Low temperature stress relaxation of nanocrystalline nickel

Stress relaxation in nanocrystalline nickel within the temperature range 523–673 K in a uniaxial compression regime is studied in the present investigation. The results obtained for coarser grained nickel are given for comparison. An average strain rate of nanocrystalline nickel within the investigated range of temperatures is 1.75 × 10^–5–3.03 × 10^–5s^–1. The presence of two types of stress relaxation dependencies are shown. The most likely strain mechanism is grain boundary sliding controlled by grain boundary diffusion for temperatures between 623 and 673 K.     

Flow behaviour and microstructural evolution in cold worked 70∶30 α-brass

Abstract

Deformation behaviour and microstructures at failure were investigated in a mill cold worked 70∶30 α-brass over the test temperature range of 298–973 K and strain rate range of 10^?5–5×10^?3 s^?1. Tensile properties as a function of temperature revealed three distinct regions, with their temperature sensitivity being maximum at intermediate temperatures (553–673 K) and much less towards the lower and higher temperature ranges. Two values of activation energy for high temperature deformation Q were obtained to be 117·5 kJ mol^?1 below 623 K and 196·4 kJ mol^?1 above this critical temperature. In the respective temperature range the values of stress exponent n were 5·6 and 3·8. Based on the values of Q and n, the deformation mechanism was suggested to be dislocation climb creep with a probable contribution from dislocation pipe diffusion on lowering the temperature. Both grain size and cavity size were found to increase with increasing test temperature, suggesting them to be interrelated and act as an alternative steps for accommodating grain boundary sliding. Static grain growth study, over the temperature range of 773 to 1073 K, led to activation energy for grain growth to be 71 kJ mol^?1, with the time exponent of 0·37.     

Developing superplasticity in a magnesium AZ31 alloy by ECAP

The processing of a magnesium AZ31 alloy by equal-channel angular pressing refines the grain size to ~2.2 μm, but annealing for 30 min at 673 K coarsens the grains to ~6.0 μm. Despite this microstructural instability, the alloy is superplastic when pulled in tension at temperatures in the range of 623–723 K with elongations up to >1000% at strain rates at and below 10^?4 s^?1. Experiments within the superplastic regime show the strain rate sensitivity is ~0.5 and the activation energy is close to the value for grain boundary diffusion. It is demonstrated by calculation that the experimental results are in good agreement with a model for superplasticity based on grain boundary sliding.     

Microstructural evolution and plastic stability during superplastic flow in a 7475 aluminium alloy

Abstract

Superplastic behaviour and microstructural evolution were examined at 788 K for strain rates in the range 2 × 10^-4–2 × 10^-3 s^-1 in a 7475 aluminium alloy of nominal composition Al–(1·2–1·9)Cu–(5·2–6·2)Zn–(1· 9–2·6)Mg (wt-%). In addition, the variation of the strain hardening and plastic stability parameters with strain was investigated based on experimental grain growth and cavitation data. The strain hardening parameter at 2 × 10^-4 s^-1 was high over a wide range of strain because of the high grain growth rate. Decrease in the strain hardening parameter due to cavitation was negligible. The highest plastic stability parameter was attained at 2 × 10^-4 s^-1, although the strain rate sensitivity was the lowest for the strain rate range investigated. This demonstrates the influence of grain growth on high plastic stability during superplastic deformation.     

High strain rate superplasticity of TiNP/2014Al composite

Superplasticity of the TiN_p/2014AI composite prepared by powder metallurgy method was investigated by tensile tests conducted at different temperatures (773, 798, 818 and 838 K) with different strain rates range from 1·7×10° to 1·7×10^?3s^?1. Results show that a maximum elongation of 351% is achieved at 818 K and 3·3·10^?1s^?1. At different deformation temperatures, the curves of m value can be divided into two stages with the variation of strain rate and the critical strain rate is 10^?1 s^?1. Superplastic deformation activation energy in the TiN_p/2014AI composite is 417 kJ mol^?1, which is related to liquid phase formation at triple points of grain boundaries and interfaces between the matrix and the reinforcement. Superplastic deformation mechanism of the TiN_p/2014AI composite is grain boundary sliding accommodate mechanism when the strain rate is lower than 10^?1 s^?1, and transfers to grain boundary sliding accommodation mechanism plus liquid phase helper accommodation mechanism when the strain rate is higher than 10^?1 s^?1     

Modified Johnson-Cook description of wide temperature and strain rate measurements made on a nickel-base superalloy

In this study, uniaxial compression experiments of a Nickel-base superalloy is conducted over a wide range of temperatures (298–1073 K) and strain rates (0.1–5200/s) to obtain further understandings of the plastic flow behaviours. The temperature and strain rate effects on the plastic flow behaviour are analysed. The flow stress decreases with increasing temperature below 673 K. Within the temperature range of about 673–873 K, the flow stress varies indistinctively, and even increases slightly with increasing temperature. As the temperature further increases, the flow stress decreases again. The flow stress of the Nickel-base superalloy displays insensitive to strain rate below 800/s and an enormous increase with increasing strain rate in excess of 800/s. Then the effects of temperature and strain rate on the microstructure are discussed. The result shows that high strain rate and high temperature may make the grain boundary of Nickel-base superalloy frail. Taking into account the anomalous temperature and strain rate dependences of flow stress, modified J–C constitutive model is developed. The model is shown to be able to accurately predict the plastic flow behaviour of Nickel-base superalloy over a wide range of temperatures and strain rates.     

Effect of grain size and indium addition on tensile characteristics of Al–1Si alloy

Abstract

The effect of grain size and indium addition on the workhardening characteristics of Al–1Si (wt-%) alloy has been investigated at room temperature (RT). The samples were preaged at different temperatures in the range 523–623 K. The yield stress, the fracture stress, the fracture time and the linear workhardening coefficient generally decreased with increasing temperature and/or grain size, while the fracture strain and dislocation slip distance increased. The yield and fracture stresses for different grain sizes at different temperatures were found to be linearly related to grain diameters. Indium addition caused general increase for all the measured strength parameters. As concluded from transmission electron microscope (TEM) investigations, In addition to Al–Si alloy may retard the coarsening of Si particles. The energies activating the operating fracture mechanisms were found to be 79·6±0·4 and 32·4±0·4 kJ mol^?1 for alloys Al–1Si and Al–1Si–0·2In respectively. This suggests a value of 47·2 kJ mol^?1 as a binding energy between Si and In atoms in Al matrix.     

Influence of deformation induced ferrite,grain boundary sliding,and dynamic recrystallisation on hot ductility of 0·1–0·75%C steels

Abstract

The influence of C on hot ductility in the temperature range 600–1000°C has been examined for three C contents (0·1, 0·4, and 0·75 wt-%). Using a strain rate of 3 × 10^?3 s^?1, tensile specimens were heated to 1330°C before cooling to the test temperature. For the 0·4%C steel, two further strain rates of 3 × 10^?2 and 3 × 10^?4 s^?1 were examined. At the strain rate of 3 × 10^?3 s^?1, increasing the C content shifted the low ductility trough to lower temperatures in accordance with the trough being controlled by the γ–α transformation. Thin films of the softer deformation induced ferrite formed around the γ grain boundaries and allowed strain concentration to occur. Recovery to higher ductility at high temperatures occurred when these films could no longer form (i.e. above Ae₃) and dynamic recrystallisation was possible. The thin films of deformation induced ferrite suppressed dynamic recrystallisation in these coarse grained steels when tested at low strain rates. Recovery of ductility at the low temperature side of the trough in the 0·1%C steel corresponded to the presence of a large volume fraction of ferrite, this being the more ductile phase. For the 0·4%C steel decreasing the strain rate to 3 × 10^?4 s^?1 resulted in a very wide trough – extended to both higher and lower temperatures compared with the other strain rates. The high temperature extension was due to grain boundary sliding in the γ. Recovery of the ductility only occurred when dynamic recrystallisation was possible and this occurred at high temperatures. At the low temperature end, thin films of deformation induced ferrite were present and recovery did not occur until the temperature was sufficiently low to prevent strain concentration from occurring at the boundaries. Of the two intergranular modes of failure grain boundary sliding produced superior ductility. At the higher strain rates there was less grain boundary sliding, which led to a lower temperature for dynamic recrystallisation. Higher strain rates also increased the rate of work hardening of deformation induced ferrite, reducing the strain concentration at the boundaries. Ductility started to recover immediately below Ae₃, resulting in very narrow troughs. Finally, it was shown that the 2% strain that occurs during the straightening operation in continuous casting is sufficient to form deformation induced ferrite in steel containing 0·1%C.

MST/1809     

Superplasticity in NiAl intermetallics macroalloyed with iron

In this paper, we aim to examine the superplastic behavior of an extruded Ni–28.5Al–20.4Fe (at.%) alloy, which consists of β+γ phases with an average linear intercept grain size of 30–50 μm. Its tensile properties were determined at temperatures from 1123 to 1323 K and initial strain rates from 1.04×10⁻² to 1.04×10⁻⁴ s⁻¹. A maximum elongation of 233% was obtained at 1123 K and a strain rate of 5.2×10⁻⁴ s⁻¹. Transmission electron microscope (TEM) observation found many dislocation-free grains adjacent to grains with a high-dislocation density and subgrains and subgrain boundaries, which indicate that dynamic recrystallization has occurred as an efficient accommodation mechanism. Scanning electron microscope (SEM) examination of the fracture sample after superplastic deformation reveals many voids on the fracture surface. By correlating with the results of TEM observation, it is suggested that the superplastic deformation in this alloy should be controlled by a grain boundary sliding-based mechanism accommodated by the movement of dislocation and dynamic recrystallization.     

High-temperature phase transitions in a quaternary lead based perovskite structured materials with negative temperature coefficient of resistance (NTCR) behavior

Impedance spectroscopy measurements were carried out on lead based, 0.25 (PbZr_0.52Ti_0.48O₃) + 0.25 (PbFe_0.50Ta_0.50O₃) + 0.25 (PbFe_0.67W_0.33O₃) + 0.25 (PbFe_0.50Nb_0.50O₃) (PZT–PFT–PFW–PFN) solid solution over a wide range of temperatures (400–650 K) and frequencies (100 Hz–1 MHz). Impedance data showed the presence of both grains and grain boundaries effects in the electrical transport properties of quaternary. The role of the grains and grain boundaries to the impedance become more prominent around the phase transition (~420 K). Two thermally activated processes were found from the temperature dependences of the relaxation time (τ). Activation energies calculated from relaxation times obtained from imaginary part of impedance were estimated ~1.21 and ~1.84 eV over 400–490 K and 490–650 K respectively. The sum of the activation energies for the grain and grain boundary resistances is basically of the same order of magnitude that is from the impedance at high temperatures. A constant phase element is used in the equivalent electrical circuits for fitting of experimental impedance data. The nature of variation of the grain and grain boundary resistance with temperature suggested negative temperature coefficient of resistance behavior.     

Thermal strain in lead thin films II: strain relaxation mechanisms

A deformation mechanism map was constructed to study the mechanisms of strain relaxation in lead thin films which were deposited on oxidized silicon wafers at room temperature and which were then thermally cycled between room temperature and liquid helium temperature. The stress level, which was calculated from the strain measured by an X-ray diffraction technique, was plotted on the map. By comparing the calculated and experimental stress levels the following observations were obtained. In the cooling process the strain was relaxed rapidly in a field of dislocation glide mechanism for films of greater than 0.2 μm thickness. In the heating process most of the strain was again believed to be relaxed by the glide mechanism. For a film 0.5 μm thick the stress (after the primary relaxation was completed) was found to be (1–1.5) × 10⁹ dyn cm^-2 for the cooling process and (0.17–0.24) × 10⁹ dyn cm^-2 for the heating process at temperatures around 200–280 K. Slow secondary relaxations were observed after the primary relaxations were completed. The measured compressive strain relaxation rate at room temperature was very close to the rate calculated on the assumption of grain boundary diffusion creep. This suggested that the secondary relaxation mechanism of compressive strain was grain boundary diffusion creep at temperatures near room temperature. These suggestions were supported by scanning electron microscopy observations in which dislocation slip lines were observed inside grains and hillocks were observed on grain boundaries.     

Creep of Pb–2·5Sb–0·2Sn alloy at low stresses

Abstract

The creep of a Pb–2·5Sb–0·2Sn alloy has been studied at stresses up to 6·5 MN m^?2 in the temperature range 318–348 K (0·53–0·58T_m) using helical specimens. At 333 K, a transition in the stress exponent from ~1 to 3 occurred at ~3 MN m^?2. The observed good agreements below the transition stress, both for experimental dE/do and predictions for Coble diffusional creep of lead, and for measured activation energy for creep and the activation energy for grain boundary self-diffusion in lead, suggest that grain boundary diffusional creep is the dominant mechanism. at low stresses. The presence of antimony does not seem to affect the magnitude of dE/do appreciably, and the results suggest that the grain boundary self-diffusivity of lead is not influenced by the presence of segregated antimony on the grain boundaries. The diffusional creep occurred above a threshold stress of magnitude ~0·5 MN m^?2, and its temperature dependence was characterised by an activation energy of ~20 kJ mol^?1, similar to the value of 23 ± 7 kJ mol^?1 typical of pure metals in the temperature range investigated. The stress exponent of ~3 observed for the power law regime suggests control by viscous glide of dislocations constrained by dragging of solute atmospheres. Preliminary tests on sagging beam specimens of as-worked material at an applied stress of 2·5 MN m^?2 and a test temperature of 333 K has provided the first direct evidence that anisotropic grain shape affects Coble creep. The specimen with the longest grain dimension along the stress axis underwent slower creep than the specimen with the longest grain dimension perpendicular to the stress axis. This observation is in qualitative agreement with theoretical predictions.

MST/1139     

Influence of oxygen on the processing maps for hot working of electrolytic tough pitch copper

Processing maps for the hot deformation of electrolytic tough pitch (ETP) copper (100 ppm oxygen) have been developed in the temperature range 600–950 °C and strain rate range 0.001–100 s^− 1, and compared with those published earlier on ETP copper with higher oxygen contents (180, 220 and 260 ppm). These reveal that dynamic recrystallization (DRX) occurs over a wide temperature and strain rate range and is controlled by different diffusion mechanisms. In ETP copper with 100 and 180 ppm oxygen, the apparent activation energy in the DRX domain occurring in the strain rate range 0.001–10 s^− 1 and temperature range 600–900 °C is about 198 kJ/mol which suggests lattice self-diffusion to be the rate-controlling mechanism. This DRX domain has moved to higher temperatures and lower strain rates in ETP copper with higher oxygen content. In the second domain occurring at strain rates in the range 10–100 s^− 1 and temperatures > 700 °C, the apparent activation energy is 91 kJ/mol and DRX is controlled by grain boundary self-diffusion. This domain is absent in the maps of ETP copper with oxygen content higher than 180 ppm and this is attributed to the pinning of the grain boundaries by the oxide particles preventing their migration.     

Tensile stress–strain and work hardening behaviour of 316LN austenitic stainless steel

Abstract

The true stress (σ)–true plastic strain (?) data of a type 316LN austenitic stainless steel tested at nominal strain rates in the range 3×10^-5–3×10^-3 s^-1 and temperatures of 300–1123 K were analysed in terms of flow relationships proposed by Hollomon, Ludwik, Swift, Voce, and Ludwigson. The applicability of the particular flow relationship is discussed in terms of ‘complete’ and ‘applicable’ range fits of the experimental σ–? data. At all strain rates, in the case of the complete range fit, the Ludwigson equation followed the stress–strain data most closely at 300 K, while in the temperature range 523–773 K, the flow behaviour was described equally well by both the Ludwigson and Voce equations. In the temperature range 823–1023 K, the Voce equation described the flow behaviour most accurately in the case of the complete range fit of σ–? data at all strain rates. The analysis of σ–? data employing the Ludwigson equation in the applicable range fit covering low and intermediate strains, and the Hollomon equation at high strains provided close simulation of the observed flow behaviour in the temperature range 823–1023 K. At high temperatures of 1073 and 1123 K, the Ludwigson equation reduces to the Hollomon equation. The variations in different flow parameters of the Ludwigson and Voce equations with temperature and strain rate exhibited anomalous behaviour at intermediate temperatures because of dynamic strain aging.     

Superplasticity and production of mechanically milled Ti–6Al–4V–0.5B

Abstract

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required. In order to minimise cycle times material with a fine grain size is required to allow either, an increase in the forming rate or a reduction in the deformation temperature. This study details the manufacture of Ti–6Al–4V–0.5B powder with a nanocrystalline grain size, which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which ～2.5 vol.-%TiB was formed by an in situ reaction between the titanium and boron. The TiB particles limited the growth of the grain size in the titanium from the nanocrystalline structure in the powder to sizes in the range 600 nm–4 µm after consolidation. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 310%was achieved when testing at 900°C at a strain rate of 6×10^-2 s^-1 compared with 220% for conventional Ti–6Al–4V sheet. However, extensive cavitation, induced by the presence of argon, occurred during high temperature deformation and limited the superplastic extensions achieved.     

Superplastic behaviour of as received superplastic forming grade IN718 superalloy

Abstract

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 ? · behaviour, the nature of the stress versus strain ? curves, ductility, and microstructure upon failure. The log σ–log ? · 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–? curves, obtained at ? · = 1 × 10^-4 s^-1 for the temperature range 1173–1273 K, and at T = 1198 K for the strain rate range 1 × 10^-4–1 × 10^-2 s^-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.     

Localized strain and heat generation during plastic deformation in nanocrystalline Ni and Ni–Fe

Room temperature tensile testing was performed on a coarse-grained polycrystalline Ni (32 μm), a nanocrystalline Ni (23 nm) and two nanocrystalline Ni–Fe (16 nm) electrodeposits at two strain rates of 10^?1 and 10^?2/s. Strain localizations and local temperature increases were simultaneously recorded during tensile testing. For all materials, higher loads or higher strain rate generally resulted in higher peak temperature with the highest temperatures recorded in the fracture regions. The maximum temperature for the nanocrystalline materials was just over 80 °C, which is significantly below the reported temperatures for the onset of thermally activated grain growth. Therefore, the previously reported grain growth observed on similar materials after tensile deformation is likely not thermally activated but a stress-induced phenomenon. Despite the wide grain range from 16 nm to 32 μm, all samples exhibited similar strain localization behavior. Local strain variations initiated in the early stage of macroscopic uniform deformation, subsequent necking and fracture took place in the region of initial strain localization. While the coarse-grained polycrystalline Ni exhibited little strain rate sensitivity, gradually increased strain rate sensitivity was observed for the 23 nm Ni and the two 16 nm Ni–Fe samples, suggesting that both dislocation-mediated and grain-boundary-controlled mechanisms were operative in the deformation of the nanocrystalline Ni and Ni–Fe samples.     

Study on dynamic strain aging phenomenon of 3004 aluminum alloy

3004 Aluminum alloy has been subjected to tension test at a range of strain rates (5.56 × 10⁻⁵ to 5.56 × 10⁻³ s⁻¹) and temperatures (233–573 K) to investigate the effect of temperature and strain rate on its mechanical properties. The serrated flow phenomenon is associated with dynamic strain aging (DSA) and yield a negative strain rate dependence of the flow stress. In the serrated yielding temperature region a critical transition temperature, T_t, was found. The critical plastic strain for the onset of serrations has a negative or positive temperature coefficient within the temperature region lower or higher than T_t. According to the activation energy, it is believed that the process at the temperature region lower than T_t is controlled by the interaction between Mg solute atom atmosphere and the moving dislocation. In the positive coefficient region, however, the aggregation of Mg atoms and precipitation of second phase decrease the effective amount of Mg atoms in solid solution and lead to the appearance of a positive temperature coefficient of the critical plastic strain for the onset of serrations.     

Influence of carbon on hot ductility of steels

Abstract

Hot ductility, measured by reduction in area, has been determined over the temperature range 550–950°C for a series of plain C–Mn steels having the same base composition except for the carbon content, which was in the range 0·04–0·65 wt-%. A ductility trough was obtained for all the steels and minimum ductility values were similar. Raising the carbon content from 0·04 to 0·28 wt–% caused the ductility trough to move to lower temperatures and this was in agreement with the observed changes in transformation temperature. Tensile fracture at the minimum ductility temperature was along thin films of ferrite which formed round the austenite grains – generally by deformation–induced transformation. The softer ferrite allowed strain concentration to cause ductile voiding at the MnS inclusions, and the voids eventually linked up to give intergranular failure. Raising the carbon content above the 0·28% level caused a change in the fracture mode. Instead of the ductility troughs moving to lower temperatures, a shift of over 100 K to higher temperatures was observed. Intergranular failure now occurred in the austenite as a result of grain boundary sliding. It is suggested that this change in fracture mode is caused by carbon increasing the activation energy, and hence the critical strain required for dynamic recrystallization, so favouring the linking of cracks formed by grain boundary sliding.

MST/366     

Creep deformation behaviour of AZ31 magnesium alloy over wide range of temperature and stress

Abstract

The creep deformation behaviour of coarse grained AZ31 magnesium alloy was examined in the temperature range from 423 to 673 K (0·46–0·73T_m) under various constant stresses covering low strain rate range from 4×10^?9 to 2×10^?2 s^?1. Most shape of the creep curve was typical of class II behaviour. However, only at low stress and low temperature, the shape of the creep curve was typical of class I behaviour. At very low stress at 673 K, the stress exponent for the secondary creep rate was ～2. At low stress level, the stress exponent was ～3 and the present results were in good agreement with the prediction of Takeuchi and Argon model. At high stress level, the stress exponent was ～5 and the present results were in good agreement with the prediction of Weertman model. The transition of deformation mechanism from solute drag creep to dislocation climb creep could be explained in terms of solute atmosphere breakaway concept.