Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

The “black pad” phenomenon, which refers to the blackening of electroless-plated nickel–phosphorus [Ni(P)] films during the immersion Au process, is reproduced using pure chemicals and its fundamental mechanisms are investigated. In the present analysis, under bump metallurgy (UBM) materials have profound effects on the black pad susceptibility, and the presence of abnormally large nodules (ALNs) is essential to the black pad occurrence. The Ni(P) films over Cu, Ag, and Au substrates all exhibit ALNs and are susceptible to black pads, while those over Ni and Co substrates do not have ALNs and therefore are not susceptible to black pad. In the former cases, submicron scale nodular variations of the surface curvature lead to variations in the P concentration in the Ni(P) films, which induces sufficiently large potential differences to drive galvanic corrosion when exposed to the electrolyte, which is a gold cyanide solution in this study. The UBM effect is ascribed to differences in the Ni(P) film growth mode, where the transition from a layer-by-layer growth mode to an island growth mode is easier over Cu, Ag, and Au UBMs.

The density-functional theory and phase-field dislocation model have been used to compute and simulate the strength of θ′ plates and precipitate-dislocation interactions in an Al-4Cu-0.05Sn (wt pct) alloy that is strengthened exclusively by coherent θ′ precipitate plates. The density-functional theory computation indicates that a 1.06 GPa applied stress is required for a dislocation to shear through a θ′ plate, which is far larger than the critical resolved shear stress increment (ΔCRSS) of the peak-aged sample of the alloy. The ΔCRSS values of the alloy aged for 0.5, 3, 48, and 168 hours at 473 K (200 °C) are computed by the phase-field dislocation model, and they agree well with experimental data. The phase-field simulations suggest that the ΔCRSS value increases with an increase in plate aspect ratio and number density, and that the change of ΔCRSS is not sensitive to the variation of the distribution of θ′ plate diameters when the average diameter of θ′ plates is fixed, and that the coherency strain of θ′ plates does not contribute much to ΔCRSS of the alloy when the θ′ number density and aspect ratio are below certain values. The simulations further suggest that, when the volume fraction of θ′ is constant, the ΔCRSS value for a random spatial distribution of the θ′ plates is 0.78 times of that for a regular spatial distribution.

To properly model the cracking susceptibility during solidification under continuous casting conditions, it is essential to have accurate data. Such data for the mechanical properties of steel during solidification are scarce if not non-existent. An experimental tool called the Mold Cracking Simulator (MCS) has been used to simulate the initial shell formation under continuous casting conditions. As part of the test, the shell is mechanically subjected to deformation. A mathematical model has been developed to translate the force and elongation measured during the MCS trials into stress–strain components. To test the model and validate the assumptions, two steel grades were tested, a peritectic steel grade and a higher-alloyed grade. The results show that the reproducibility of the test is very good and the stress–strain curves are consistent with the steel composition. Moreover, the metallographic and fractographic analysis of the deformed MCS samples shows that the microstructure is comparable to that of a continuously cast product and the cracks generated are interdendritic, i.e., hot tears.

The stress-strain behavior and the development of microstructure between 850 °C and 1150 °C in an austenitic stainless steel, 22Cr-13Ni-5Mn-0.3N, were investigated by uniaxial compression of cylindrical specimens at strain rates between 0.01 and 1 s-1 up to a strain of one. The measured (anisothermal) and corrected (isothermal) flow curves were distinctly different. The flow stress at moderate hot working temperatures, compared to a number of other austenitic alloys, was second only to that of alloy 718. Both static and dynamic recrystallization were observed. Recrystallization was sluggish in comparison to alloy 304L, apparently due to the presence of a fine Cr- and Nb-rich second-phase dispersion, identified as Z phase, which tended to pin the high-angle grain boundaries even at a high temperature of 1113 °C. Recrystallization may also be retarded by preferential res-toration through the competitive process of recovery, which is consistent with the relatively high stacking-fault energy for this alloy. It is concluded that this alloy must be hot worked at temperatures higher than usual for austenitic stainless steels in order to minimize flow stress and refine grain size.

A new creep model is applied to rationalize the stress and temperature dependences of minimum creep rate for a Fe-20Cr-25Ni (wt pct) austenitic stainless steel (Alloy 709). The creep activation energy determined on the basis of this model does not depend on stress and the stress exponent not on temperature. Consequently, it can be used together with the Mankman–Grant relationship to predict the long-term creep strengths and lifetimes at different temperatures using short-term creep rupture data.

Dynamic strain aging at different temperatures and its effects on the strain hardening behavior, dislocation substructure and fracture morphology in a stainless steel grade 430 was investigated. Sheet type specimens were subjected to tensile tests performed at a temperature range of 298 K to 873 K. Subsequently, the strain hardening behavior of the material was depicted via modified Crussard–Jaoul analysis, strain hardening rate, and instantaneous strain hardening exponent curves. Changes in the dislocation substructure during the tests were characterized by means of X-ray diffraction and transmission electron microscopy. Scanning electron microscopy was used to investigate the fracture morphology of the specimens. The results indicated the occurrence of dynamic strain aging from 523 K to 773 K by the presence of the Portevin–Le Chatelier effect. These results were reinforced by the strain hardening analysis that revealed a three staged behavior at most of the studied temperatures, except during the dynamic strain aging regime, which presented an extra stage. Different substructures were observed as a function of the test temperatures: cellular dislocation substructure in the samples deformed at 298 K and 673 K, an array of straight and parallel dislocations in conjunction with a cellular substructure at 673 K, and finally a subgrained substructure with fine precipitates was formed at 873 K. A ductile surface fracture presenting a network of dimples and voids was present at all investigated temperatures, with a dimple size refinement being observed during the dynamic strain aging regime.