Constant stress creep and constant true strain-rate tensile tests of the superplastic alloy PbSn

Strain-time curves and stress-strain curves have been obtained for the superplastically deformed PbSn eutectic tested in creep under constant stress and in tension under true strain-rate conditions at temperatures ranging between –44 and 30° C. It is shown that the flow stress does not depend on strain and time and is only a function of the true strain rate, of the temperature and of the initial grain size. The result is that this superplastic alloy does not strain-harden and that the grain size is constant. The apparent activation energy does not depend on stress and temperature and is equal to 11.5±0.5 kcal/mole.     

High temperature compressive behaviour of as-cast Al-Cu alloys of varying composition

Al-Cu alloys with different copper contents ranging from 10 to 45 wt% were deformed in compression in the as-cast condition. Measurements of the strain-rate sensitivity parameterm using the crosshead speed jump technique show that alloys of composition close to the eutectic become superplastic after about 25% strain, withm increasing from about 0.2 at the beginning of deformation to about 0.5 or more, depending on composition. This transition to the superplastic state is accompanied by an important decrease of the flow stress during compression and it is associated by the breakdown of the initially lamellar structure of the eutectic phase in the highly deformed regions of the specimen. Moreover for the alloy with 37 wt% copper, the transition corresponds also to the degeneration of the dendritic primary phase and this alloy shows a particularly high value ofm and a low steady state flow stress in spite of the large volume fraction of the hard CuAl₂ compound. Alloys of composition far away from the eutectic do not exhibit superplastic behaviour during compression, at least in the range of strain rate investigated and this is due to the large grain size and the important grain growth that occurs during deformation.     

The variation in flow stress and microstructure during superplastic deformation of the Al-Cu eutectic

Stress-strain curves have been obtained for the superplastically deformed Al-Cu eutectic tested in tension under constant true strain-rate conditions. It is shown that constant flow stress conditions do not obtain and that, after an initial transient, the flow stress is linearly related to natural tensile strain. Optical metallography has been employed to follow the variation of both inter-phase particle separation and -Al grain size with strain and it is concluded that the observed strain hardening is due mainly to grain coarsening.     

The tensile properties of a superplastic aluminium bronze

The high temperature tensile properties of a micrograin Cu-9.5% Al-4% Fe alloy, which is superplastic at 800° C, have been determined. Elongations at fracture of greater than 700% are achieved when the nominal strain-rate is in the range 3.9×10^–2 min^–1 to 7.9×10^–2 min^–1. The nature of plastic instability in superplastic materials is considered and it is shown that the amount of strain at the onset of plastic instability is inversely related to the applied strain-rate and is relatively independent of the strain-rate sensitivity exponent, m. The onset of plastic instability during a tensile test results in an increase of local strain-rate at the point of minimum cross-section and this, together with the existence of a triaxial stress state in the necked region, may produce errors in the m versus strain-rate plot if m is determined by the change-rate method. The initial strain-rate for maximum elongation is lower than the strain-rate for maximum m. This may be ascribed either to the influence of plastic instability or the formation of cavities at the higher strain-rates.     

High strain rate superplasticity of hot extruded and hot rolled AA 6013/20 vol.-%SiCp composite

Abstract

High strain rate superplasticity(HSRS)of an AA 6013/20SiC_pcomposite, produced by powder metallurgy and then hot extruded or hotrolled, was evaluated by means of tensile tests carried out over a range of initial strain rates from 1 × 102 to 3.8 × 10^-1 s^-1 and temperatures from 520 to 590 ° C. A maximum elongation to failure of 370% was achieved in a hot rolled composite deformed at 1 × 10^-1 s^-1 and 560 ° C. Substantially lower elongations were achieved in hot extruded composites, with a maximumof200% at1 × 10^-2 s ^-1 and 580 ° C. The lower elongations in the hot extruded composite could be related to the large quantity of intermetallic compounds, shown by TEM analyses, which probably hinder large superplastic elongations. In both hot extruded and hot rolled composite, the flow stress was strongly dependenton temperature and strain rate; a steady state flow stress region was observed in the specimen that exhibited the maximum elongation to failure. The strain rate sensitivity index m reached a maximum ofabout 0.4 for the hot rolled composite, and about 0.35 for the hot extruded composite. Analyses of the fracture surfaces of hot rolled composite deformed at the maximum elongation, were characterised by the presence of many filaments or 'whiskers', which are generally considered as evidence of a liquid phase present at grain boundaries or interfaces.     

The impact behaviour of paints

The split Hopkinson pressure bar has been used to study the impact behaviour of a selection of single and multi-layer paint systems in the form of films of thickness 0.04 mm. Stress-strain curves are presented for systems comprising three coatings, coating A, coating B and coating C, in compression for strain rates of the order 5×10³ s^–1. A comparison is made between the high strain-rate behaviour and that seen at quasi-static strain rates. All tests were carried out at 23 °C. The coatings studied are shown to be strain-rate sensitive, exhibiting almost a two-fold increase in flow/yield stress between the two strain-rate regimes. At low strain rates, all the coatings deformed uniformly with no sign of fracture. At high strain rates, both coating A and coating C underwent catastrophic failure which is indicative of their susceptibility to chipping. However, this was not the case with coating B which shows no signs of fracture at high strain rates for strains up to 45%. However, a combination of coating A and coating B in alternate layers led to catastrophic fracture of the resulting two-coat multi-layer system at high strain rates.     

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.     

无取向硅钢形变储能取向依赖性及其对再结晶织构的影响

采用晶体塑性有限元模拟与实验相结合的方式,研究无取向硅钢冷轧过程中不同初始织构组分的取向流动与形变储能累积。结果表明:冷轧后形成了较强的α,γ形变织构和较弱的λ形变织构。再结晶织构由γ,α,η和λ织构组成,其取向密度依赖于冷轧压下率。随冷轧压下率增大,λ再结晶织构逐渐增强,η织构先增强后减弱,γ织构先减弱后增强,α织构稍有弱化。冷轧过程中形变储能累积具有明显的初始取向依赖性,初始γ取向储能累积速率在低于50%压下率时与初始α取向接近,高于50%压下率时则明显大于后者,初始λ取向储能累积速率始终显著低于γ和α取向,转至同一形变取向的不同初始取向间的储能累积也会产生差异。冷轧过程中不同初始织构组分的取向流动与形变储能累积规律,决定了无取向硅钢再结晶织构组分的发展。     

Stress-strain rate relations for high-temperature deformation of two-phase Al-Cu alloys

Al-Cu alloys containing 6, 11, 17, 24 and 33 wt% Cu, annealed for 0.5–100 h, were deformed by the differential strain-rate test technique over a strain-rate range of 4×10^–6 to 3×10^–2s^–1 at temperatures ranging from 460–540°C. Superplastic behaviour, with strain-rate sensitivity, m0.5, and activation energy, Q=171.5 kJ mol^–1, is shown by the Al-24Cu and Al-33Cu alloys at lower strain rates and higher temperatures. All the alloys show m0.20 at higher strain rates, but the average activation energy for deformation of the Al-6Cu, Al-11Cu, and Al-17Cu alloys is evaluated to be 480.7 kJ mol^–1, in contrast to a lower value of 211 kJ mol^–1 for the Al-24Cu and Al-33Cu alloys. Instead of grain size, the mean free path between particles is suggested to be a more appropriate microstructural parameter for the constitutive relationship for deformation of the Al-Cu alloys.     

Constant strain- and stress-rate compressive strength of columnar-grained ice

Unconfined compressive strength of transversely isotropic columnar-grained ice has been investigated for loads applied normal to the longitudinal axis of the columns at the high homologous temperature of 0.96 T _m (T _m is the melting temperature) under truly constant strain and stress rates. A closed-loop, servo-hydraulic test system inside a cold room was used. Both the strain- and stress-rate dependences of upper yield stress can be expressed in terms of power laws. The observed strain-rate dependence of strength was found to be numerically the same as the dependence of viscous-flow rate on stress in constant stress creep tests at the same temperature. It is shown that the strain-rate sensitivity of yield strength compares well with previous results (obtained under constant cross-head rates using a conventional machine) only if the average strain rate to yield is used as the independent variable instead of the conventional nominal strain rate. The paper also discusses the strain and time aspects of the tests. It shows interdependence among values for compressive yield strength, strain rate, failure strain and time very similar to the interdependence among the corresponding values in tensile creep failures in metals, alloys and other polycrystalline materials at high temperatures. It is emphasized that the splitting type of brittle-like premature failure depends on the stiffness of the test system and should not be considered to be a fundamental material property. The concept of failure modulus is proposed for examining the ductile to brittle transition.     

The influence of texture on superplasticity of the titanium alloy VT 6

This paper describes a specific influence of crystailographic texture on tensile properties of the titanium alloy VT6 under superplastic flow conditions. The texture effect has been examined on alloy states with identical microstructures but with different preferred grain orientations. Tests over wide temperature and strain-rate intervals have shown that the formation of a strong texture as a result of pretreatment of a titanium alloy leads to a decrease in flow stress, an increase in plasticity and a shift of the optimum strain-rate region to higher rates. Data on the effect of texture on the anisotropy of properties of the VT6 alloy are also presented. The present results suggest that the influence of preferred orientation on the characteristics of superplastic flow is a general phenomenon for finegrained superplastic materials.     

Hot deformation and annealing of cube oriented aluminium single crystals

Abstract

Single crystals of the {001}〈100〉 orientation of an Al–0.05Si single phase alloy have been deformed in plane strain compression at temperatures of 300–500°C and strain rates of 0.5–50 s^-1, and annealed in a salt bath at temperatures of 300–450°C. Quantitative texture measurements by electron backscatter diffraction (EBSD)show that, in agreement with previous work, the cube orientation is stable at lower strain rates and higher temperatures (lower Zener–Hollomon parameter Z), whereas this orientation is unstable at higher values of Z. During annealing of the deformed crystals there is a competitive migration of subgrain boundaries of a wide range of orientations, and recrystallisation starts preferably at deformation bands of high orientation gradient. Measurement of subgrain growth has enabled the dependence of the mobility of low angle grain boundaries on misorientation to be determined. The results are in accord with those obtained for lower angle (<6°)boundaries in the same material.     

A metallographic study of superplasticity in a micrograin aluminium bronze

The microstructures of micrograin Cu-9 to 10% Al-0 to 4% Fe alloys, which are superplastic at 800° C, have been determined. Metallographic studies after deformation at 800° C over a range of strain-rates encompassing the three stage strain-rate hardening behaviour common to superplastic materials show that in the low strain-rate range, below that for high values of the strain-rate sensitivity exponent (m), clumps of grains slide together as units with considerable flow in the matrix close to sliding interfaces. After deformation in this low strain-rate range there is no evidence for dislocation motion within the grains. With increasing strain-rate, through and beyond the strain-rate range where peak values ofm are recorded, evidence for dislocation motion steadily increases; the tendency for clumps of grains to slide together diminishes; and there is decreased flow in the matrix about the sliding interfaces. The strain-rate for maximumm shows a strong dependence on the proportion of phase in the microstructure and the presence of iron which acts to refine the grain size. These observations are explained in terms of a flow mechanism whereby the high strain-rate sensitivity range occurs intermediate between a low strain-rate range, where sliding is accommodated by diffusion, and a high strain-rate range, where accommodation is by dislocation movement through the matrix.Formerly Graduate Student, Department of Mechanical Engineering, University of Waterloo, Ontario, Canada.     

Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 m to 10 m after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers     

Effects of initial grain size on hot deformation behavior of commercial pure aluminum

The effects of initial grain size of commercial pure aluminum on hot deformation behavior were investigated using hot compression tests. The hot compression tests were carried out on the pure aluminum samples with the initial grain sizes of 50, 150 and 450 μm using various strains, strain rates and different deformation temperatures. It was found that the hot deformation behavior of used material was sensitive to deformation conditions and initial microstructure. Results indicate that the initial grain size has significant effect on the flow stress. Flow stress decreases when the grain size decreases from 450 to 50 μm and when strain rate is lower than 0.05 s⁻¹. This procedure is reversed at strain rate of 0.5 s⁻¹. Furthermore, effects of other parameters like the strain rates and deformation temperatures on the flow stresses and hardening rates were investigated. It was also found that the inhomogeneity of microstructure distribution at different positions of the deformed specimens depended on the amount of deformation concentration at particular points and other processing parameters such as initial grain sizes, strain rates and deformation temperatures. In addition the geometric dynamic recrystallization (GDRX) was observed in the specimens highly strained (0.7) at elevated temperature (500 °C) using polarized light microscope and sensitive tint (PLM + ST).     

High temperature deformation of nickel base superalloy Udimet 520

Abstract

The high temperature deformation behaviour of nickel base superalloy Udimet 520 was characterised using hot compression isothermal tests. Hot compression tests were conducted between 900 and 1150°C with strain rates of 0.001, 0.01, 0.1 and 1 s^-1. Testing at ≤ 950°C led to sample fracture for all the applied strain rates. The flow behaviour at 1000, 1050 and 1075°C indicated the occurrence of dynamic recovery. For specimens tested at 1100, 1125 and 1150°C, recrystallisation is the softening mechanism. The strain rate sensitivity factor m was estimated for various thermomechanical histories. The activation energy for the hot deformation was determined to be 780 kJ mol¹. The Zener–Hollomon parameter was also determined and its variation with grain size was studied with deformation conditions. The microstructures of all samples were examined by both optical and scanning electron microscopy. The presence and variations in the morphology and size distribution of deformed and recrystallised grains were determined and related to the deformation conditions.     

Grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper

This work illustrates the grain size effect on strain-rate dependence of mechanical properties of polycrystalline copper through experimental characterisations and numerical calculations. The as-received and annealed samples with different grain sizes are prepared. Mechanical properties at high strain rates are experimentally attained by using a split Hopkinson pressure bar device. With the increase of grain size, dynamic flow stress decreases, but strain-rate dependence of flow stress increases. Johnson–Cook constitutive model is applied to carry out a numerical analysis about the grain size effect on strain-rate dependence of mechanical data and the quantificational illustrations are given.     

Superplasticity in a 7055 aluminum alloy subjected to intense plastic deformation

Abstract

Superplasticity in a 7055 aluminum alloy subjected to intense plastic straining through equal channel angular extrusion (ECAE) was studied in tension over a range of strain rates from 1.4 × 10^-5 to 5.6 × 10^-2 s^-1 in the temperature interval 300 - 450 °C. The alloy had a grain size of ~ 1 μm. A maximum elongation to failure of ~750% occurred at a temperature of 425 °C and an initial strain rate of 5.6 × 10^-4 s^-1, with a strain rate sensitivity coefficient m of about 0.46. The highest m value was ~0.5 at a strain rate of 1.4 × 10^-3 s^-1 and T≥ 425 °C. Moderate superplastic properties with a total elongation of about 435% and m of ~0.4 were recorded in the temperature interval 350 - 400 °C; no cavitation was found. It was shown that the main feature of superplastic behaviour of the ECAE processed 7055 aluminum alloy is a low yield stress and strong strain hardening during the initial stages of superplastic deformation. Comparing the present results with the superplastic behaviour of the 7055 Al subjected to thermomechanical processing (TMP), the highest tensile elongation in the ECAE processed material occurred at lower temperatures because ECAE produces a finer grained structure.     

Powder compaction interpretation using the power law

The effects of strain hardening and axial strain rate, over a wide range of rates (10^–3 to 10⁵ s^–1), on the compaction properties of a variety of pharmaceutical powders have been investigated. The powders tested are: Di Pac sugar, paracetamol d.c., Avicel and lactose. These materials have been assessed using the constants derived from the power law as a criterion to describe their behaviour. All the materials tested show, with varying degrees, a non-linear increase in the yield pressure (flow stress), the constantG, the strain rate exponentm and the strain hardening exponentn as the strain rate increases. These variations are more clear in the materials known to deform plastically, such as Avicel. This is attributable to a change either from ductile to brittle behaviour or a reduction in the amount of plastic deformation due to the time-dependent nature of the plastic flow. This, however, is explained in terms of dislocation and diffusion processes involved in the plastic deformation mechanisms during the compaction process. As the speed of compaction increases the characteristics of deformation, including the value of the strain rate exponent, the shape of the creep curve and the nature of creep rate, suggest that the creep behaviour is therefore controlled by some form of diffusion process. Meanwhile, the creep characteristics of the low and medium rate tests appear to be consistent with dislocation climb and viscous glide. For the materials tested, Avicel is found to be the most strain-rate sensitive material, while paracetamol d.c. is found to be the least strain-rate sensitive material.