Cavity mediated strain localization and overall ductility in eutectic tin-lead alloy

A combined experimental and numerical study is undertaken to examine the effects of pre-machined holes on strain localization and overall ductility in eutectic tin-lead alloy. Thin-sheet specimens with equal-sized holes aligned in the tensile loading direction are used. The tensile tests were performed at room temperature with a nominal strain rate of 0.001 s⁻¹. The specimen, containing one hole, showed a significant reduction in ductility compared to the control (no-hole) specimen. With an increasing number of holes, however, the overall strain-to-failure increases and fracture tend to follow shear bands generated locally from the hole edges. Finite element analyses, taking into account the viscoplastic response, were carried out to provide a mechanistic rationale to corroborate the experimental findings. The dispersion of plastic deformation and the effect of hole interaction are both found to contribute to the observed behavior. The local maximum equivalent plastic strain decreases with increasing number of holes, resulting in more delayed fracture. Plastic deformation becomes more intense inside the shear band when the holes are spaced more closely, which explains the increasing propensity of fracture along the shear bands in specimens containing more pre-machined holes.     

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     

Achieving superplastic properties in a Pb–Sn eutectic alloy processed by equal-channel angular pressing

Experiments were conducted on a Pb-62% Sn eutectic alloy containing 160 ppm of Sb. The alloy was processed by equal-channel angular pressing (ECAP) through 1 to 5 passes at room temperature and then tested in tension at a temperature of 423 K using initial strain rates from 1.0 × 10⁻⁴ to 1.0 × 10⁻¹ s⁻¹. Excellent superplastic elongations were achieved at intermediate strain rates with a maximum elongation to failure of 2,665%. It is shown that, for processing through similar numbers of ECAP passes, these elongations are higher than in an earlier investigation using a Pb-62% Sn alloy of higher purity. The results are presented pictorially in the form of a deformation mechanism map by plotting normalized grain size against normalized stress at a temperature of 423 K.     

Tensile flow and fracture behaviour of a superplastic Al-Ca-Zn alloy

The tensile flow behaviour in the range 275 to 550 ° C of an ultra-fine-grained superplastic Al-Ca-Zn alloy is reported. Under certain conditions of temperature and strain rate, superplastic ductility could be established. Fracture surfaces of tensile specimens tested in the above temperature range were examined by scanning electron microscopy and a correlation could be obtained between the ductility, as revealed by the tension tests, and the fracture behaviour. The fractographic studies also suggested a transition in the deformation process from grain deformation (mainly slip) at the lower temperatures to grain-boundary deformation (predominantly grain-boundary sliding) in the vicinity of 425 ° C.     

Cavitation during superplastic flow of ternary alloys based on microduplex Pb-Sn eutectic

The effect of phases having a range of hardnesses on the superplastic tensile behaviour of microduplex Pb-Sn eutectic has been studied. The presence of relatively hard particles induced cavitation at particle/matrix interfaces during deformation in an otherwise noncavitating system, and the growth and interlinkage of cavities led to brittle superplastic fractures. Density measurements showed that cavitation increased as the volume fraction, hardness and size of intermetallic particles was increased. Increasing strain rate and decreasing deformation temperature also led to an increased level of cavitation. Cavity nucleation was attributed to the limited ability of the relatively hard phases to contribute to the accommodation processes occurring during superplastic flow.     

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.     

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.     

Formation of a separated eutectic in Al-Si eutectic alloy

The phenomenon of a separated eutectic in Al-Si eutectic alloy unidirectionally solidified with electromagnetic stirring was investigated. It was found that the eutectic silicon could be swept away from the solid/liquid (s/l) interface by the flow, and transferred by the secondary flow to the wall of the crucible, forming a silicon-rich layer on the periphery of the ingot.Nomenclature B Magnetic induction vecto - B _o Magnetic induction magnitude on the periphery of liquid column - E Electrical field vector - f _r,f Radial and azimuthal components of electromagnetic force - g Acceleration due to gravity - H Height of liquid column - H Magnetic field - J Current - N Rotation rate of the melt - P Pressure - r Radial coordinate - r _o Radius of liquid column - u _r,u ,u _z Radial, azimuthal, and axial components of flow velocity - V Speed of solidification - Z Axial coordinate - _o Magnetic permeability - v Viscosity of liquid - Density of liquid - Electrical conductivity - Angular frequency of the current.     

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.     

Effect of relatively hard particles on caviation of microduplex Pb-Sn eutectic during superplastic flow

Journal of Materials Science -     

The interdiffusion in melts of tin-lead system of eutectic and near-eutectic composition

The method of gamma-raying of samples by a narrow beam of gamma radiation is used to measure the interdiffusion coefficients in melts of a tin-lead system, which contain 17.6, 26.1, 29.3, and 33.9 at.% Pb, at temperatures from 500 to 1000 K. The results are compared with literature data. It is found that the concentration dependence of diffusion coefficient exhibits no special features in the vicinity of eutectic composition.     

Fracture toughness of eutectic Al-Si casting alloy with different microstructural features

The static fracture toughness of a series of eutectic Al-Si casting alloy with different microstructural features has been evaluated. The dominant influence of eutectic silicon in controlling the fracture toughness is thus clarified. The relationship between the fracture toughness and the microstructure was established. Fracture toughness was found to be strongly associated with the size and morphology of silicon particles. The other feature which greatly influences the fracture toughness is the ratio (/DE)_Si, i.e. the silicon particle spacing divided by the equivalent particle diameter, rather than the silicon particle spacing, _Si. Fracture toughness also correlates well with the void growth parameter, VGP (=_y (/DE)_Si), proposed by the authors. The results of the present work can be used to develop an understanding of the variation of fracture toughness with the microstructural features of eutectic Al-Si alloys.