Friction stir welding, Whisker growth, and ductile fracture

Back-of-the-envelope calculations in friction stir welding – Velocities, peak temperature, torque, and hardness

A Arora et al

Given the complexity and resource requirements of numerical models of friction stir welding (FSW), well-tested analytical models of materials flow, peak temperatures, torque, and weld properties are needed. Here an approximate analytical technique for the calculation of three-dimensional material flow during FSW is proposed considering the motion of an incompressible fluid induced by a solid rotating disk. The accuracy of the calculations is examined for the welding of three alloys. For the estimation of peak temperatures, the accuracy of an existing dimensionless correlation is improved using a large volume of recently published data. The improved correlation is tested against experimental data for three aluminum alloys. It is shown that the torque can be calculated analytically from the yield stress using estimated peak temperatures. An approximate relation between the hardness of the thermomechanically affected zone and the chemical composition of the aluminum alloys is proposed.

Interface flow mechanism for tin whisker growth

H P Howard et al

Tin coatings, widely used in electronics, are susceptible to the spontaneous eruption of fine metal filaments or “whiskers”. Tin whiskers are a serious reliability issue in microelectronics, as they can cause short circuits and device failure. While it is generally accepted that whiskers grow to relieve compressive stresses, the specific mechanism for whisker formation is yet unknown. Data are presented to support an interface-transport mechanism for whisker nucleation and growth. This mechanism, involving the formation of a viscous layer at the interface between substrate and coating, could explain the extremely rapid growth of whiskers that has been observed experimentally.

In situ observation of ductile fracture using X-ray tomography technique

H Toda et al

Fast microtomography combined with local crack driving force analysis has been employed to analyze crack-tip stress/strain singularities in an aluminum alloy. The application of fast microtomography has made it possible to observe real crack initiation and propagation behaviors without intermediate unloading. The details of a crack and its local propagation behaviors are readily observed with this technique along with evidence of microstructure/crack interactions. After a preliminary investigation of the achieved spatial resolution, we show that conventional stationary and growing crack singularities can be quantitatively validated by deriving the local crack opening displacement. This is to our knowledge the first three-dimensional validation of conventional fracture mechanics during a real time continuous experiment that has been mainly developed via surface observations so far. We also reveal that there is a spatial transition from a stationary crack singularity to a growing crack singularity in addition to the well-known temporal transition that occurs with the onset of crack propagation. Local crack propagation behaviors are also discussed on the basis of this validation. To separate the effects of complex crack geometry from those of microstructure, we also perform an image-based numerical simulation.

Plasticity, shape memory and strain accommodation

Crystal plasticity finite-element analysis versus experimental results of pyramidal indentation into (0 0 1) fcc single crystal

B Eidel

Pyramidal microindentation into the (0 0 1) surface of an face-centered cubic (fcc) single crystal made of a Ni-base superalloy is analyzed in experiment and crystal plasticity finite-element simulations. The resultant material pile-up at the surface reflects the material’s symmetry and turns out to be insensitive to different loading scenarios as induced by (i) different azimuthal orientations of the pyramidal indenter, (ii) different indenter shapes (sphere or pyramid) and (iii) the elastic anisotropy. Experiments and simulations are in agreement and suggest that pile-up deformation patterns merely depend on the geometry of discrete slip systems but are invariant to different anisotropic stress distributions as induced by (i)–(iii). The local adaption of pile-up to the pyramidal indenter leads to convex or concave indent shapes corresponding to the indenter orientation. We contrast the present findings for curved indent shapes of fcc single crystals to similar, well-known observations for quasi-isotropic polycrystals. Although phenomenologically similar in kind, the driving mechanisms are different: for the single crystal it is the discrete and anisotropic nature of plastic glide in certain slip systems; for isotropic polycrystals it is the rate of strain-hardening caused by the cumulative response of dislocations.

Phase transformation in free-standing SMA nanowires

F R Phillips et al

The primary focus of this work is on determining if the phase transformation of shape memory alloy (SMA) nanowires exhibits a critical size below which the phase transformation is inhibited. The SMA nanowires are fabricated through the use of the mechanical pressure injection method. The mechanical pressure injection method is a template-assisted nanowire fabrication method in which an anodized aluminum oxide (AAO) template is impregnated with liquid metal. The fabrication of SMA nanowires with different diameters is accomplished through the fabrication of AAO templates of varying diameters. The phase transformation behavior of the fabricated SMA nanowires is characterized using transmission electron microscopy. By analysis of the fabricated SMA nanowires, it is found that the phase transformation of SMA nanowires is not affected for nanowires ranging in diameter from 650 to 10 nm.

The role of strain accommodation during the variant selection of primary twins in magnesium

J J Jonas et al

Samples of magnesium alloys AM30 and AZ31 were deformed in tension at room temperature and a strain rate of 0.1 s−1 to strains of 0.08 and 0.15. Of the numerous contraction twins that formed, the orientations of 977 were determined by electron backscatter diffraction techniques. The orientations of their host grains were also measured, so that the Schmid factors (SFs) applicable to each of the six contraction twins that could potentially form in each grain could also be calculated. About half of the observed twins were of the “high SF” (0.3–0.5) type, while nearly half had “low” SFs (0.15–0.30). Furthermore, 5% of the observed twins had associated Schmid factors of only 0.03–0.15, i.e. these were of the “very low SF” type. Of particular interest is the observation that many potential “high Schmid factor” twins did not form. The presence of the low and very low SF twins and the absence of many potential high SF twins are explained in terms of the accommodation strains that are or would be required to permit their formation. These were calculated by rotating the twinning shear displacement gradient tensor into the crystallographic reference frame of the neighboring grain. It is shown that the very high plastic anisotropy of Mg grains permits the “easy” accommodations to take place but conversely prevents accommodation of the potential twinning shears when these are “difficult” (when these involve high critical resolved shear stresses). The twins that appear require little or no “difficult” accommodation.

Magnetocrystalline anisotropy

Magnetocrystalline anisotropy in Fe-Mn-Ga magnetic shape memory alloy

T Omori et al

Magnetocrystalline anisotropy of the martensite phase was investigated in a single crystal of Fe44Mn28Ga28 shape memory alloy. It was found that the martensite phase of Fe44Mn28Ga28 alloy has a tetragonal distorted L21 with a = 0.5368 nm and c = 0.7081 nm. An approximately single variant was obtained by thermomechanical treatment, and magnetization measurement revealed that the c-axis is the magnetic easy axis. The magnetocrystalline anisotropy constant Ku was determined to be 7.6 × 105 J m-3 at 300 K.

From Acta

[1] Theory of the Kirkendall effect during grain boundary interdiffusion

L Klinger and E Rabkin

A grain boundary interdiffusion in a semi-infinite bicrystal under the conditions of negligible bulk diffusion is considered. We show that the inequality of the intrinsic grain boundary diffusion coefficients of the two components leads to plating out of additional material at the grain boundary in the form of a wedge of extra material, which generates an elastic stress field in the vicinity of the grain boundary. We solved a coupled diffusion/elasticity problem and determined the time-dependent stress field and concentration distribution in the vicinity of the grain boundary. We show that diffusion of embrittling impurities along the grain boundary generates tensile stresses at the boundary which are high enough to cause intergranular fracture.

[2] Evolution of the microstructure of Sn–Ag–Cu solder joints exposed to ultrasonic waves during solidification

R K Chinnam et al

Cu/SAC405/Cu solder joints were fabricated using a modified reflow-soldering procedure. The samples were first maintained at 260 °C for 320 s, following the conventional reflow-soldering methodology. The reflow process was then interrupted and the samples were exposed to ultrasonic waves (USW) while they were cooling in air. Compared with a sample reflow-soldered conventionally, the solidification of the Sn–Ag0.04–Cu0.005 (SAC405) solder filler metal alloy under the influence of USW resulted in significant changes to the microstructure of the solder joints. The thickness of the Cu6Sn5 intermetallic layer at the Cu/SAC interface of the USW-solidified soldered joint decreased by as much as 76%. The β-Sn dendrite width was also reduced by as much as 67%, and the SAC matrix was filled with bundles of acicular Cu6Sn5 crystals. The formation of Ag3Sn plates was also prevented, and the size of the rod-like SAC ternary eutectic matrix was reduced from 700 nm to 50 nm. This behaviour is attributed to the effect of cavitation and liquid metal streaming induced by the USW on nucleation and the whole solidification process. The presence of Cu6Sn5 bundles and the refined eutectic and β-Sn dendrites in the matrix led to an average improvement in the hardness of the solder bulk by 45%.