Magnus Hörnqvist Colliander

Structural Characterization of Phase Separation in Fe-Cr: A Current Comparison of Experimental Methods

Self-assembly due to phase separation within a miscibility gap is important in numerous material systems and applications. A system of particular interest is the binary alloy system Fe-Cr, since it is both a suitable model material and the base system for the stainless steel alloy category, suffering from low-temperature embrittlement due to phase separation. Structural characterization of the minute nano-scale concentration fluctuations during early phase separation has for a long time been considered a major challenge within material characterization. However, recent developments present new opportunities in this field. Here, we present an overview of the current capabilities and limitations of different techniques. A set of Fe-Cr alloys were investigated using small-angle neutron scattering (SANS), atom probe tomography, and analytical transmission electron microscopy. The complementarity of the characterization techniques is clear, and combinatorial studies can provide complete quantitative structure information during phase separation in Fe-Cr alloys. Furthermore, we argue that SANS provides a unique in-situ access to the nanostructure, and that direct comparisons between SANS and phase-field modeling, solving the non-linear Cahn Hilliard equation with proper physical input, should be pursued.

Strain rate effects on fracture behavior of Austempered Ductile Irons

In this work, the mechanical behavior of the austempered ductile iron (ADI) JS/1050-6 was investigated, with particular attention to the strain rate effects on the material ductility. Tensile tests at different strain rates (up to 103 s(-1)) and temperatures (ranging from 213 to 343 K) were performed. Samples with different geometries, smooth and round notched bars, were used to evaluate the effect of the stress triaxiality level on the strain at fracture. For each configuration, the evolution paths of stress and strain were extracted in the point where failure is expected performing numerical analyses at the continuum scale. Stress histories were used as input in a micromechanics analysis aimed to analyze the heterogeneous state of stress, determined by the presence of the graphite nuclei, under the different loading conditions obtained in the experiments. The main result is that, under dynamic conditions, the stress field redistribution, due to the adiabatic condition, postpones the failure occurrence, regardless temperature and strain rate effects on the matrix ductility.

Deformation and Failure of OFHC Copper under high strain rate shear compression

Hat-shaped specimen geometries were developed to generate high strain, high-strain-rates deformation under prescribed conditions. These geometries offer also the possibility to investigate the occurrence of ductile rupture under low or negative stress triaxiality, where most failure models fail. In this work, three tophat geometries were designed, by means of extensive numerical simulation, to obtain desired stress triaxiality values within the shear region that develops across the ligament. Material failure was simulated using the Continuum Damage Model (CDM) formulation with a unilateral condition for damage accumulation and validated by comparing with quasi-static and high strain rate compression tests results on OFHC copper. Preliminary results seem to indicate that ductile tearing initiates at the specimen corner location where positive stress triaxiality occurs because of local rotation and eventually propagates along the ligament.

Crack Growth Studies in a welded Ni-base superalloy

It is well known that the introduction of sustained tensile loads during high-temperature fatigue (dwell-fatigue) significantly increases the crack propagation rates in many superalloys. One such superalloy is the Ni-Fe based Alloy 718, which is a high-strength corrosion resistant alloy used in gas turbines and jet engines. As the problem is typically more pronounced in fine-grained materials, the main body of existing literature is devoted to the characterization of sheets or forgings of Alloy 718. However, as welded components are being used in increasingly demanding applications, there is a need to understand the behavior. The present study is focused on the interaction of the propagating crack with the complex microstructure in Alloy 718 weld metal during cyclic and dwell-fatigue loading at 550 °C and 650 °C.