No abstract

During dynamic tensile extrusion (DTE) the material is subjected to a complex deformation history, including high strain rates, large strains and elevated temperatures. This technique provides unique means to explore material performance under extreme conditions. In this work, the microstructural evolution of 99.98% commercially pure copper during DTE test was investigated by means of electron backscatter diffraction (EBSD). The investigation was focused on the segment of the extruded jet that remained in the die, since numerical simulation showed that material points along the longitudinal axis of such segment correspond to different stages of a common temperature compensated deformation history. Therefore, post mortem microstructure information extracted at different locations along the centre line is equivalent to in situ real-time measurement during the deformation process. EBSD investigations along the centre line showed a progressive elongation of the grains, and an accompanying development of a strong <001>+<111> dual fibre texture. Meta-dynamic discontinuous dynamic recrystallization (DRX) occurred at larger strains, and it was demonstrated that nucleation occurred during straining, while subsequent grain growth took place during post-deformation cooling in the die. According to strain energy minimization arguments, the recrystallization resulted in an increased <001> texture component. The critical strain for recrystallization was well predicted from a power-law dependence on the Zener-Hollomon parameter, including grain size dependence and a temperature dependent activation energy. In addition, it was shown that <001> and <111> oriented grains develop different dislocation substructures during straining, exhibiting elongated cells/micro-bands and typical cell structures, respectively. The present results also confirm that dynamic tensile ductility increases with decreasing initial grain size as a result of grain refinement and lowering of dislocation and twin densities during DRX.

A physical-based model for the flow stress of bcc metals is presented. Here, thermally activated and viscous drag regimes are considered. For the thermally activated component of the flow stress, the diffusion-controlled regime at elevated temperature is also taken into account assuming the non-linear dependence of the activation volume on temperature. The model was applied to A508 (16MND5) steel showing the possibility to accurately describe the variation of the flow stress over the entire temperature range (from 0 K to Tm) and over a wide strain-rate range.

The dynamic tensile extrusion (DTE) test offers unique possibility to probe material response under very large plastic strain, high strain rate and temperature to support constitutive modelling development. From the computational point of view, the DTE test is particularly challenging and a number of issues need to be assessed before proceeding with material modelling verification. In this work, an extensive and detailed computational work was carried out in order to provide the guidelines for accurate simulation of DTE test. Two constitutive models, the first phenomenological the latter physically-based, were used to simulated the behavior of fully annealed OFHC copper in dynamic extrusion at different velocities. Material models parameters were calibrated using uniaxial test data at different strain rates and temperatures. The number, size and shape of the ejected fragments at different velocity were used as validation metrics for the selected constitutive models. Results indicate that material behavior under dynamic extrusion can be accurately predicted limiting the influence of numerical parameters not related to the constitutive model under investigation. The physically based modelling allows a more accurate prediction of the material response and the possibility to incorporate microstructure evolution processes, such as dynamic recrystallization, which seems to control the response of OFHC copper in DTE tests at higher velocity.

Some results of materials characterization activities, dedicated to classical and notch mechanics fatigue and elastoplastic properties, have already been published for some Ferritic-Pearlitic Ductile Iron, including the patented heat treated Isothermed (IDI) and Austempered Ductile Iron (ADI) grades. Others have not yet been published. The possible use of all of these results in new standards is discussed in this paper. It is proposed that new standards should provide a criterion that is able to measure the process quality that represents more accurately the actual market needs and manufacturing capabilities. Classification of grades, considered by existing standards, is based on minimum properties for strength and ductility parameters that are separately evaluated. A different approach that is based on a quality index, which considers strength and ductility all in one, is proposed. However, this new proposed approach may not be sufficient to provide a satisfactory classification for the ADIs. This is because their fracture mechanical behavior and machinability can be correlated with their austenite stability. It could also be insufficient for the classification of the recent High Silicon Solid Solution Strengthened Ductile Irons that exhibit a decreasing ultimate tensile strength/proof stress ratio with increasing Si. For construction steels, fracture mechanics properties are sometimes believed to be related to the Charpy impact energy. This paper introduces an innovative practical and inexpensive data analysis, performed on the tensile test curve, which appears to be a potential estimator of fracture mechanical properties, at least for ADIs, where said properties could be correlated with the austenite stability.