Accelerated Spheroidization of Cementite in Sintered Ultrahigh Carbon Steel by Warm Deformation

Evolution of microstructure and hardness in quenched ultrahigh carbon steel Fe-0.85Mo-0.6Si-1.4C by warm compression on a Bähr plastometer-dilatometer at 775 °C and at 0.001 to 1 s−1 strain rate range is reported. The material was prepared via powder metallurgy: cold pressing and liquid phase sintering. Independent of strain rate, the initial martenstic microstructure was transformed to ferrite and spheroidized cementite. Strain rate had an effect on size and shape of spheroidized Fe3C precipitates: the higher the strain rate, the smaller the precipitates. Morphology of the spheroidized carbides influenced hardness, with the highest hardness, 362 HV10, for strain rate 1 s−1 and the lowest, 295 HV10, for the lowest strain rate 0.001 s−1. Resultant microstructure and ambient temperature mechanical properties were comparable to those of the material that had undergone a fully spheroidizing treatment with increased time and energy consumption, indicating that it can be dispensed with in industrial processing. All our results are consistent with the Hall–Petch relation developed for spheroidized steels.

Emission of Dislocations from Grain Boundaries and Its Role in Nanomaterials

The Frank-Read model, as a way of generating dislocations in metals and alloys, is widely accepted. In the early 1960s, Li proposed an alternate mechanism. Namely, grain boundary sources for dislocations, with the aim of providing a different model for the Hall-Petch relation without the need of dislocation pile-ups at grain boundaries, or Frank-Read sources inside the grain. This article provides a review of his model, and supporting evidence for grain boundaries or interfacial sources of dislocations, including direct observations using transmission electron microscopy. The Li model has acquired new interest with the recent development of nanomaterial and multilayers. It is now known that nanocrystalline metals/alloys show a behavior different from conventional polycrystalline materials. The role of grain boundary sources in nanomaterials is reviewed briefly.

Microstructure and Strengthening Model of Cu–Fe In-Situ Composites

The tensile strength evolution and strengthening mechanism of Cu–Fe in-situ composites were investigated using both experiments and theoretical analysis. Experimentally, the tensile strength evolution of the in-situ composites with a cold deformation strain was studied using the model alloys Cu–11Fe, Cu–14Fe, and Cu–17Fe, and the effect of the strain on the matrix of the in-situ composites was studied using the model alloys Cu–3Fe and Cu–4.3Fe. The tensile strength was related to the microstructure and to the theoretical strengthening mechanisms. Based on these experimental data and theoretical insights, a mathematical model was established for the dependence of the tensile strength on the cold deformation strain. For low cold deformation strains, the strengthening mechanism was mainly work hardening, solid solution, and precipitation strengthening. Tensile strength can be estimated using an improved rule of mixtures. For high cold deformation strains, the strengthening mechanism was mainly filament strengthening. Tensile strength can be estimated using an improved Hall–Petch relation.