimodal grain distribution resulted in a high yield strength value of 1620 MPa and high plasticity of 45%.