The Effect of Strain Reversal during High Pressure Torsion on the Evolution of Microstructure, Texture and Hardness Properties of Aluminum Alloys

The present work aims to investigate the effect of strain reversal during high pressure torsion on the evolution of microstructure, texture and hardness properties of two different materials with different dynamic recovery behavior, namely, high purity Aluminum (>99%, designated as 2N-Al) and Aluminum-Magnesium (Al-2.5%Mg) alloy. For this purpose, 2N-Al and Al-2.5%Mg were subjected to monotonically (CW) and strain reversal (CW-CCW) deformation by High Pressure Torsion (HPT). The samples were subjected to a series of rotations in monotonically and strain reversal deformation with same equivalent strains of 1, 4, 12, 24 and 60 under an applied load of 6 GPa and with 1 rpm under quasi-constrained conditions

Microstructure evolution of medium carbon low alloy steel during ingot breakdown process

High-strength steels used for the manufacturing of large size components such as turbine shafts are produced by ingot casting, followed by forging, quenching, and tempering operations to breakdown the as cast structure and to optimize the properties. The first step of the deformation process, called ingot breakdown, is carried out well above the paraequilibrium Ae3 temperature (temperature at which ferrite to austenite transformation accompanied by interstitial diffusion reaches equilibrium, and therefore produces major microstructural changes. Moreover, the microstructure developed during the ingot break down process has a determining influence in the design of the subsequent thermomechanical treatments to achieve the desired properties in the wrought product. The main aim of this thesis is to investigate and understand the mechanisms involved in the evolution of microstructure during break down of as-cast structure of a medium carbon low alloy steel used as a die material in the transport industry. In addition, constitutive behavior of the as-cast structure was modelled and the material model best describing the behavior of the investigated steel during the ingot breakdown process of very large ingots was determined for the first time. The developed material model was implemented in a finite element (FEM) code. The first part of this study provides the details of the development of a precise model in order to predict the flow behavior of the material under different deformation conditions. The microstructure evolution from as-cast to wrought condition needs to be accurately quantified and the optimum material model and constants need to be identified. The model which can better anticipate the flow curves during the deformation of an as-cast structure is determined. Specifically, Hansel-Spittel and Arrhenius constitutive models were developed using hot compression tests and then incorporated in the FEM code Forge NxT 1.0®. The simulation results thus generated were further utilized to analyze the adiabatic heating and force vs. time analysis. To determine and quantify the accuracy of the developed models, a comparison of the Arrhenius and Hansel-Spittel model is done. The accuracy and the reliability of both models were compared in terms of correlation coefficient (R) and the average absolute relative error (ARRE). The values of R and ARRE for Arrhenius model are 0.978 and 1.76%, whereas, for the Hansel Spittel model these values are 0.972 and 3.17 % respectively. Stress-strain predictions by both equations reveal that Hansel-Spittel model is unable to predict the dynamic softening of the material and also proved insignificant in predicting the curves of other range of deformation parameters. Finite Element Modeling (FEM) was used by integrating the two models in Forge NxT 1.0® software for the design and optimization of the ingot breakdown processes. The simulation results showed good agreement with the theoretical and simulated values of adiabatic heating using both models, whereas only Arrhenius model was able to accurately predict the force vs. time values. The overall results indicate that the Arrhenius model is more accurate and efficient in predicting the deformation behavior of the cast structure than the Hansel-Spittel model. The second part of this study analyzes the occurrence of dynamic transformation during ingot break down process. This investigation focuses on fundamental mechanisms responsible for the evolution of the cast microstructure towards a wrought one. As-cast homogenized medium carbon steel with a grain size of ~ 200 μm was used for the investigation. Hot deformation was carried out using Gleeble 3800 thermomechanical simulator at a temperature 1150 °C and 1200 °C under strain rates of 0.25, 1 and 2 s-1. The selected hot deformation temperatures were 450-500 °C above Ae3. The double differentiation technique was employed to analyze stress-strain curves and a combination of optical, SEM, and Electron Back Scattered Diffraction (EBSD) technique were used for microstructure characterization. Kernel average misorientation (KAM) was used to measure internal misorientation to separate out the fraction of dynamically transformed (DT) ferrite grains. Results indicated that with the increase in the deformation temperature, ferrite fraction increased, whereas it decreased with increase in strain rate. The investigation on the effect of strain rate on the occurrence of DT ferrite and its growth was determined using pipe diffusion coefficient. It was found that the strain rate has significant role in the microstructure evolution in DT. At higher strain rate, Widmanstätten ferrite plates were observed and at lower strain rates, quasi polygonal ferrite was observed. The change in morphology was related to variation of carbon diffusivity and diffusion distance w.r.t strain rate. A mechanism for this transformation is proposed and shown schematically. The third part of this study focusses on the effect of the addition of Chromium (Cr) on the dynamic transformation (DT) of austenite to ferrite at temperature above Ae3 up to 430 °C in as-cast medium carbon low alloy steel. Calculating the driving force and the barrier energy to the transformation, the results pointed to a fact that although Cr increases the driving force of the transformation, the barrier energy is also increased with its addition. To further understand the fundamental behind this at the atomistic level, diffusion analysis was performed by considering stress due to deformation. It was found that, with the increase in stress, the diffusivity of Cr, increased. Upon comparing the diffusion results with other alloys like carbon and silicon, it was found that the diffusivity and the diffusion distance of Cr was significantly lower than that of carbon and silicon. This finding gave an insight that with the addition of elements like Cr, the austenite matrix becomes stronger in terms of work done in shear and dilatation energy which is related to the sluggish diffusion of Cr and thereby, results as a barrier to the transformation. The forth part of this study presents the results of the investigation on the effect of double hit deformation on Metadynamic Recrystallization (MDRX) kinetics. Ingot breakdown process often consists of several successive deformation steps with high interpass times, during which MDRX occurs. Two-stage isothermal compression tests were carried out at 1150 °C and 1200 °C with strain rates of 0.25-2 s-1 and interpass times of 5-25 s. Based on the experimental results, a material model for MDRX is proposed. The constitutive model was implemented in Forge NxT 1.1® software to simulate the multistage compression. The predicted results from the material model were found to be consistent with the numerical analysis and experimental results, which indicated that the proposed kinetic equations can give precise softening behavior for hot deformed as-cast medium carbon low alloy steel

On the short-time thermal phase-stability of as-cast AlCoCrFeNi2.1 eutectic high entropy alloy

The authors would like to gratefully acknowledge the kind support of Clodualdo Aranas, who the NSERC Discovery Grant supported by the Natural Sciences and Engineering Research Council of Canada (RGPIN 04006). Also, JPO acknowledges Fundação para a Ciência e a Tecnologia (FCT – MCTES) for its financial support via the project UID/00667/2020 (UNIDEMI). JPO also acknowledges the funding of CENIMAT/i3N by national funds through the FCT-Fundação para a Ciência e a Tecnologia, I.P., within the scope of Multiannual Financing of R&D Units, reference UIDB/50025/2020–2023. JS acknowledges the China Scholarship Council for funding the Ph.D. grant (CSC No. 201808320394).The present work deals with the short-time thermal phase-stability of the as-cast eutectic AlCoCrFeNi2.1 high entropy alloy. Toward this end, the effect of different temperatures (800-1000 °C) and soaking times (15-60 min) on the stability of primary dendritic regions and formation of the ordered phases was explored. Microstructural characterization was supported by thermodynamic calculations and assessment of the subsequent mechanical properties. Upon the increase in annealing temperature and soaking time, the primary FCC dendritic areas grown and destabilized owing to elemental partitioning. This was followed by dendrite fragmentation and formation of needle shape B2 ordered phases within the primary FCC regions. Despite the unstable nature of the primary constituent phases, the material hardness increased considerably to a peak point corresponding to the moderate soaking time of 45 min. The variation of the subsequent mechanical properties was discussed relying on the characteristics of the ordered and primary phases.publishersversionpublishe

Modeling Metadynamic Recrystallization of a Die Steel during Ingot Breakdown Process

Ingot forging processes often consist of several successive deformation steps with high interpass times, during which metadynamic recrystallization (MDRX) occurs. Two-stage isothermal compression tests were carried out at 1150°C and 1200°C with strain rates of 0.25-2s−1 and interpass times of 5-25s. Based on the experimental results, a material model for MDRX is proposed. The constitutive model was implemented in Forge NxT 1.1® software to simulate the multistage compression. Results from the material model are consistent with the numerical analysis and experimental results

Modeling Metadynamic Recrystallization of a Die Steel during Ingot Breakdown Process

Ingot forging processes often consist of several successive deformation steps with high interpass times, during which metadynamic recrystallization (MDRX) occurs. Two-stage isothermal compression tests were carried out at 1150°C and 1200°C with strain rates of 0.25-2s−1 and interpass times of 5-25s. Based on the experimental results, a material model for MDRX is proposed. The constitutive model was implemented in Forge NxT 1.1® software to simulate the multistage compression. Results from the material model are consistent with the numerical analysis and experimental results

Influence of ECAP processing temperature and number of passes on hardness and microstructure of Al-6063

Equal-channel angular pressing (ECAP) is one the most efficient techniques of severe plastic deformation for grain refinement and improving mechanical properties. In this study, aluminium alloy 6063 is used due to its wide range of applications. The ECAP process depends on die geometry, number of passes, processing temperature, following routes, plunger speed, strain and frictions. In this study, cylindrical billets of Al-6063 are processed at two different temperatures, at room temperature and at elevated temperature (250°C) through route BC. In the present research, specimens deformed after first pass, third pass and the sixth pass are considered for analysing the microstructure evolution and hardness values. Optical microscopy and electron back scattered diffraction (EBSD) techniques are used for microstructural study. Hardness test is carried out for hardness measurement with test load 100 g. The hardness is increased up to 85 HV after six passes at room temperature. Hardness increases up to 83% only after one pass at room temperature. EBSD result shows the low-angle grain boundaries are 91.06% where high-angle grain boundaries are only 8.9% of sample with one pass at 250°C. The elevated processing temperature influences both hardness and microstructure. © 2021 Informa UK Limited, trading as Taylor & Francis Group