Hamid Azizi-Alizamini

Biaxial Deformation of the Magnesium Alloy AZ80

The multiaxial deformation of magnesium alloys is important for developing reliable, robust models for both the forming of components and also analysis of in-service performance of structures, for example, in the case of crash worthiness. The current study presents a combination of unique biaxial experimental tests and biaxial crystal plasticity simulations using a visco-plastic self-consistent (VPSC) formulation conducted on a relatively weak AZ80 cast texture. The experiments were conducted on tubular samples which are loaded in axial tension or compression along the tube and with internal pressure to generate hoop stresses orthogonal to the axial direction. The results were analyzed in stress and strain space and also in terms of the evolution of crystallographic texture. In general, it was found that the VPSC simulations matched well with the experiments. However, some differences were observed for cases where basal 〈a〉 slip and

Phase Field Modelling of Austenite Formation in Low Carbon Steels

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Through Process Modelling of Extrusion for AA3xxx Alloys

extension twinning were in close competition such as in the biaxial tension quadrant of the plastic potential. The evolution of texture measured experimentally and predicted from the VPSC simulations was qualitatively in good agreement. Finally, experiments and VPSC simulations were conducted on a second AZ80 material which had a stronger initial texture and a higher level of mechanical anisotropy. In the previous case, the agreement between experiments and simulations was good, but a larger difference was observed in the biaxial tension quadrant of the plastic potential.

The Effect of Mn on Microstructure Evolution during Homogenization of Al-Mg-Si-Mn Alloys

Abstract In this paper, austenite formation in an Fe – C system was simulated with a phase field approach. The model deals with a detailed description of morphological changes during austenite formation from ultrafine ferrite – cementite aggregates isothermally annealed in the intercritical region to form dual phase microstructures. Long-range diffusion of carbon is explicitly considered. The model is capable of resolving carbide particle sizes of about 100 nm to simulate the morphological complexity during austenite formation. Simulations were carried out in two- and three-dimensions. It was observed that morphological aspects of austenite formation depend significantly on spacing and distribution of cementite particles that provide suitable nucleation sites for austenite. This dependency can primarily be attributed to overlapping diffusion fields and curvature effects.

Microstructure Refinement of a Low Interstitial Ferritic Stainless Steel by Cold Rolling and Annealing of Martensite

There is renewed interest in the investigation of austenite formation due to the development and increased use of advanced high strength steels for automotive applications. Intercritical annealing is an essential processing step for cold rolled and coated steel products with multi-phase microstructures. During intercritical annealing the initial ferrite-pearlite microstructure transforms partially to austenite. Models for the austenite formation are critical to predict the austenite fraction as a function of the thermal cycle thereby facilitating the design and control of robust processing paths. Modelling the austenite formation is challenging because of the morphological complexity of this transformation. Phase field models are a powerful tool to describe the evolution of microstructures with complex morphologies, e.g. formation of finger-type features during austenite formation. The present paper gives an overview of model approaches for the austenite formation. Phase field simulations are presented for two scenarios: (i) austenite formation from a fully pearlitic structure with a lamellar arrangement of carbide aggregates and (ii) austenite formation from ferrite-pearlite microstructures. Simulation results are compared with experimental observations for pearlitic steels. The challenges are delineated for the development of austenite formation models with predictive capabilities.

Formation of Ultrafine Grained Low Carbon Steels via Martensite Deformation and Annealing

A phase field model (PFM) is applied to simulate the effects of microsegregation, cooling rate, and austenite grain size on banding in a C-Mn steel. The PFM simulations are compared with experimental observations of continuous cooling transformation tests in the investigated steel. Using electron probe microanalysis, the microsegregation characteristics of Mn were determined and introduced into the model. Ferrite nucleation is assumed to occur at austenite grain boundaries, and ferrite growth is simulated as mixed-mode reaction for para-equilibrium conditions. The driving pressure for the austenite to ferrite transformation depends on Mn concentration and thus varies between the alternating microsegregation layers. In agreement with experimental observations, the simulation results demonstrate that by increasing the cooling rate and/or austenite grain size, banding tends to disappear as the transformation shifts to lower temperatures such that ferrite also forms readily in the layers with higher Mn levels. Further, a parametric study is conducted by changing thickness and Mn content of the bands. In accordance with experimental observations, it is shown that for sufficiently large band thickness, band splitting takes place where ferrite grains form close to the center of the Mn-rich band. Changing the degree of Mn segregation indicates that a segregation level of 0.2 wt pct is necessary in the present case to achieve banded microstructures.

A novel technique for developing bimodal grain size distributions in low carbon steels

A through process model for high temperature extrusion of AA3xxx aluminum alloys is presented. An overview of the various chemistry dependent sub-models, e.g. homogenization, extrusion, cold deformation and annealing, are described with an emphasis on the linkages between the models. Examples are presented to illustrate the importance of including the linkages between the sub-models. For example, there is a strong linkage between dispersoids which form during homogenization on subsequent recrystallization behavior after high temperature extrusion or ambient temperature deformation. Finally, a number of observations are presented on the prognosis (opportunities and challenges) for the future of through process modelling.

Austenite Formation in Plain Low-Carbon Steels

A study was conducted on the evolution of microstructure during homogenization for two Al-Mg-Si alloys with different Mn levels, i.e. 0 and 0.5wt%. The homogenization treatment was conducted over a wide range of temperatures above the Mg2Si solvus. The holding time at the peak temperature ranged from 2 hour to one week. Microstructure evolution of the constituent particles and Mn dispersoids were characterized by means of optical microscopy and FEG-SEM. The Mn content in and out of solution was estimated using the Thermo-calc (TTAl6 database) and resistivity measurements. The micro-segregation and distribution of the main alloying elements before and after homogenization were systematically studied by electron probe micro analysis (EPMA). It was found that the Mn content together with the homogenization practice had a significant influence on the microstructure evolution. By combining all the measurements, a comprehensive quantitative dataset describing microstructure evolution during homogenization was developed.