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Austenite grains that have undergone a martensitic transformation are typically composed of 24 variants that can be categorized by their Bain axis of transformation. There are 3 〈001〉 axes for Bain transformations, therefore the (001) pole figure of a prior austenite grain displays 3-fold symmetry. However, we observed superficially similar prior austenite grains containing 6-fold symmetry in the (001) pole figure. This paper introduces evidence of this 6-fold symmetry and explores the crystallographic origins.

Aged alloys of Al-0.5Si-0.5Ge (at. pct) contain diamond-cubic-A4 precipitates in a dispersion that is much finer than is found in alloys with Si or Ge alone. To help understand this aging behavior, the present work was undertaken to determine alloy composition as a function of aging temperature. The composition was estimated theoretically using a CALPHAD approach, and measured experimentally with energy-dispersive spectroscopy (EDS) in a high-resolution electron microscope. Theory and experiment are in reasonable agreement. As the aging temperature rises, the precipitates become enriched in Si, changing from 50 at. pct in the low-temperature limit to about 80 at. pct Si as the temperature approaches 433 °C, the high-temperature limit of the precipitate field.

The Y modification of a two-phase (γ+α2) TiAl-(Mn,Mo,C) alloy was studied with an aim to improve, mainly, the oxidation resistance and the mechanical properties in a high-temperature air environment. The experimental alloy was prepared by the elemental powder metallurgy (EPM) method. The addition of up to 0.6 at. pct Y resulted in a significant improvement in tensile properties and compressive yield strength and an anomalous yielding phenomenon withal. Two structural characteristics were identified: first, microstructural refinement in terms of the grain size as well as the interlamellar spacing, and second, precipitation of fine oxides that might scavenge harmful oxygen. Deformation was found to be mainly provided by 1/2

The present work addresses the response of Si-Ge precipitates in Al-0.5Si-0.5Ge (at. pct) to thermalcycling treatments of the sort known to refine the shapes of Ge precipitates in binary Al-Ge alloys. Alloys aged at 250 °C contained both small, platelet precipitates on {111} planes and larger, equiaxed precipitates that were heavily twinned. Thermal cycling between 250 °C and 360 °C led to partial or complete dissolution of the platelets. However, the equiaxed particles coarsened at an essentially constant shape; the shape refinement that led to untwinned, single-variant octahedral precipitates in binary Al-Ge did not occur. The apparent reason is the heavy twinning of the Si-Ge precipitates, which produces particles with a nearly spherical shape and rounded, incoherent interfaces and is, hence, a viable mechanism for relaxing the large misfit strain of the precipitate structure. The twinned particles undergo normal coarsening at an essentially constant shape. After thermal cycling, the precipitates contain Si and Ge in the approximate ratio of 70Si-30Ge, which is in the composition range expected for the cycling temperature.

Al−Cu−Si−Ge alloys display a unique combination of ultrarapid aging response, high peak hardness, and extended-aging microstructural stability. The purpose of this work is to explain these properties in terms of the role that the Si−Ge additions have on modifying the conventional Al−Cu aging sequence. In both AlCu and AlCuSiGe, the room-temperature microstructure consists of both Guinier-Preston (GP) zones and ϕ″ precipitates. Upon aging at 190°C, Al−Cu displays the well-known precipitation sequence: the slow dissolution of GP zones and ϕ″ and the gradual formation of ϕ′. In the quaternary alloy, Si−Ge particles quickly nucleate and grow during elevated-temperature aging (they are detected after as little as 30 minutes at 190 °C). The Si−Ge particles then act as nucleation sites for ϕ′ precipitates, resulting in a peak-aged microstructure consisting of a dense distribution of ϕ′ attached to Si−Ge.

The work reported here concerns the effect of Si on the precipitation of θ′ phase (metastable Al2Cu) during the isothermal aging of Al-2Cu-1 Si (wt pct). The binary alloys Al-2Cu and Al-1 Si were studied for comparison. Only two precipitate phases were detected: essentially pure Si in Al-1 Si and Al-2Cu-1 Si, and θ′ (metastable Al2Cu) in Al-2Cu and Al-2Cu-1Si. On aging the ternary alloy at 225 °C, Si precipitates first and catalyzes the θ′ phase. The precipitates in the ternary alloy are smaller, are more densely distributed, have lower aspect ratios, and coarsen more slowly than those in the binary Al-2Cu aged at the same temperature. While the shapes of individual θ′ precipitates in binary Al-2Cu are strongly affected by the kinetic problem of nucleating growth ledges, which produces a significant scatter in the aspect ratio for samples of given thickness, the overall evolution of particle shape with size follows the predictions of the Khachaturyan-Hairapetyan (KH) thermoelastic theory, which reduces to κ=L/d ∞ √L at large sizes. The KH theory provides an estimate for the interfacial tension of the broad Al-θ′ interface of 85 to 96 mJ/m2, which is near the values for other low-energy interfaces in Al, such as the twin boundary energy (100 mJ/m2) and the antiphase boundary energy in δ′ Al3Li (70 mJ/m2). Si and θ′ precipitates in Al-2Cu-1 Si have a strong elastic interaction because of their compensating strain fields. This elastic interaction promotes the nucleation of θ′ precipitates on Si, decreases the expected aspect ratio of θ′, and inhibits coarsening. Finally, Si precipitation in ternary Al-2Cu-1 Si differs from that in binary Al-1 Si in that the Si precipitates are coarser, more equiaxed, and more extensively twinned. These changes appear to be effects of Cu, which increases the solubility of Si in Al and adsorbs on the Si-Al interface, promoting twinning by a “step-poisoning” effect at the interface.

The effects of isothermal tempering on the coarsening behavior of hexagonal M2C precipitates and the secondary hardening reaction in ultrahigh-strength AerMet 100 steel were investigated. The tempering temperatures were 468 °C, 482 °C, and 510 °C, and the tempering time spanned the range from 1 to 400 hours. Experimental studies of the coarsening behavior of the carbides were made by utilizing transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffractometry (XRD). The hardness at the secondary hardening peak was about HRc 55. The average length and diameter of M2C carbides were 4 to 8 nm and 1.5 to 2.5 nm, respectively, at all three tempering temperatures; hence, the aspect ratio was almost 3, an equilibrium value in this case. The size of the M2C carbides increased monotonically with time, but the growth kinetics did not exactly follow the classical coarsening behavior. The amount of precipitated austenite increased with tempering time and temperature. M2C precipitates were still relatively fine even after 200 hours of tempering. This feature seemed to be closely related to the high hardness maintained after prolonged tempering.

This article reports the results of computer simulation studies of reversible, athermal martensitic transformations in idealized, two-dimensional crystals. The transformation is accomplished by se-quentially transforming elementary cells. The model accounts for the elastic strain developed during the transformation, assuming homogeneous elastic constants, negligible interfacial tension, and no external stress. The effects of frictional resistance to the transformation and plastic relaxation of the elastic strain are included in a simple way. The model is used to study the sources of hysteresis in the temperature-transformation (TT) curve and in the microstructural transformation path when the transformation is reversed. The central result is that some hysteresis is inevitable in a transformation of the type studied here. Even in the absence of friction and plastic relaxation, the transformation follows a path in which sequential elements of martensite relax the elastic strain of those that have previously formed. This causes the martensite to form in bursts and has the consequence that the reverse transformation does not reverse the path of the forward transformation. Friction and plastic relaxation increase hysteresis.

This article reports research on the initiation and growth of small fatigue cracks in a nickel-base superalloy (produced commercially by INCO as INCOLOY* 908) at 298 and 77 K. The experimental samples were square-bar specimens with polished surfaces, loaded in fourpoint bending. The crack initiation sites, crack growth rates, and microstructural crack paths were determined, as was the large-crack growth behavior, both at constant load ratio (R) and at constant maximum stress intensity (Kmax). Small surface cracks initiated predominantly at (Nb,Ti)xCy, inclusion particles, and, less frequently, at grain boundaries. Small cracks grew predominantly along {111} planes in individual grains and were perturbed or arrested at grain boundaries. For values of ΔK above the large-crack threshold, ΔKth, the average rate of smallcrack growth was reasonably close to that of large cracks tested under closure-free conditions. However, short-crack growth rates varied widely, reflecting the local heterogeneity of the microstructure. The threshold cyclic stress (Δσth) and the threshold cyclic stress intensity (ΔKσth) for small surface cracks were measured as functions of the crack size, 2c. The results suggest that a combination of the fatigue endurance limit and the threshold stress intensity for closure-free growth of large cracks can be used to define a fatigue-safe load regime.

This study was conducted in order to determine and understand the effect of substrate on the behavior of eutectic In-Sn. Samples for mechanical testing were produced with either bare Cu or Ni on Cu substrates. Both the microstructure and the mechanical behavior are strongly dependent on substrate. When eutectic In-Sn is joined to bare Cu, Cu diffusion into the joint causes the alloy to become off-eutectic, giving a nonuniform and irregular microstructure. The addition of a layer of Ni acts as a diffusion barrier, preventing Cu diffusion sufficiently such that a uniform, normal colony-based eutectic forms. Deformation is more uniform in the In-Sn on Ni, while it is concentrated along the length of the joint in the In-Sn on Cu. This distinction is reflected in the different shapes of shear stress-strain curves between In-Sn on Cu and In-Sn on Ni. The stress exponents and activation energies for creep also vary with substrate. Creep deformation is governed by the In-rich β phase for In-Sn on Cu and by the Sn-rich γ phase for In-Sn on Ni. If In-Sn on Ni samples are aged, the microstructure coarsens and the mechanical behavior changes to resemble that of the as-cast In-Sn on Cu.

The effects of low Au concentrations on the creep properties of a eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90 °C, 0, 0.2, and 1.0 wt pct Au at 65°C, and 0.2 wt pct Au at 25 °C. In the absence of Au, the creep of finegrained eutectic Sn/Pb is dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.

Ni/Au metallization layers are used with increasing frequency to protect Cu substrates in ball grid array microelectronic packaging. The external Au layer provides oxidation and corrosion resistance during storage prior to assembly, while the intermediate Ni layer acts as a diffusion barrier that inhibits the formation of a thick Cu-Sn intermetallic layer during aging. During soldering with eutectic Pb-Sn, the Au dissolves into the molten solder and forms fine, needle-shaped AuSn{sub 4} intermetallic precipitates that are retained in a dense distribution in the bulk of the solder joint after it has solidified. However, recent research by Mei et al. has revealed a new and potentially problematic phenomenon that seems to be peculiar to the Ni/au metallization. They found that after extensive aging (150 C for 2 weeks in their case), the Au-Sn intermetallic redeposited onto the solder-substrate interface. The reconstituted interface was significantly weakened and failed by brittle fracture along the surface between the redeposited Au-Sn and the Ni{sub 3}Sn{sub 4} layer that formed during reflow. While the interfacial redeposition of Au-Sn intermetallics has been observed previously, the mechanism remains unknown. The present work was undertaken to identify the mechanism of this phenomenon and explore methods for controlling it.

The creep rate of Sn solder joints is noticeably affected by joint metallization. Cu|Sn|Cu joints have significantly higher creep rates than Ni|Sn|Cu joints, which, in turn, have higher creep rates than Ni|Sn|Ni joints. Replacing Ni by Cu on both substrates increases the creep rate at 333 K (60 °C) by roughly an order of magnitude. The increased creep rate appears with no apparent change in the dominant creep mechanism; the change in the constitutive equation for creep (the Dorn equation) is in the pre-exponential factor. The decreased creep rate on substituting Ni is accompanied by an increase in the hardness of the polygranular solder but a decrease in the nanohardness of the grain interiors. The source of the strong influence of the Ni substrate appears to be the introduction of an array of Ni3Sn4 intermetallic precipitates along the grain boundaries. These precipitates inhibit grain boundary sliding, boundary reconfiguration, and grain growth during creep. The intermediate creep rate of the asymmetric Ni|Sn|Cu joint has two causes: a decrease in grain boundary mobility due to precipitate decoration and a restriction in the free volume of the joint due to rapid intermetallic growth from the substrate on the Ni side. The sources of this anomalous intermetallic growth are discussed.