A microstructural examination of the flow behaviour of a superplastic copper alloy

There are important differences in the microstructures of specimens of a superplastic copper alloy deformed in the three regions of flow associated with superplasticity. There is very extensive dislocation activity at high strain rates in Region III, whereas at intermediate and low strain rates in Regions II and I the dislocation density is low and many of the grains appear to be dislocation-free. Measurements show that grain-boundary sliding is important in Region II but decreases in magnitude in the less superplastic Regions I and III.     

Tensile and fatigue fracture behaviour of a spot welded Al- Mg alloy

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

The fracture characteristics of a superplastic single phase copper alloy

A superplastic single phase copper alloy exhibits a sigmoidal relationship between strain rate and stress at 823 K, dividing the behaviour into three regions. Maximum elongation to fracture (380%) occurs at intermediate strain rates at the lower end of region II, and there is a decrease in total elongation at both low (region I) and high (region III) strain rates. No necking is observed in regions I and II, and there is only very slight necking in region III. Internal cavities are formed at all strain rates, but the appearance of the cavities depends critically on the imposed strain rate. At high strain rates, the cavities are small and lie in strings parallel to the tensile axis; but as the strain rate is reduced the cavities become larger, more rounded, and essentially randomly distributed. The mode of failure is ductile rupture in region III, but void growth and interlinkage become increasingly important with decreasing strain rate.     

Grain refinement and superplastic behaviour of a modified 6061 aluminium alloy

Abstract

The superplastic properties and microstructure evolution of a 0.15%Zr and 0.7%Cu modified 6061 aluminium alloy were examined in tension at temperatures ranging from 475 to 600°C and strain rates ranging from 7 × 10^-6 to 2.8 × 10^-2 s^-1. The refined microstructure with an average grain size of about 11 μm was produced in thin sheets by a commercially viable thermomechanical process. It was shown that the modified 6061 alloy exhibits a moderate superplastic elongation of 580% in the entirely solid state at 570°C and ? = 2.8 × 10^-4 s^-1. Superior superplastic properties (elongation to failure of 1300% with a corresponding strain rate sensitivity coefficient m of about 0.65) were found at the same strain rate and a temperature of 590°C, which is higher than the incipient melting point of the 6061 alloy (~575°C). The microstructural evolution during superplastic deformation of the 6061 alloy has been studied quantitatively. The presence of a slight amount of liquid phase greatly promotes the superplastic properties of the 6061 alloy, reducing the cavitation level.     

The deformation behaviour of a dispersion strengthened superplastic zinc alloy

The deformation behaviour of a new dispersion strengthened superplastic zinc alloy was investigated. A significant long range internal stress was observed at all strain-rates. The activation volume of deformation decreased very rapidly with a decrease in the true effective stress. The maximum strain-rate sensitivity corresponds to a region of change from this high stress dependence of the activation volume to a much lower stress dependence. The observation of a metallographic halo effect shows that apart from dislocation movement, diffusive creep plays a role during superplastic deformation. It is stipulated that both these processes aid boundary sliding which accounts for the largest part of the strain.     

The fracture process in a fine-grained superplastic 7475 Al alloy

In order to contribute towards alloy design and therefore an improvement in fracture toughness of engineering materials in general, the effect of temperature, strain rate and strain level on the superplastic deformation, cavity nucleation and growth, and fracture behaviour are studied in an important rate-sensitive structural engineering material, 7475 Al, in the light of current models and thinking. The efficacy of hydrostatic pressure in reducing cavitation during superplastic deformation is considered.     

Tensile fracture behaviour of microstructurally engineered Cu bearing high strength low alloy steel

Abstract

An attempt has been made to highlight the influence of precipitation and microstructural constituents on tensile fracture behaviour in Cu bearing HSLA 100 steel. Variations in the microconstituents have been incorporated in the steel by engineering the microstructures through thermal treatments consisting of solutionising, water quenching and aging at various temperatures. The microstructure in quenched condition consists of mainly lath martensite, bainite and acicular ferrite besides little amount of retained austenite, carbides and carbonitrides. Aging up to 500°C facilitated fine coherent ?-Cu precipitation that lost its coherency at >550°C. Simultaneously, recovery and recrystallisation of martensite and acicular ferrite occurred at higher temperatures. The formation of new martensite islands occurred on aging at >650°C. Carbides, carbonitrides and retained austenite remained essentially unchanged. Tensile tests were conducted at a slow strain rate to study the tensile fracture behaviour of the steel. Microstructural and fractographic evidences indicating that coherent Cu precipitate causes the brittleness in the material in initial stages of aging whereas loss of coherency of Cu precipitate in later stages results in the reappearance of ductility in the material.     

The stress-strain rate behaviour of superplastic Zn-Al eutectoid alloy

The stress-strain rate behaviour of a superplastic Zn-22% Al alloys was investigated by the differential strain rate and constant load cycling tests on an Instron machine. A region with a rate sensitivity approaching unity was observed at low strain rates. On increasing the strain rate, a transition to the superplastic region occurred through an intermediate region of lower m (0.35). These observations are interpreted in terms of a transition from diffusional creep to superplastic flow with a threshold stress for superplastic deformation.     

Low-stress creep behaviour of superplastic Zn-22% Al alloy

Low-stress creep behaviour of microduplex Zn-22% Al alloy was studied using spring specimen geometry. The average phase size in the specimens investigated was 0.87, 1.48 and 1.98 m. Experiments were conducted in the temperature range 393–473 K at stresses below about 1.0 MN m^–2. The present study has established that the stress exponent of the creep rate is unity and, therefore, a viscous creep process dominates the flow in Region I superplasticity. The activation energy corresponds to that for boundary diffusion. However, the phase-size exponent was found to be –2 instead of –3, as predicted by the Coble creep theory. Further, the measured creep rates are three to four orders of magnitude slower than those predicted by the Coble theory. Transmission electron microscopy revealed precipitation, along / grain interfaces, whose inhibiting action on plastic flow should at least be partly responsible for the lower values of measured creep rates. There also exist two other interfaces, namely / and /, whose comprehensive role in diffusion creep is not yet fully understood. Therefore, it seems illogical to describe the creep behaviour of Zn-22% Al by the classical Coble theory, originally developed for single-phase polycrystals.     

Fracture of a tin-lead eutectic alloy in Region III of superplastic flow

With the aid of scanning electron microscopy, cavitation and fracture behaviour in the Sn-Pb eutectic alloy, whose reduction in area of cross-section before failure is close to 100%, has been investigated in Region III of superplastic flow (where both the elongationto-fracture and the strain-rate sensitivity index decrease with increasing strain rate). It has been demonstrated that, although it decreases, grain-boundary sliding persists in this range as the strain rate is increased. At all strain rates the final failure was due to tearing by plastic flow of the inter-cavity ligaments, but the interlinkage of cavities along the graininterphase boundaries decreased with increasing strain rate. The features of cavitation and fracture did not differ much from an earlier study on a pseudo-single phase copper alloy, although copper alloys usually fail non-ideally, i.e., a large area of cross-section is present at fracture.     

Tensile and impact-toughness behaviour of cryorolled Al 7075 alloy

The effects of cryorolling and optimum heat treatment (short annealing + ageing) on tensile and impact-toughness behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its mechanical properties were studied by using tensile testing, hardness, and Charpy impact testing. The microstructural characterization of the alloy was carried out by using field emission scanning electron microscopy (FE-SEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain structure as observed from its FE-SEM micrographs. It is observed that the yield strength and impact toughness of the cryorolled material up to 70% thickness reduction have increased by 108% and 60% respectively compared to the starting material. The improved tensile strength and impact toughness of the cryorolled Al alloy is due to grain refinement, grain fragments with high angle boundaries, and ultrafine grain formation by multiple cryorolling passes. Scanning electron microscopy (SEM) analysis of the fracture surfaces of impact testing carried out on the samples in the temperature range of −200 to 100 °C exhibits ductile to brittle transition. cryorolled samples were subjected to short annealing for 5 min at, 170 °C, and 150 °C followed by ageing at 140 °C and 120 °C for both 40% and 70% reduced samples. The combined effect of short annealing and ageing, improved the strength and ductility of cryorolled samples, which is due to precipitation hardening and subgrain coarsening mechanism respectively. On the otherhand, impact strength of the cryorolled Al alloy has decreased due to high strain rate involved during impact loading.     

Tensile properties and fracture behaviour of polypropylene-nickel-coated carbon-fibre composite

Polypropylene-based composites reinforced with nickel-coated carbon fibres have been prepared and the effect of filler content on tensile properties and fracture behaviour at different temperatures and strain rates was investigated. The elastic modulus of such composites is enhanced by two orders of magnitude while the tensile strength and strain-to-break are lowered. The fracture toughness parameters,G _c andK _c, are also enhanced with filler content. The yield stress of this composite showed strain rate and temperature dependence. Activation energy and volume of a single rate-activated yielding process, at relatively high strain rates, were determined. The variations of the measured physical quantities are discussed in terms of the observed composite morphology.On sabbatical leave from the Physics Department, University of Jordan, Amman, Jordan.     

Effect of prestrain on the superplastic behaviour of a fine grained 7475 AI alloy

The effect of prestraining at a fast strain rate (region III) on the subsequent superplastic behaviour (region II) of a 7475 AI alloy has been studied. The results show that prestraining causes a decrease in the elongation to failure as compared to the non-prestrained (as-received) samples. This decrease in elongation is postulated to be associated with the growth of cavities formed during prestraining as well as grain growth during deformation in region II. Prestraining in region III did not lead to any observable inhomogeneities in strain distribution during subsequent deformation.     

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.     

Grain growth behaviour of the Al-Cu eutectic alloy during superplastic deformation

Grain growth behaviour of the Al-Cu eutectic alloy was investigated as a function of strain (ε), strain rate and deformation temperature (T) over = 10⁻² s⁻¹ and T=400 to 540°C. The grain size increases with increase in strain and temperature. Upon deformation to a fixed strain, the grain growth is generally seen to be more at lower strain rates. The rates of overall grain growth and due to deformation alone , however, increase with increasing strain rate according to and , respectively. The increase in the grain growth rate with strain rate is attributed primarily to the shorter time involved at higher strain rate for reaching a fixed strain. The activation energy for grain growth under superplastic conditions is estimated to be 79 kJ mol⁻¹.     

Cavitation and fracture in the superplastic Al-33% Cu eutectic alloy

Experiments were conducted on the Al-33% Cu eutectic alloy in both an annealed and an as-extruded condition. For both conditions, the relationship between flow stress and strain rate is sigmoidal with maximum ductilities occurring at intermediate strain rates in the superplastic Region II. Specimens fail by necking at the faster strain rates in Region III, but the severity of necking is reduced with decreasing strain rate and the necks are very diffuse in Region II. There is extensive internal cavitation in the fractured specimens, especially at lower strain rates and in the vicinity of the fracture tip. It was observed that cavities form preferentially on the- interphase boundaries. It is shown by calculation that the observed change from small rounded cavities to large cavities elongated along the tensile axis is reasonably consistent with the theories of cavity growth in fine-grained superplastic alloys