Volume 93, Issue 12

Bulk and grain boundary (GB) diffusion of Nb in α-Fe were studied in the temperature interval from about 700 to 1100 K by the radiotracer method combined with serial sectioning. A non-Arrhenius temperature dependence of Nb bulk diffusion was established below the Curie temperature T c . This observation was discussed in relation to the well-known magnetically-induced diffusion anomaly in α-Fe and to the contribution of dislocation diffusion. In contrast, a practically linear Arrhenius temperature dependence was found for Nb GB diffusion. At T c , bulk and GB diffusion of Nb are about one order of magnitude faster than the corresponding self-diffusivities of α-Fe. The effect of matrix purity on Nb GB diffusion in α-Fe was studied.

Fatigue lives of grain boundaries (GBs) and component crystals were measured by using [123]/[335] and [5 9 13]/ [579] Cu bicrystals with a perpendicular GB at room temperature in air. The results show that the GBs were always the preferential sites for fatigue crack initiation and propagation in the two groups of bicrystals under cyclic tensiontension loading as well as push-pull straining control. When an [123]/[335] bicrystal with reduced section area was tested by cyclic tension-tension loading, the [335] component crystal always had a relatively higher fatigue life than the [123] component crystal. As a result, the fatigue life increased in the order of GB, [123] and [335] component crystals. When the two groups of bicrystals were subjected to cyclic push-pull straining, the GB of the [5 9 13]/[579] bicrystal showed a higher fatigue life than that of the [123]/[335] bicrystal, which was suggested to be partially attributed to the difference in the component crystal orientations in the two bicrystals. By using scanning electron microscopy and electron channeling contrast techniques, the fatigue damage features on the surfaces and fatigue fractography were examined. Based on the present results, the fatigue cracking mechanisms along GBs and persistent slip bands in Cu bicrystals are discussed.

The morphology and growth kinetics of the intermetallic compound formed during the reaction between liquid Sn-20In-2.8Ag solder and Ag substrate are studied in the temperature range 225–325 °C. The results indicate that the intermetallic compound Ag 2+ x (In, Sn) of scallop shape appears at the interface. The intermetallic thickness versus reaction time shows a parabolic relation, indicating that the growth of this intermetallic is diffusion-controlled. The activation energy calculated from the Arrhenius plot of the intermetallic growth rate is 41.6 kJ/mol. By marking the original interface with a Ta thin film, the dissolution of Ag into solder and the growth mechanism of the intermetallic compound have been clarified.

A neural network model for the calculation of the phase regions of the continuous cooling transformation (CCT) diagram of engineering steels has been developed. The model is based on experimental CCT diagrams of 459 low-alloy steels, and calculates the CCT diagram as a function of composition and austenitisation temperature. In considering the composition, 9 alloying elements are taken into account. The model reproduces the original diagrams rather accurately, with deviations that are not larger than the average experimental inaccuracy of the experimental diagrams. Therefore, it can be considered an adequate alternative to the experimental determination of the CCT diagram of a certain steel within the composition range used. The effects of alloying elements can be quantified, either individually or in combination, with the model. Nonlinear composition dependencies are observed.

The neural network model of Van der Wolk et al. [1] describes the effect of composition on the phase regions of the continuous cooling transformation (CCT) diagram, yet does not consider the fractions of microstructural components and the hardness data that are often quoted in CCT diagrams. In the present paper, the construction of two more neural network models, one for the fractions of ferrite, pearlite, bainite and martensite in the microstructure, and one for the hardness after cooling, using the data of 338 and 412 diagrams, respectively. The accuracy of each model was found to be similar to the expected experimental error; moreover, the models were found to be mutually consistent, although they have been constructed independently. Furthermore, the trends in these properties for alloying elements can be quantified with the models, and are largely in line with metallurgical expectations.

The detailed analysis of X-ray diffraction data obtained from intercritically annealed and isothermally transformed low-alloy FeCMnSi TRIP-aided steels reveals that the microstructure contains athermal plate martensite and Fe 2 C η carbide in addition to ferrite, bainite and residual austenite. Neutron diffraction shows that athermal plate martensite can be formed at room temperature in the isolated austenite phase. Whereas the formation of athermal martensite leads to compressive strains in the austenite, the formation of strain-induced martensite results in tensile straining of the austenite. The strain-induced transformation leads to the formation of a martensite of low tetragonality. Low-temperature annealing leads to the formation of η carbide in both the athermal and strain-induced martensite.

A novel X-ray diffraction method, allowing the position-resolved imaging of a polycrystalline specimen using the diffracted radiation, was applied for in situ investigation of recrystallization of cold-rolled copper. A large area of the specimen could be observed simultaneously, yielding information about nucleation and growth of many individual crystallites. The recrystallization process showed a stochastic behavior which can be described by the model of self-organized criticality.

The strain softening effect is exemplified on Al rods (99.8 wt.% purity) torsioned after pre-deformation by rolling, where that effect is most distinct. Observations of thin foils in Al rods, parallel to cross – sectional plane as well as along longitudinal section, were carried out on samples deformed up to the shear deformation φ = 0.645, i. e., near the end of the stress softening range, and on samples deformed up to the shear deformation φ = 5.193, i. e., near the maximum of the strain hardening range. In the strain softening range we have noted: (a) a decrease in dislocation density in the grain interior and in the grain boundary, and (b) formation of a strong (001)[110] shear texture component. Growing dislocation density and formation of axial ⟨111⟩+⟨100⟩ texture components were observed during torsion in the strain hardening range.

The microstructure of as-cast hypereutectic Al-(17 or 25) Si- x La (x - 0, 0.4, 1, 3, 6 wt.%) alloys and the effect of La on the Si phase have been investigated. The microstructure was examined using optical and electron microscopy. The modification effects of La addition on the morphology and size of both the primary and the eutectic Si were observed. However, such effects are uneven and there seems no systematic change for the effectiveness of La modification with increasing La contents. The modification of La may be explained by the suppression of the growth of Si crystals through reduction of the eutectic reaction temperature or through enveloping of primary Si crystals, or additional twinning. Large particles of La-rich compounds are always observed associated with large primary Si crystals and unmodified eutectic Si crystals, while small La-rich particles are associated with small primary Si crystals and modified eutectic Si crystals. The overall concentration of La-rich compounds, formed in the Al–Si alloys containing more than 1 wt.% La, could be represented as a ternary AlSi 2 La 2 phase, which actually contains the LaSi 2 phase and an unknown ternary Al x Si y La phase.

A modified mixing and casting procedure was used for the preparation of Al-1.2 Si-0.8 Fe–Gr composite (wt.%). Graphite and gray cast iron powders were blended and dispersed in Al-1.2 Si matrix alloy melt and poured in a cast iron mould. Gray cast iron powder worked as carrier for graphite particles. The presence of graphite improves the ultimate tensile strength, tensile and compressive stress but beyond 1 % of graphite all properties deteriorate. However, hardness continuously increases and percentage elongation reduces with graphite. In wear studies, different mechanisms: mild-oxidative, oxidative-metallic and severe-metallic, are operative in the different combinations of sliding velocities and applied loads.

Conventional pressure die-casting was used to produce δ-Al2O 3 (Saffil) short fiber reinforced Zn–Al based metal matrix composites, containing 10, 15, 20, 25 and 30 vol.% fibers. The temperature dependence of the elasticity modulus of the composites and the matrix material were measured at temperatures from 20 to 200 °C, at 20 °C intervals. It was found that the modulus of elasticity of the composites gradually increased with increasing fiber volume, but decreased with increasing temperature. The rate of change in both, however, differed depending on the fiber volume content of the composites in such a manner that the higher the volume content of fibers the lower the rates of decrease in the modulus of elasticity values with increasing fiber volume and temperature. In addition, the data obtained from the Young’s modulus measurements over the temperature range studied were compared with theoretical results predicted by the shear-lag and Nielsen–Chen models and found to be in a better agreement with those of the later model.