Numerical modeling of dual phase microstructure behavior under deformation conditions on the basis of digital material representation

Development of a digital material representation (DMR) model of dual phase steel is presented within the paper. Subsequent stages involving generation of a reliable representation of microstructure morphology, assignment of material properties to component phases and incorporation of the model into the commercial finite element software are described within the paper. Different approaches used to recreate dual phase morphology in a digital manner are critically assessed. However, particular attention is placed on innovative identification of phase properties at the micro scale by using micro-pillar compression tests. The developed DMR model is finally applied to model influence of micro scale features on failure initiation and propagation under loading conditions.

Quenched and annealed microstructures of hot worked magnesium AZ31

The microstructures of magnesium AZ31 are examined following hot compression testing and annealing. The grain size, fraction dynamically recrystallized and, in a couple of cases, the crystallographic texture are reported. The progress of dynamic recrystallization and the recrystallized grain size were sensitive to processing conditions, as expected. This effect was more marked in the former than in the latter, compared to other metals. It was also found that, for structures containing between 80 and 95% dynamic recrystallization, abnormal grain growth occurred during annealing. Irrespective of the whether or not abnormal grain growth occurred, the annealing step weakened the crystallographic texture.

Method for determining reaction rate of mild steel containers during melting of magnesium-aluminium alloys and effect of aluminium content on directionally solidified microstructures

A magnesium alloy of eutectic composition (33 wt-'%Al) was directionally solidified in mild steel tubes at two growth rates, 32 and 580 mum s(-1,) in a temperature gradient between 10 and 20 K mm(-1). After directional solidification, the composition of each specimen varied dramatically, from 32'%Al in the region that had remained solid to 18%Al (32 mum s(-1) specimen) and 13%Al (580 mum s(-1) specimen) at the plane that had been quenched from the eutectic temperature. As the aluminium content decreased, the microstructure contained an increasing volume fraction of primary magnesium dendrites and the eutectic morphology gradually changed from lamellar to partially divorced. The reduction in aluminium content was caused by the growth of an Al-Fe phase ahead of the Mg-Al growth front. Most of the growth of the Al-Fe phase occurred during the remelting period before directional solidification. The thickness of the Al-Fe phase increased with increased temperature and time of contact with the molten Mg-Al alloy. (C) 2003 Maney Publishing.

Influence of grain size on hot working stresses and microstructures in Mg-3A1-1Zn

The influence of the grain size on the deformation of Mg–3Al–1Zn was examined in compression at 300 °C. At low strains the flow stress increases with increasing grain size. This is interpreted in terms of dynamic recrystallization. Empirical models of dynamic recrystallization are developed and employed to generate a microstructure map.

Formation of ultrafine grained microstructures in steel through strain induced transformation during single pass hot rolling

In the present study, wedge-shape samples were used to study the effect of strain induced transformation on the formation of ultrafine grained structures in steel by single pass rolling. The results showed two different transition strains for bainite formation and ultrafine ferrite (UFF) formation in the surface layer of strip at reductions of 40% and 70%, respectively, in a plain carbon steel. The bainitic microstructure formed by strain induced bainitic transformation during single pass rolling was also very fine. The evolution of UFF formation in the surface layer showed that ferrite coarsening is significantly reduced through strain induced transformation combined with rapid cooling in comparison with the centre of the strip. In the surface, the ferrite coarsening mostly occurred for intragranular nucleated grains (IG) rather than grain boundary (GB) ferrite grains. The results suggest that normal grain growth occurred during overall transformation in the GB ferrite grains. In the centre of the strip, there was significantly more coarsening of ferrite grains nucleated on the prior austenite grain boundaries.

The formation of ultrafine grained steel microstructures through thermomechanical processing.

The formation of ultrafine grained steels is an area of intense research around the World. There are a number of methods to produce grain sizes of approximately 1 µm, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. This paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. It is shown that this requires deformation within the Ae3 to Ar3 temperature range for a given alloy. The formation of ultrafine ferrite involves a dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation arises from the introduction of additional intragranular nucleation sites. It is possible that the deformation also hinders the growth or coarsening of the ferrite and may also lead to dynamic recrystallization of the ferrite. The most likely commercial exploitation of ultrafine ferrite would appear to rely on the formation of a critical volume fraction of dynamic strain induced ferrite followed by controlled cooling to ensure this is maintained to room temperature and to also form other secondary phases, such as martensite, bainite and/or retained austenite to improve the formability.

A mechanical approach to quantify dynamic recrystallization in polycrystalline metals

A new method is proposed to quantify progress of dynamic recrystallization in polycrystalline metals during deformation. This approach utilises the stress–strain curve of the material to quantify the progress of dynamic softening. The outcome of this method showed a good agreement with experimental results for alloys of this study.

A comparative study of microstructures and mechanical properties obtained by bar and plate rolling

Microstructures and mechanical properties of a low carbon steel were studied after plate rolling and bar rolling. Plate rolling is characterized as a monotonic compressive loading, while bar rolling is characterized as a cross-compressive loading. A four-pass plate rolling and bar rolling experiment was designed so that the material experiences the same amount of strain at each pass during rolling. The rolling experiment was performed at moderately high temperatures (450, 550 and 650 °C). The microstructures and mechanical properties of the low carbon steel acquired from the two types of rolling experiments were compared. The results revealed that differences of loading path attributed by monotonic loading (plate rolling) and cross loading (bar rolling) significantly influenced the microstructures and mechanical properties such as yield stress, ultimate tensile stress, strain hardening exponent and elongation of the low carbon steel.

Non-Schmid behaviour during secondary twinning in a polycrystalline magnesium alloy

The present work combines electron backscattering diffraction and Schmid analysis to investigate secondary twinning in the magnesium alloy Mg–3Al–1Zn. Inspection of the misorientations between the parent and ^{{1011} - {1012}} doubly twinned volumes reveals that there are four possible variants. One of these variants (the one that forms a misorientation with the matrix characterized by 38°^⟨1210⟩ ) is favoured much more than the others. This variant involves the activation of secondary twinning systems quite inconsistent with Schmid-type behaviour. For the secondary twin to grow significantly it must take on a shape enforced by the primary twin. This is non-optimal for strain compatibility. It is argued that the 38°^⟨1210⟩ variant occurs most because it provides the closest match between the primary and secondary twinning planes, thus minimizing the compatibility strain.

Microstructures and bond strengths of the calcium phosphate coatings formed on titanium from different simulated body fluids

Calcium phosphate (Ca-P) coatings were deposited on Ti substrates by a biomimetic method from m-SBF and 10× SBF, respectively. Comparative study of microstructures and bond strengths of the Ca-P coatings deposited from those different SBFs was carried out. Effect of the surface roughness of the substrates on the bond strength of the Ca-P coatings was also studied. Scanning electron microscopy (SEM), X-ray diffractometry (XRD), Fourier transformed infrared spectroscopy (FTIR), inductive coupled plasma spectrometry (ICP) and thermogravimetry (TG) were used to characterize the Ca-P coatings. The bond strengths between the coatings and Ti substrates were measured using an adhesive strength test. Results indicated that the ionic concentrations of the SBFs and the surface roughness of the substrate had a significant influence on the formation, morphology and bond strength of the Ca-P precipitates. The induction period of time to deposit a complete Ca-P layer from the m-SBF is much longer, but the Ca-P coating is denser and has higher bond strength than that formed from the 10× SBF. The Ti with a surface roughness of R_a 0.64 µm and R_z 2.81 µm favoures the formation of a compact Ca-P coating from the m-SBF with the highest bond strength of approximately 15.5 MPa.