Volume 95, Issue 8

A thorough review and critical evaluation of phase equilibria and thermodynamic data of the nickel – sulfur binary system at 1 bar pressure has been made over the entire composition range for temperatures from 25°C to above the liquidus. The Gibbs energies of nine phases have been modeled, and optimized model parameters have been obtained which reproduce all data simultaneously within experimental error limits. For the liquid phase, the recently extended modified quasichemical model is applied. Two-sublattice models within the framework of the Compound Energy Formalism are used for the high-temperature heazlewoodite and monosulfide pyrrhotite solutions. The Gibbs energies of six stoichiometric compounds are also modeled.

Two unconventional approaches of controlling the size and distribution of primary particles in aluminium and magnesium alloys are suggested. The first approach uses the ultrasonic cavitation treatment for multiplication of solidification sites for primary crystals by activating catalytic impurities that are present in the melt. The second way, on the contrary, involves elimination of active solidification nuclei by melt overheating with subsequent nucleation of primary particles at a higher undercooling. Both approaches result in refinement of primary crystals.

The densities of liquid copper, nickel and iron and their binary alloys have been measured over a wide temperature range, including the undercooled regime. A non-contact technique was used, consisting of electromagnetic levitation and optical dilatometry. In all cases the density was a linear function of temperature. On the other hand, the concentration dependence was only linear for Fe –Ni, whereas Fe –Cu and Cu–Ni are characterized by a positive or negative excess volume respectively.

After Onsager the thermopower of a material is related to the ratio either of two transport coefficients or of the gradients of the electrochemical potential of the electrons and the temperature. In metals the potential is a unique function of the temperature and this ratio can be replaced by the differential quotient, a function obtainable from the density of states of the metal as illustrated on a thermopower isotherm of Nb–Mo alloys.

This paper describes a study of the structural and mechanical properties of Zn-matrix composites discontinuously reinforced by 7 wt.% SiC particles of size 0.7 µm. Commercial Zn-rich Zn–Al –Cu (ZA27) alloy was gas atomized, the resulting powder was mixed with a SiC powder and hot compacted into cylindrical pellets. Metal powder and powder mixture were pressed at 230°C for 60 min, under a pressure of 170 MPa. The compressive strength testing was performed in the temperature range from 20°C to 170°C with a deformation rate of 2.4 × 10 –3 s –1 . Detailed microstructure characterization of samples has been evaluated by optical and scanning electron microscopy.

The thermodynamic properties of Zn 13 La were investigated by calorimetry. The standard entropy of formation at 298 K, ΔfS298° ${{\Delta }_{\text{f}}}S_{298}^{{}^\circ }$ was determined from measuring the heat capacities, C p , from near absolute zero (2 K) to 300 K by the relaxation method. The standard enthalpy of formation at 298 K, ΔfH298° ${{\Delta }_{\text{f}}}H_{298}^{{}^\circ }$ was determined by solution calorimetry in hydrochloric acid solution. The standard Gibbs energy of formation at 298 K, ΔfG298° ${{\Delta }_{\text{f}}}G_{298}^{{}^\circ }$ was determined from these data. The coefficient, γ , of the electronic term contributing to the heat capacity of Zn 13 La was similar to the composition average of the values of pure Zn and La, indicating that the 5d- and 4f-electron states for La are localized in the vicinity of the Fermi energy level.

Bake hardenability in ultra low carbon bake hardening (ULC-BH) steels is primarily influenced by the (i) solute carbon content in the matrix, (ii) dislocation density and (iii) grain boundaries. Grain boundaries in fine-grained steels significantly change the dislocation distribution and carbon-concentration profiles from grain interior to the grain boundary and correspondingly, the bake hardening response may change. The present study examines the effect of different grain sizes (66–15 µm) on the bake hardening behavior of a ULC-BH steel (total carbon content 24 wt.ppm) as a function of tensile strain (2–5%) and aging temperatures (50–170°C). The carbon concentration profiles across the grains of different sizes have been evaluated both through modeling calculations and experimental measurements. Results showed that the bake hardening decreases with decreasing grain size in general, but there is a critical fine grain size corresponding to a specific amount of tensile strain, at which the bake hardening increases significantly due to the contribution of grain boundary carbon and increased dislocation density adjacent to the grain boundaries.

Growth kinetics of Widmanstätten austenite in ferrite in a Fe–C–Mn–Nb high strength low-alloy steel is studied. It is based on the model developed by Ivantsov, Horvay and Cahn, and Trivedi (HIT theory) that describes diffusion-controlled growth of precipitates with shapes approximating to needles or plates. The calculated results using HIT theory with experimental values of the radius of the advancing tip agree well with the experimental observations. It is found that the ratio between calculated and experimental values of the radius of the advancing tip is inversely proportional to the degree of supersaturation. The influence from transformation strain/stress and/or anisotropy of interphase energy on precipitate morphology, which was ignored in the development of HIT theory, is discussed.

The transformation behavior of Ni 50.5 Mn 30.4 Ga 19.1 and Ni 53.7 Mn 26.4 Ga 19.9 was studied with ac magnetic susceptibility, differential scanning calorimetry, in-situ optical microscopy and X-ray diffractometry. The liquidus, solidus and L2 1 ⟶B2’ temperatures were detected for the alloys. The transformation enthalpies of melting, solidification and martensitic/reverse reactions were determined. In Ni 53.7 Mn 26.4 Ga 19.9 the magnetic transition was clearly below the parent to martensite phase transformation and did not have hysteresis. The transformation sequence is suggested as P param ⟶T param ⟶ T ferrom with reverse reactions in respective order. In Ni 50.5 Mn 30.4 Ga 19.1 the temperatures of the magnetic transition and structural phase transformations are close to each other and the Curie temperature shows a hysteresis of 9 K during cooling-heating cycle. Based on the combined measurements, the transformation sequences are assumed to be during cooling P param ⟶ 7M param ⟶ 7M ferrom ⟶ T ferrom and T ferrom ⟶ T param ⟶ 7M param ⟶ P param during heating. Consequently, the hysteresis in the magnetic transition is explained with the intermartensitic reaction and the difference of the Curie points of 7M and T phases.