Hot deformation characteristic and strain dependent constitutive flow stress modelling of Ti + Nb stabilized interstitial free steel

Abstract An effort has been made to establish a relation between Zener–Hollomon parameter, flow stress and dynamic recrystallization (DRX). In this context, the plastic flow behavior of Ti + Nb stabilized interstitial free (IF) steel was investigated in a temperature range of 650–1100 °C and at constant true strain rates in the range 10−3–10 s−1, to a total true strain of 0.7. The flow stress curves can be categorized into two distinct types, i.e. with/without the presence of steady-state flow following peak stress behavior. A novel constitutive model comprising the strain effect on the activation energy of DRX and other material constants has been established to predict the constitutive flow behavior of the IF steel in both α and γ phase regions, separately. Predicted flow stress seems to correlate well with the experimental data both in γ and α phase regions with a high correlation coefficient (0.982 and 0.936, respectively) and low average absolute relative error (7 and 11%, respectively) showing excellent fitting. A detailed analysis of the flow stress, activation energy of DRX and stress exponent in accord with the modelled equations suggests that dislocation glide controlled by dislocation climb is the dominant mechanism for the DRX, as confirmed by the transmission electron microscopy analysis

Influence of Zr on structure development in rapidly solidified AI-Si alloys

The method of rapid solidification from the melt enables a nano-structured zone to develop that is clearly visible in cross-section of ribbon-like specimens. The study of this zone (referred to earlier as structureless) is performed on a model Al-Si eutectic alloy with different Zr-additions. Due to the low Zrconcentration in equilibrium Al-solid solution, rapidly solidified ribbons are obtained and processed. The structure of a ribbon reveals two different zones. The first (near to the cooling surface) has shown nanosized intermetallic phases and Si-particles, which are easy to change during thermal treatment. Zr-additives to the Al-Si eutectic influence the solidification mechanism, and change the quantity, the composition and crystallography of the phase's comparing its state as-cast and after thermal treatment. There is evidence for the processes obtained by DSC analyses and by light microscopy. Thermal treatment at elevated temperature was used to detect changes, which could take place if hot-extrusion processing was used to compact chopped ribbon to bulk material. The micro-hardness tests were used to reveal new properties of compacts and oxidation experiments

The formation of supersaturated solid solutions in Fe–Cu alloys deformed by high-pressure torsion

Fully dense bulk nanocomposites have been obtained by a novel two-step severe plastic deformation process in the immiscible Fe–Cu system. Elemental micrometer-sized Cu and Fe powders were first mixed in different compositions and subsequently high-pressure-torsion-consolidated and deformed in a two-step deformation process. Scanning electron microscopy, X-ray diffraction and atom probe investigations were performed to study the evolving far-from-equilibrium nanostructures which were observed at all compositions. For lower and higher Cu contents complete solid solutions of Cu in Fe and Fe in Cu, respectively, are obtained. In the near 50% regime a solid solution face-centred cubic and solid solution body-centred cubic nanograined composite has been formed. After an annealing treatment, these solid solutions decompose and form two-phase nanostructured Fe–Cu composites with a high hardness and an enhanced thermal stability. The grain size of the composites retained nanocrystalline up to high annealing temperatures

Stability of the ultrafine-grained microstructure in silver processed by ECAP and HPT

The high-temperature thermal stability of the ultrafine-grained (UFG) microstructures in low stacking fault energy silver was studied by differential scanning calorimetry (DSC). The UFG microstructures were achieved by equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) at room temperature (RT). The defect structure in the as-processed samples was examined by electron microscopy and X-ray line profile analysis. The stored energy calculated from the defect densities was compared to the heat released during DSC. The sum of the energies stored in grain boundaries and dislocations in the ECAP-processed samples agreed with the heat released experimentally within the experimental error. The temperature of the DSC peak maximum decreased while the released heat increased with increasing numbers of ECAP passes. The released heat for the specimen processed by one revolution of HPT was much smaller than after 4–8 passes of ECAP despite the 2 times larger dislocation density measured by X-ray line profile analysis. This dichotomy was caused by the heterogeneous sandwich-like microstructure of the HPT-processed disk: about 175 lm wide surface layers on both sides of the disk exhibited a UFG microstructure while the internal part was recrystallized, thereby yielding a relatively small released heat