The rate dependent response of a titanium alloy subjected to quasi-static loading in ambient environment

A dual phase Ti-6A1-4V alloy was tested in uniaxial tension over a large quasi-static loading range (10^–5–10^–1 s^–1) in ambient environment. As strain rate increases, strength of the alloy was found to increase at the expense of ductility. In the low strain-rate region, strain rate sensitivity of the material experienced a gradual decrease during plastic deformation. In the high strain-rate region, strain-rate sensitivity of the material was largely constant for most part of the plastic deformation. The different rate dependent behaviours are believed to be caused by a change of governing plastic deformation mechanism from dislocation slip at low strain rates to twinning at the highest strain rate. Strong fractographic and metallographic evidence was obtained to understand the micromechanisms of plastic deformation.     

Strain-rate effect on high temperature low-cycle fatigue deformation of AISI 304L stainless steel

The effect of strain rate on the behaviour of high temperature low-cycle fatigue is investigated for AISI 304L stainless steel. Regardless of the test temperature of 873 or 973 K, the fatigue life is saturated in the strain-rate range of slower than 4 × 10^–3 sec^–1. Also it is interesting to note that serrated flow, which is evidence of the occurrence of dynamic strain ageing, is clearly observed in the load-elongation hysteresis loops for strain rates that are slower (at 873 K) and faster (at 973 K) than 4 × 10^–3 sec^–1. Since the combination of temperature and strain rate is concerned with the phenomenon of dynamic strain ageing, it is considered that the above-mentioned saturated fatigue life at 873 K is caused by dynamic strain ageing and that the hardening effect due to dynamic strain ageing abnormally increases the fatigue life. However, even though the behaviour of fatigue life under strain rates slower than 4 × 10^–3 sec^–1 at 973 K has nothing to do with the dynamic strain ageing, it has been found that the failure life is also saturated in this slower strain-rate range. This behaviour is considered to be caused by the effect of creep, because the deformation under the low strain-rate activates the recovery process and as a result it causes saturation of the inelastic strain range.     

Dynamic strain-rate effect on uniaxial tension deformation of Ti5Al2.5Sn α-titanium alloy at various temperatures

The uniaxial tensile experiments for Ti5Al2.5Sn alloy were performed at strain rates ranging from 10^?3–10⁺³ s^?1 and test temperatures of 153–873 K. Experimentally measured stress-strain responses indicate the yield strength exhibits positive strain-rate dependency, while the yield strength increases as the test temperature is decreased. To understand the thermomechanical coupling of dynamic plastic deformation, a specially developed single-tensile-pulse loading technique was used, and the isothermal stress-strain curves for the rates of 180 and 450 s^?1 were obtained at temperatures of 203, 298 and 573 K. The plastic strain hardening measurements obtained here are essentially athermal and largely independent of strain rate, consistent with titanium and its alloys being bcc-structure-like in mechanical behaviour. Based on the experimentally obtained plastic deformation features of the alloy, the physically based Voyiadjis-Abed constitutive relationship was modified to model the dynamic tensile deformation of the Ti5Al2.5Sn alloy at low and high temperatures.     

Microstructure and texture of high manganese steel subjected to dynamic impact loading

ABSTRACT

Dynamic impact response of high Mn-steel at a strain rate of 3000?s^?1 was investigated using the Split Hopkinson Pressure bar. The investigated steel depicted continuous yielding at high strain rates. Additionally, the yield stress displayed a positive strain-rate sensitivity with an increasing strain rate. Microstructural evaluations displayed that strain-induced martensitic transformation and dislocation multiplication during slip were dominant plastic deformation mechanisms in the absence of deformation twinning which contributes to the strain hardening. Adiabatic shear band and martensite to austenite reversion or dynamic recrystallisation were also attributed to strain softening during impact deformation. The {001}<110> R-cube, {011}<110> R-Goss, and ({111}<110>) E texture components were strengthened after impact loading compared with as-received condition, while the intensities of Cube, Cupper, Brass, and S texture components were decreased.     

Deformation kinetics by crazing in glassy polymers

In order to confirm the proposed theory of the plastic deformation caused by crazing, the theoretical deformation curves derived from the dislocation analogue were calculated using the data of craze behaviour measured under static tension and compared with the experimental results of creep and constant strain-rate tension in PMMA and PVC plates wetted by kerosene as a crazing agent. Favourable agreement between the theoretical and experimental results may provide evidence that the proposed method is valuable for estimating the amount of the plastic strain caused by crazing.     

Effect of oxidation on creep data: Part 2 – A procedure for assessing the effect of oxidation on constant-stress or constant-load creep curves using the Theta-projection concept

Abstract

A procedure based on Theta-methodology coupled with the knowledge of oxidation kinetics is envisaged for quantitatively assessing the effect of oxidation on creep curves. The procedure is based on the generation of a series of real constant-stress creep curves, at different stress and temperature levels, in inert atmosphere, where the effects due to oxidation are kept to a minimum level. Stress enhancement factors due to the effect of area reduction on specimen cross-section with plastic deformation and oxidation are defined for constant-stress or constant-load creep testing. These factors can be used in the integration of the strain-rate equation related to the 4θ - parameter analysis, to derive constant-stress or constant-load curves in air using either the strain hardening or time hardening theories. Although systematic constant-stress creep data in vacuum are not yet available to test the methodology effectively, a preliminary simulation is done to demonstrate how the model works, to check its performance and the possibilities of analysis.     

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.     

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.     

Uniaxial strain rate effects in pharmaceutical powders during cold compaction

The results of uniaxial compression tests on some pharmaceutical powders subjected to strain rates of between 10^–3 and 10⁵ s^–1 are given. The tests fall into three main categories: low-strain-rate tests (10^–3–10 s^–1) performed on a servohydraulic variable speed-compression machine at constant compression rate; medium-strain-rate tests (10²–10³ s^–1) carried out on a drop hammer; and high strain rate tests (10³–10⁵ s^–1) performed on a high-pressure air projectile launcher compaction apparatus. Axial and radial pressures, as well as displacement-time measurements, are made. Powders tested include: Dipac sugar; sodium chloride; potassium bromide; lactose; paracetamol d.c.; avicel; calcium phosphate; and copper sulphate. The influence of compression rate on the form of the characteristic pressure-density and radial-axial pressure relationships during uniaxial straining is presented. The investigation showed that the general tendency for all powders tested, except for paracetamol d.c., is to exhibit increased compaction pressure with strain rate up to 10⁵ s^–1. Due to morphological and compositional effects, paracetamol d.c. softens with the rate of straining up to about 10² s^–1 and at higher rates it behaves like other powders. Also the mean radial pressure at the die wall (obtained by a pin-type transducer) shows that the friction conditions are variable during the process, and their effect tends to decrease as the speed of compaction increases, resulting in more uniform density compacts. Finally, by observing the decay of both axial and radial pressures with time under constant volume conditions, a reasonably linear behaviour is obtained for all materials tested, particularly the axial relaxation curves, over the period recorded.     

The use of an Elastic Recovery Index as a Criterion of Compactional Behaviour of Some Direct Compression Bases

Abstract

The effects of compression force and holding time on computer logged strain movements (at constant stress) and elasticviscoelastic expansions (on load release) of compacts made from eight pharmaceutical powders are reported. The feasibility of using an elastic recovery index (the ratio of elastic recovery to viscoelastic strain movements) to predict the quality of a material is also discussed.

All compacts continued to consolidate by viscoelastic and plastic flow when held under constant stress during the holding period. The “compressible” bases Avicel PH-101 and Sta-Rx 1500 exhibited more time dependent movements than powders such as Paracetamol DC, Paracetamol, Emcompress and magnesium stearate. Poorly compressible materials such as magnesium carbonate and paracetamol showed the greatest elastic expansions on load release.

It has been found that the elastic recovery indices of the compressible materials were lower than those of poorly compressible powders.     

Deformation behaviour of Fe-3Si steel

Abstract

The deformation behaviour of an Fe-3Si (wt-%) steel was studied in the temperature range 400-900°C over six orders of magnitude of strain rate. It was found that the Fe-3Si steel exhibits a threshold behaviour. A correlation between the deformation behaviour and the temperature dependence of the threshold stress was etermined. An analysis in terms of the threshold stress showed that two modes of deformation behaviour exist in the power law creep regime. At normalised strain rate ?kT/(D ₁ Eb) ranging from 2 × 10^-6 to 10^-3, the value of the true stress exponent n is equal to 7, and at lower values of ?kT/(D ₁ Eb) the value of n is ~ 5. The true activation energy for plastic deformation Q _c increases from 250 ± 15 to 290 ± 30 kJ mol^-1 with increasing temperature from 550 to 700°C, and remains virtually unchanged at high temperatures. The relationship between rate controlling mechanisms of plastic deformation and mechanisms of interaction between lattice dislocations and dispersoids is discussed.     

Superplastic behaviour of a ceramic-based kappa/alpha Fe-10Al-1.9C material

A fine microstructure has been developed in a Fe-10Al-1.9C material made from rapidly solidified powders. The compacted material had a microstructure containing about 50 vol% kappa phase (Fe₃AlC _x ) and 50 vol% alpha phase. The creep behaviour of the material was investigated using tension and compression change in strain rate tests and elongation to failure tests. Stress exponents of 2–3 were obtained over a wide range of strain rates, and an average activation energy for creep of 245 kJ mol^–1 was determined. A maximum elongation to failure of 1120% was obtained at 900°C. The tensile ductility as a function of strain rate was found to follow the same behaviour of other ceramic materials wherein the elongation to failure decreases sharply with an increase in strain rate.     

High strain-rate compressive deformation behavior of the Al0.1CrFeCoNi high entropy alloy

High entropy alloy is a new class of structural metallic materials. No work, so far, has been carried-out to understand high strain-rate plastic deformation behavior and resulting microstructure. This work focuses on understanding the deformation behavior of an Al_0.1CrFeCoNi HEA at high strain-rate (HSR). HSR plastic deformation in compression mode was carried out using split-Hopkinson pressure bar. The pre- and post-deformation microstructures were studied using electron microscopes. A high strain-rate sensitivity of yield strength, significant work hardening, and profuse twinning are main characteristics observed during deformation of the alloy at HSR. Overall, the deformation behavior of the alloy was consistent with low stacking fault energy materials.     

Tensile behaviour and strain hardening characteristics of constrained groove pressed nickel sheets

In this study commercially pure nickel sheets are severe plastically deformed at room temperature by constrained groove pressing (CGP) technique and the effect of pass number on the room temperature mechanical behaviour is investigated. Increase in strength observed after first pass is much higher than the increase observed during subsequent passes. Mechanical behaviour of constrained groove pressed sheets indicated negligible strain hardening ability during initial passes; gain in strain hardening ability is observed during latter passes resulting in enhanced ductility. The observation of shortened uniform elongation phase during tensile testing of CGP processed sheets could be linked to the lack of strain hardenability and change in deformation mechanism. Constitutive mechanical behaviour in uniform plastic deformation regime of nickel sheets in annealed condition obeys Hollomon relation whereas severely deformed sheets obey Voce relation closely. Strain hardening characteristics of groove pressed sheets analysed by Kocks–Mecking approach revealed stage-III hardening behaviour associated with high initial hardening rate when compared to annealed sheets. The influence of pass number on dislocation density evolution is assessed by Taylor’s expression. The synergistic effect of dislocation generation and recovery on the evolution of constitutive mechanical behaviour in the uniform elongation regime is described by applying single parameter based Kocks model. The kinetics of dislocation storage and dislocation annihilation in severely deformed nickel sheets during deformation corroborated with mechanical properties and dislocation density indicates the dominance of dislocation generation during earlier passes.     

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.     

A case for Taylor hardening during primary and steady-state creep in aluminium and type 304 stainless steel

Previous elevated-temperature experiments on 304 stainless steel clearly show that the density of dislocations within the subgrain interior influences the flow stress at a given strain rate and temperature. A re-evaluation shows that the hardening is consistent with the Taylor relation if a linear superposition of solute hardening ( ₀, or the stress necessary to cause dislocation motion in the absence of a dislocation substructure) and dislocation (Gb ^1/2) hardening is assumed. The same Taylor relation is applicable to steady-state structures of aluminium if the yield stress of annealed aluminium is assumed equal to ₀. New tests on aluminium deforming under constant-strain-rate creep conditions show a monotonic increase in the dislocation density with strain. This and the constant-stress creep trends are shown to be possibly consistent with Taylor hardening.     

Effect of aging on work hardening behaviour of cold rolled Nimonic C-263 alloy

Abstract

Experimental true stress–true strain data of Nimonic C-263 alloy in solution treated as well as aged condition have been analysed using different flow relationships. Ludwigson relationship provides the best fit of the data for all the conditions investigated. The transition in macroscopic flow behaviour of the alloy with plastic strain, in solution treated condition, can be correlated with the transition in deformation mode from low strain regime to high strain regime. Although aging does not appear to alter the macroscopic flow behaviour, it causes a considerable change in flow parameters of the Ludwigson relationship and substructural evolution. On the other hand, the effect of sheet thickness is marginal. The flow data of the aged alloys fitted according to Ludwigson model not only yield a unique set of flow parameters for each aging condition but also exhibit a systematic trend with aging time. The transition in macroscopic flow behaviour of the alloy with strain, in aged conditions, can be correlated with a change in dislocation mechanism from dislocation–precipitate interaction at lower strains to dislocation–dislocation interaction at higher strains leading to formation of a dense dislocation tangled networks in the matrix regions surrounding the precipitates. The alloy in both solution treated and aged conditions exhibits three fairly distinct stages of strain hardening. The strain hardening rate decreases in regime I, remains constant in regime II and begins to fall again in regime III. Furthermore, it is observed that the alloy specimen with longitudinal orientation (L, i.e. parallel to rolling direction), exhibits marginally highest strain hardening rates, while specimens with long transverse orientation exhibit lowest strain hardening rates both in solution treated and aged conditions. However, for all other in-plane orientations (i.e. L+30°, L+45° and L+60°), the strain hardening rate data are fairly very close and lie in between those of longitudinal and long transverse orientations.     

Das Verhalten metallischer Werkstoffe unter mechanischer Beanspruchung

The Deformation Behaviour of Metallic Materials Under Mechanical Loading Simple loading conditions are reviewed with load, reaction-diagrams and characteristic mechanical data as well as typical surface appearances. The uniaxial tensile test, the uniaxial creep test, the fracture toughness test and the single-stage fatigue test are dealt with in detail. The basis of plastic deformation is discussed and the reasons for the formation of definite dislocation structures and the type of glide bands are considered. The influence of different parameters of structure on the yield stress is explained in examples together with different hardening mechanisms. The influence of temperature and strain rate on the yield stress is described quantitatively. The deformation processes which are represented in deformation mechanism maps. Finally the influence of environments on the mechanical behavior of materials is considered. In examples, an insight is given into the activity of electrolytes, gaseous environments, organic liquids and liquid metals on the deformation behaviour at typical loading conditions.     

Dislocation network models for recovery creep deformation

The development of dislocation network models for recovery creep and the important results which arise from the application of the models are discussed. These models are basically aimed at describing the two simultaneous processes, namely strain hardening and recovery, which occur during high-temperature creep deformation. These processes are modelled using detailed dislocation mechanisms which occur in the deforming crystalline materials. The present models, although still being approximations, are reasonably well able to describe high-temperature recovery creep deformation of crystalline materials.