Department of Metallurgical & Materials Engineering

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IIT Jodhpur

The derivation of efficiency of powder dissipation for PM compacts is illustrated. An accurate evaluation of the efficiency parameter for any flow stress data is described. The workability parameters are determined for PM Rene 95 alloy by considering the flow stress data. For the power law stress distribution, the efficiency of power dissipation depends only on the strain rate sensitivity parameter. The dynamic recrystallization occurs completely in the domain of maximum workability parameters.

World University root node

Powder Metallurgy

Application of dynamic material model for hot isostatic pressing of powder compacts

The influence of tempering-induced microstructural changes on the micromagnetic parameters such as magnetic Barkhausen emission (MBE), coercive force (Hc), residual induction (Br), and maximum induction (Bmax) has been studied in 0.2 pct carbon steel, 2.25Cr-1Mo steel, and 9Cr-1Mo steel. It is observed that, after short tempering, the micromagnetic parameters show more or less linear correlation with hardness, which is attributed to the reduction in dislocation density, but long-term tempering produces nonlinear behavior. The variation in each of these parameters with tempering time has been explained based on the changes in the size and distribution of ferrite laths/grains and precipitates. It has been shown that the individual variation in the microstructural features such as size and distribution of laths/grains and precipitates during tempering can be clearly identified by the MBE parameters, which is not possible from the hysteresis loop parameters (Hc and Br). It is also shown that the MBE parameters can not only be used to identify different stages of tempering but also to quantify the average size of laths/grains and second-phase precipitates.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Insight into the microstructural characterization of ferritic steels using micromagnetic parameters

Tensile specimens of superplastic forming grade IN718 superalloy, containing banded microstructure in the as received state, were deformed at high temperatures T to investigate the stress σ versus strain rate ε behaviour, the nature of the stress versus strain ε curves, ductility, and microstructure upon failure. The log σ-log ε plot for the range ∼5 × 10-6-3 × 10-2 s-1 at T=1173-1248 K exhibited a strain rate sensitivity index m = 0·62 at low strain rates and m = 0·26 at high strain rates, representing region II and III behaviour, respectively. The activation energies were estimated to be 308 and 353 kJ mol-1, respectively. All the σ-ε curves, obtained at ε = 1 × 10-4s-1 for the temperature range 1173-1273 K, and at T= 1198 K for the strain rate range 1 × 10-4-1 × 10-2s-1, exhibited initial flow hardening, followed by flow softening. The microstructures revealed dynamic recrystallisation, grain growth, cavitation, and a variation in the amount of second phase particles. Grain growth and cavitation were found to increase with temperature in region II. Excessive grain growth at 1273 K led to the elimination of region II. Grain growth and cavitation were both found to be less pronounced as the strain rate increased in region III. © 2000 IoM Communications Ltd.

Materials Science and Technology

Superplastic behaviour of as received superplastic forming grade IN718 superalloy

A simple condition for metallurgical instability useful in the development of processing maps for analyzing the high temperature forming of metals, is used. This condition is deduced following a criterion based on continuum principles as applied to large plastic flow proposed by Ziegler, which can be applied for any type of flow stress versus strain rate curve. This criterion has been validated using the flow stress data of a α2/O titanium aluminide alloy Ti-25Al-15Nb with microstructural observations. The optimum hot working conditions are suggested for this material based on the processing map.

Zeitschrift fuer Metallkunde/Materials Research and Advanced Techniques

Development of a processing map for the hot working of Ti-25Al-15Nb

Tensile specimens of the Al-Cu alloys containing 5.6 to 33.4 wt.% Cu were deformed to failure for investigating the effects of strain rate, test temperature, initial grain size and the volume fraction of θ phase on ductility, grain growth and cavitation level. The ductility increases with the increase in temperature and decrease in grain size. The other variables, however, influence the ductility more in an interdependent manner. There occur grain growth and cavitation to varying extent depending on test condition; but the latter appears to show a systematic effect in limiting the ductility.

Variation in tensile ductility and terminal microstructure of the binary two-phase Al-Cu alloys at elevated temperatures

Tensile specimens of a Zr-2.5 wt% Nb pressure tube alloy were deformed at strain rates (ε) of 3.2×10-4-2.9×10-2 s-1 and test temperatures (T) of 625-800 °C to investigate the nature of stress-strain curves and the optimum condition for superplasticity. The maximum ductility of 1384% was obtained at T = 700°C and intermediate strain rate of 3.4×10-3 s-1. The stress-strain curves were found to exhibit the dominance of flow hardening, especially in the early part of deformation, followed by flow softening. The variation in ductility and flow stress as a function of test condition is explained by the observed microstructural evolution and fracture behavior.

Journal of Nuclear Materials

Effect of strain rate and test temperature on superplasticity of a Zr-2.5 wt% Nb alloy

Zr-2.5 wt% Nb alloy sheet, obtained by unfolding and straightening a pressure tube, was further cold rolled up to 39% reduction in thickness to investigate the effect of cold working on the stress (σ)-strain rate behaviour over a strain rate range of approx. 2 × 10-5 to 5 × 10-3 s-1 and a temperature range of 625 to 700°C.

Journal of Materials Science

Effect of prior cold rolling and test temperature on stress-strain rate behaviour of a Zr-2.5Nb alloy

Tensile tests were performed on specimens in quenched and tempered (Q + T) and thermally aged (TA) conditions over a wide temperature range (300-873 K) to assess the influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel. Prior thermal ageing at 793 and 873 K for durations ranging from 10 to 5000 h did not cause a significant change in room temperature tensile properties. However, a marginal decrease in yield strength and reduction in area were observed for specimens aged for longer durations at 793 and 873 K. Prior thermal ageing at 793 K for 5000 h and at 873 K for 1000 and 5000 h produced significant reduction in strength values at intermediate temperatures (523-723 K) compared to that observed at high temperatures. At intermediate temperatures, the alloy in all heat treatment conditions exhibited serrated flow, a manifestation of dynamic strain ageing (DSA). The significant loss of strength in thermally aged conditions at intermediate temperatures has been attributed to reduced propensity to DSA. The elongation to fracture values at temperatures in the range 300-873 K were affected little by prior thermal ageing, whereas the reduction in area exhibited a decrease in the value with increasing thermal ageing. The fracture mode remained transgranular ductile at all test conditions investigated in the present study. However, specimens aged for longer durations exhibited chisel fracture at room and intermediate temperatures due to split in the martensite lath boundaries. The influence of thermal ageing on room temperature tensile properties of the forging remained similar to that reported for thin section 9Cr-1Mo steel.

Influence of prior thermal ageing on tensile deformation and fracture behaviour of forged thick section 9Cr-1Mo ferritic steel

The influence of prior tensile deformation on the magnetic Barkhausen emission (MBE) and the hysteresis (B-H) curve has been studied in 0.2% carbon steel and 2.25Cr-1Mo steel under different tempered conditions. This study shows that the micromagnetic parameters can be used to identify the four stages of deformation, namely (i) perfectly elastic, (ii) microplastic yielding, (iii) macroyielding and (iv) progressive plastic deformation. However, it is observed that the MBE profile shows more distinct changes at different stages of tensile deformation than the hysteresis curve. It has been established that the beginning of microplastic yielding and macroyielding can be identified from the MBE profile which is not possible from the stress-strain plot. The onset of microplastic yielding can be identified from the decrease in the MBE peak height. The macroyielding can be identified from the merging of the initially present two-peak MBE profile into a single central peak with relatively higher peak height and narrow profile width. The difference between the variation of MBE and hysteresis curve parameters with strain beyond macroyielding indicates the difference in the deformation state of the surface and bulk of the sample.

Acta Materialia

Effect of tensile deformation on micromagnetic parameters in 0.2% carbon steel and 2.25Cr-1Mo steel

Tensile tests were performed on specimens in quenched and tempered and thermally aged conditions over a wide temperature range (300-873 K) to assess the occurrence of serrated flow, a manifestation of dynamic strain aging (DSA), in 9Cr-1Mo ferritic steel, with an emphasis on the influence of prior thermal aging on serrated yielding. The alloy exhibited jerky/serrated flow in the load-elongation curves at intermediate temperatures. Types A, B, and C serrations were observed, depending on the test temperature and applied strain rate. The apparent activation energy of 83 kJ mol-1 measured for serrated flow suggests that diffusion of an interstitial solute such as carbon is responsible for dynamic strain aging in 9Cr-1Mo steel. Prior thermal aging at 793 K for 5000 h and at 873 K for 1000 and 5000 h resulted in a significant decrease in the height of serrations, i.e. the magnitude of the stress drop, as well as an increase in the critical strain for the onset of serrations. Both of these observations indicate reduced propensity to DSA as a result of increased precipitate sinks as well as a reduced carbon concentration in solid solution owing to an increased density of carbides in the thermally aged conditions. Reduced propensity to DSA resulted in a significant reduction in the strength values at intermediate temperatures. © 1999 IoM Communications Ltd.