Influence of σ Phase on the Allotropic Transformation of the Matrix in Co-Re-Cr-Based Alloys with Ni Addition

Co-Re-Cr alloys are being developed for high-temperature application in gas turbines. In these alloys, the Cr2Re3-based σ phase is stable when the Cr content is higher than 20 atomic %. The addition of Ni is being studied to partially substitute Cr, which aims to suppress σ formation without sacrificing the benefit of Cr in the oxidation resistance of the alloy. The microstructure of the alloys with varying Cr (18–23%) and Ni (8–25%) was investigated by electron microscopy in the present study, primarily to look into the stability of the σ phase and its influence on the Co matrix phase transformation. The σ phase is mainly found in two morphologies in these alloys, where at high temperature only blocky σ phase is present at grain boundaries but cellular σ is formed through a discontinuous precipitation within the grains at lower heat treatment temperatures. The presence of fine cellular σ phase influences the alloy hardness. Moreover, the σ precipitation, which depletes the matrix in Re, also influences the allotropic transformation of the Co matrix.

Investigation on performance characteristics and metallographic transformation on drilling aluminium/titanium sandwich

Materials sandwiches of aluminium/titanium are widely used in structural aerospace, especially in the surface coating and skeletons of aircrafts. Due to their inhomogeneous behavior and poor machinability, drilling aluminium/titanium sandwich in one-shot operation has brought great challenges to the modern manufacturing community. In this paper, a series of experiments were conducted to understand and characterize the process of drilling aluminium/titanium sandwich. Experimental results have shown that the serrated chip is not common in drilling. Only the outer edge of the spiral chip has saw teeth because of the variable cutting speed along the cutting edge. In addition, a better understanding of the phase transformation on surface and subsurface of drilled hole has been provided. Two kinds of phase transformations on the surface of drilled hole have been observed: one is the allotropic transformation from α to β phase and the other is that from β to minor phases α′ (hexagonal martensites). All these are mainly due to the interaction of mechanical force and high temperature.

Physical Simulation of Thermo-Mechanical Processing of Ferritic-Bainitic Dual Phase (FBDP) Steel

The present work is dealing with a physical simulation of thermo-mechanical processing of ferritic-bainitic dual phase (FBDP) steel alloy containing 0.1% C, 0.3% Si, 0.9% Mn and 0.7% Cr. The microstructure changes and allotropic transformations during thermo-mechanical simulation are investigated. A series of heating – cooling cycles to detect the critical and allotropic transformation temperature by dilatation were carried out on the thermo-mechanical simulator (Gleeble 3500). On the other hand, five – consecutive hits were used during the physical simulation of hot rolling process. Two hits were representing the roughening stage followed by three ones representing finish rolling. Holding at 500°C for 5, 7, 10, 12 and 15 min. after last hit has been applied and then followed by air cooling. Dilation curves appear that Ac1= 766 °C, while Ac3 was detected as 883 °C. Baintic allotropic transformation temperatures were clearly noticed as 618 °C for Bs and 542 °C for Bf. The recrystallization temperature was also detected as 1035 °C. Holding for 5-7 min. at 500 °C was concluded as the optimum for creation a bainite volume fraction. Rough hot deformation a higher temperature above the recrystallization temperature is essential, where no strain hardening and possibility for achieving high strains without excessive loads. Finishing deformation at temperature lower than Tr would create fine bainitic structure. The flow curve of the steel ensures continuous strain hardening. The strain hardening rate (σf/ε) was directly proportional to temperature difference from pass to pass.