Papers by Keyword: Strain Path

Abstract: The microstructure evolution of copper multicrystalline sheets, undergoing plastic deformation in the sequences of strain paths rolling – tension and tension – rolling, was studied in the present work. For both sequences, two different types of change of strain path were studied: the tensile and rolling directions were parallel and normal to each other. Samples submitted to these four complex strain paths were investigated by transmission electron microscopy (TEM). TEM observations have shown the typical dislocations microstructures for the prestrain paths in tension and rolling. The dislocation microstructures observed during the second path were analysed and discussed as a function of the sequence and of the type of strain path change (parallel and normal sequential paths). Special microbands features were observed during the second path, for both sequences, rolling – tension and tension – rolling. The appearance of such microstructural features is discussed in terms of the sequence and type of strain path change and it is linked with the slip activity during the second deformation mode.

Abstract: The effects of strain path reversal under hot working conditions on the microstructure and crystallographic texture in the near-α titanium alloy Timetal-834 have been studied using high resolution electron backscatter diffraction (EBSD). The main objective of the work was to investigate the potential effect strain path may have on breaking up the well known clustering of similarly orientated primary alphas grains in the alloy, which significantly reduces its low cycle dwell fatigue lifetime. Deformation was carried out using forward torsion to an equivalent strain of 0.9 and forward/reverse torsion of two equal steps to produce a total strain of 0.9. The tests were performed at a typical industrial forging condition of 990°C (~50% alpha, ~50% beta) at an equivalent tensile strain rate of 2s-1. Investigation of the microstructure showed the primary alpha grains to align with the direction of torsion for the forward test and return to an equiaxed shape on strain reversal, though a significant numbers of deformation twins are formed and retained after the full strain reversal. Analysis of the texture of the starting material showed the typically clustering of primary alpha grains, which upon forward torsion and forward/reverse torsion did not break down. This indicates that during a typical forging operation the clustering of similarly orientated primary alpha grains inherited from the as-received billet will not be reduced. This suggests that improved in-service performance of this alloy can only be achieved by reducing the clustering upstream in the manufacture of the billet.

Abstract: For the formation of ultrafine grain in Al alloys, various means have been investigated based on a process of continuous recrystallization using a high-strain technique that employs rigorous plastic working. However, utilization for practical application is difficult for small specimens that require constraining. In this study, the effects were studied of the use of constraining die walls in the multi-axial alternative forging process (MAF) on the formation of ultrafine grains and microstructural homogeneity. This technique has possible for scaling up to a practical scale. Our results showed that tensile strength and yield stress in these fabricated materials were tripled over those of the initial materials when strain was applied. The average grain size after strain application was 0.5 µm. We conclude that a loading technique that uses different applied directions is the key determinant in creating ultrafine grains.

Abstract: The strain path undergone by a material can have a significant influence on the deformation behaviour, recrystallisation kinetics, grain size and crystallographic texture. To study the influence of strain path, samples of an aluminium-1%manganese alloy have been subjected to a number of strain path changes. These have been achieved using combinations of plane strain and free compression to give strain paths of 0, 90, and 180°. The development of the dislocation substructure resulting from each stage of deformation was studied using Electron Back Scatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). Following deformation annealing was carried out to determine the effects of the strain path changes on the recrystallisation kinetics and grain size. Results have shown that the strain path angles cannot be used to satisfactorily describe the material behaviour when mixtures of deformation modes are used.

Abstract: It has been demonstrated recently that when a reversion of the strain is applied during the hot working of a Nb-microalloyed steel, the subsequent static recrystallization kinetics is significantly affected. However, depending on the magnitude of the reversion, the static softening kinetics can be accelerated or delayed. This relates to the substructure dissolution taking place by the effect of the reversal. In the present work, new microstructural results obtained by EBSD on an austenitic stainless steel hot deformed by torsion is used to explain better the observed effect of the reversion of the strain.