Materials Science and Technology

Q1N samples with different degrees of tempering were produced and changes in MBN energies in non-stressed and stressed samples was investigated. It was observed that while MBN energy decreased with increasing degree of tempering, it increased with applied uniaxial stress and the two opposing effects cannot be easily separated. MBN energy increased with stress along RD and decreased in the transverse direction (TD). Therefore, the ratio of MBN energy in RD to TD was examined. The value of the ratio at 0.3 T was successfully shown to be strongly correlated with applied uniaxial tensile stress level, regardless of the degree of tempering (pinning site density) in Q1N samples.

The performance of ferrite-martensite (DP 600) and ferrite-pearlite (SPFH 590) steels with a similar tensile strength of ∼650 MPa under cyclic loading was investigated. This work presents the high cycle fatigue properties of DP 600 and SPFH 590 steel in the presence and absence of a hole. In presence of the hole, the fatigue strength for both the steels drops significantly, with drop being more for SPFH 590 steel. While the fatigue strength in absence of a hole is a strong function of yield strength of material, the behaviour in presence of the hole depends on the steel microstructure. The presence of martensite in case of DP 600 steel delayed the crack propagation by branching and crack tip blunting. Additionally, martensite  also increases the local strain, low angle grain boundary and dislocation density ahead of the crack tip, resulting in an increased  amount of plastic deformation and blunting of the crack tip.

The phase transformations and twinning in a nano-sized face-centred cubic (FCC) crystal of iron with different carbon contents are studied by molecular dynamic (MD) simulations. The simulation cells are stretched uniaxially along the direction. It is found that the FCC matrix transforms into the body-centred cubic (BCC) phase following the Nishiyama–Wassermann orientation relationship (OR). Then, the ‘BCC hexagonal close-packed (HCP)’ and ‘BCC FCC’ transformations occur following different variants of the Burgers OR and the Pitsch OR, respectively. The twins form between two neighbouring BCC grains. Additionally, the carbon atoms can destabilise the BCC phase, inhibit the phase transformation and pin phase interfaces, leading to tiny BCC twins.

In this investigation, microstructural variation across the dissimilar weld joint between modified 9Cr-1Mo ferritic steel and 316LN stainless steel with nickel-based filler metal was fabricated and characterized by various techniques. It was observed that the heterogeneous microstructure and non-uniform microhardness were developed across the dissimilar weld joint. Carbon-depleted soft zone, carbon-enriched hard zone, type I and type II boundaries are evident at the P91 heat-affected zone (HAZ) – buttering layer interface. An unmixed zone was observed at the weld metal-316 LN stainless steel HAZ interface. Distinct microstructures were developed across the dissimilar weld joint are due to the mismatch in chemical composition, crystal structure and melting temperature difference between base metals and filler metal.

In the present study, the influence of aging temperature on precipitation hardening in a non-equiatomic Al_0.2Co_1.5CrFeNi_1.5Ti_0.3 high-entropy alloy has been evaluated. The aging curves have been deduced for different aging temperatures (650, 750, and 850°C) as a function of time. The aging response was found to be markedly different with varying aging temperatures. A combined FESEM and transmission electron microscope studies have been performed to understand the evolution of precipitates and their characteristics with aging temperature and time. The variation of precipitate morphology, number density and size of precipitates with aging temperature were correlated with the age-hardening response. Additionally, the role of lattice misfit strain on precipitate morphology has been evaluated.

GRAPHICAL ABSTRACT

Novel Cu–24Pb–xAl alloys with dispersed and rodlike Pb-rich secondary phases (PSPs) were designed and prepared with the guidance of CALPHAD. Through primary binary-phase diagram analysis and thermodynamic calculation, Al was proven to be a suitable alloying element in Cu–Pb system to promote the formation of dispersed and rodlike PSPs during solidification by introducing a (L₁+L₂+α-Cu) phase region in Cu–Pb–xAl systems. The Cu–24Pb–xAl (wt-%, x = 0, 2, 4) alloys were then prepared in order to verify the reliability of the alloy design strategy. Cu–24Pb–2Al alloy with rodlike PSPs was successfully designed and prepared. A novel strategy to design Cu–Pb–(X) immiscible alloy with optimised microstructure is thus proposed in this work which can help establish a design criterion for other immiscible alloys.