Grain Orientation Spread in Dynamically Recrystallized Austenitic Steel

Some feature of discontinuous dynamic recrystallization (DRX) in an Fe-0.4%C-18%Mn austenitic steel during isothermal compression tests at temperatures of 973-1373 K and strain rates of 10-3-10-1 s-1 were studied. The DRX microstructures consisted of various grains, i.e., DRX nuclei, growing DRX grains, and work-hardened DRX grains, which differentiated with the grain orientation spread (GOS). DRX was commonly promoted by a decrease in temperature-compensated strain rate, i.e., Zener-Hollomon parameter (Z), corresponding to an increase in deformation temperature and/or a decrease in strain rate. In contrast, the GOS distribution varied non-monotonously with Z. The large area fraction of DRX grains with small GOS below 1° appeared at definite temperature/strain rate conditions. The large fraction above 0.6 of DRX grains with small GOS was observed in DRX microstructures with a large ratio of CSL Σ3 boundary fraction to low-angle subboundary fraction. The GOS distribution in the DRX microstructures is discussed in terms of the DRX grain nucleation and growth rates.

Hydrogen behaviour at twist {110} grain boundaries in α -Fe

The behaviour of hydrogen at structural defects such as grain boundaries plays a critical role in the phenomenon of hydrogen embrittlement. However, characterization of the energetics and diffusion of hydrogen in the vicinity of such extended defects using conventional ab initio techniques is challenging due to the relatively large system sizes required when dealing with realistic grain boundary geometries. In order to be able to access the required system sizes, as well as high-throughput testing of a large number of configurations, while remaining within a quantum-mechanical framework, an environmental tight-binding model for the iron–hydrogen system has been developed. The resulting model is applied to study the behaviour of hydrogen at a class of low-energy {110}-terminated twist grain boundaries in α -Fe. We find that, for particular Σ values within the coincidence site lattice description, the atomic geometry at the interface plane provides extremely favourable trap sites for H, which also possess high escape barriers for diffusion. By contrast, via simulated tensile testing, weakly trapped hydrogen at the interface plane of the bulk-like Σ3 boundary acts as a ‘glue’ for the boundary, increasing both the energetic barrier and the elongation to rupture. This article is part of the themed issue ‘The challenges of hydrogen and metals’.

Variation of Σ3 and Coherent Σ3 Boundary Fraction with Thickness in Nanometric Cu Films

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Dynamic Recrystallisation during Isothermal Hot Deformation in a Titanium Modified Austenitic Stainless Steel

The paper discusses the microstructural evolution during dynamic recrystallisation (DRX) of a titanium-modified austenitic stainless steel (alloy D9). Isothermal hot compression tests were conducted in a Gleeble thermo-mechanical simulator in the temperature range 1173-1373K to various strains at a constant strain rate of 0.1 and 1 s-1. The extent of DRX increased with increase in strain and temperature. Nucleation of new DRX grains was found to occur by bulging of parent grain boundary. A continuous sub-grain rotation around the original grain boundaries, which would lead to the formation of DRX nucleus in sub-grain structures, could not be confirmed from the present study. Fractions of Σ3 boundaries increased almost linearly with increase in area fraction of DRX. The generation of this Σ3 boundary was accounted for in the formation of annealing twins during DRX. The possible role of annealing twins on DRX in alloy D9 is also discussed.

Characterization of Grain Boundaries in Recrystallized L12-Type Intermetallic Alloys

Microstructural feature of the recrystallized Co-based (Co3Ti) and Ni-based (Ni3(Si,Ti) and Ni3Fe) ordered alloys with L12 structure was investigated by the electron backscatter diffraction (EBSD) method, with emphasis on grain boundary character distribution (GBCD). For comparison, the GBCDs of the recrystallized Co-Ni, Ni-Fe and 70/30 brass disordered alloys, and also copper, nickel and aluminum pure metals with A1 (fcc) structure, which have widely different stacking fault energies, were also determined. The frequency of Σ3 boundary for the Co-based alloys was higher than that for the Ni-based alloys, regardless of ordered L12 alloy or disordered fcc alloy, indicating that the frequency of Σ3 boundary was primarily dominated by stacking fault energy. Furthermore, the effect of ordering energy on structure and energy of the grain boundaries including Σ3 boundary in the ordered L12 alloys was discussed.