Nonequilibrium phases in rapidly quenched Fe-Al-C ternary alloys

Nonequilibrium phases of austenite(Y), ordered austenite (γ′) and hcp epsilon (ε) have been found in Fe-Al-C ternary alloys quenched rapidly from the melt. The formation ranges of these single phases are 2 to 6 pct Al and 1.8 to 2.1 pct C for the 7 phase, 6 to 12 pct Al and 1.7 to 2.1 pct C for the γ′ phase and 2 to 5 pct Al and above 4 pct C for the e phase. The lattice parameter varies from 0.361 to 0.365 nm for the γ phase and from 0.361 to 0.367 nm for the γ′ phase with increasing carbon and aluminum contents and is abouta = 0.264 nm andc = 0.434 nm for the e phase. Among these non-equilibrium phases, the austenite is so ductile that no crack is observed even after closely contacted bending test. The austenite phase has fine subgrains of 0.1 to 0.4 μm diam and the Vickers hardness, yield strength and tensile fracture strength are about 360 DPN, 940 MPa and 995 MPa, respectively, for Fe-4.0 pct Al-2.0 pct C alloy. Thus, due to relatively high hardness and strength combined with good ductility, the nonequilibrium austenite found in Fe-Al-C system is attractive as high-strength materials whose useful dimensions may be limited by critical rapid cooling rates. Formerly Graduate Student of Tohoku University, Formerly Graduate Student of Tohoku University     

Fine structure in quenched Fe-Al-C steels

The origin of “extra” spots in electron diffraction patterns taken from retained austenite and martensite in Fe-7 pct Al-2 pct C has been investigated. The present interpretation differs from that of previous investigators. It is concluded that a high density of fine carbide precipitates 50Å in diameter is responsible for the extra reflections. The precipitates have the perovskite structure and grow epitaxially in the austenite during quenching. When martensite forms at a lower temperature, the precipitates are transformedin situ.     

Fragmentation of the grain structure of quenched rails

Transmission electron microscopy permits layer-by-layer structural analysis (along the central axis and in the direction of the rounded corner) of bulk-quenched and differentially quenched rails at distances of 0, 2, and 10 mm from the working surface. Regardless of the direction and the distance from the working surface, the structure of all the rails consists of plate-pearlite grains and ferrite grains, containing cementite particles of different shape (ferrite–carbide mixture) and grains of structure-free ferrite (ferrite grains that do not contain carbide phase, grain-boundary ferrite). The morphology and defect substructure of the phases are studied; the locations of the stress concentrators are established. Formulas are derived for the fragmentation parameters of the grains in the ferrite–carbide mixture as a function of the heat-treatment conditions and the distance from the working surface.     

On the structure of multiple frank loops in quenched aluminium-silicon alloys

Multiple Frank dislocation loops in quenched aluminium-silicon alloys are analysed by means of transmission electron microscopy. Contrast analysis of intermediate configurations observed during unfaulting, and observation of the shapes during in situ annealing give evidence for a structure in next-nearest layers.     

The interpretation of structure functions in quenched binary alloys

We study the segregation process in quenched binary alloys by analyzing and comparing the time evolution of the structure function and of the grain distribution obtained from computer simulations on a model system. We find good agreement between cluster sizes and densities determined directly on the computer sample and ones obtained by the Guinier method from the structure function. We then describe a graphical method for determining the scaling behaviour of the structure function S(k, t) which gives good statistics because the whole curve S(k, t) vs k is used. This yields very good agreement between the scaling function (scaled with the Guinier radius) obtained from the computer simulations and from a variety of real experiments. This function shows a universal behaviour independent of the alloy composition, the temperature and even the substance investigated. Our results are also not consistent with the more recent theoretical work (Binder et al., Furukawa et al.) which give alternate derivations and extensions of the Guinier formulas.     

Misfit accommodation in a quenched

Quenched and aged specimens of the Al-1.3 at. pct Cu-19.1 at. pct Si alloy were studied by X-ray diffraction. Since this alloy contains a high volume percentage (~20 vol pct) of second- phase Si particles, it is regarded as a model for a metal matrix composite (MMC). During isothermal aging of the solid-quenched Al-1.3 at. pct Cu-19.1 at. pct Si alloy, the Cu and Si atoms precipitate. This causes the Al-rich phase lattice parameter to increase from a value lower to a value higher than the lattice parameter of pure unstrained aluminum. Due to thermal misfit after quenching from heat-treatment temperatures, all lattice parameters are influenced by re- sidual stresses. A model describing the elastic/plastic accommodation of a misfitting spherical inclusion in an infinite matrix is adapted for the case of misfitting inclusions in a finite matrix. This model describes the measured lattice parameter shifts of the Si phase reasonably well. Comparison of the model for elastic accommodation and the model for elastic/plastic accom- modation with measured stresses shows significant discrepancies for the low-temperature range (ΔT < 200 K). These discrepancies may be related to the volume effect of defects (dislocations, vacancies) created in the plastic zone.     

Thermal analysis of the compositional shift in a transient liquid phase during sintering of a ternary Cu-Sn-Bi powder mixture

In this work, differential scanning calorimetry (DSC) and microstructural analysis were used to study the transient-liquid-phase sintering (TLPS) of a Cu-Sn-Bi powder mixture. During sintering, the liquid phase shifts from a Sn-rich (i.e., ∼90 wt pct Sn) to a Bi-rich (i.e., >78 wt pct Bi) composition. In addition, the presence of Bi creates two melting events: a Sn:Bi eutectic reaction at 139 °C and a reaction involving the melting of (Bi) at 191 °C. The Sn:Bi eutectic melting event was fully transient. The melting event at 191 °C was consistent with the formation of a terminal Bi-rich liquid phase. The rate of compositional shift toward this terminal liquid phase at 260 °C was dependent on the rate of the reaction of the Sn with the Cu powder to form intermetallic phases. For mixtures made with medium and fine Cu powder, the terminal Bi-rich composition was reached after isothermal hold times of 20 and 15 minutes, respectively. This resulted in a new melting point for the mixture of 191 °C. For coarse Cu powders, the rate of the compositional shift toward a Bi-rich composition was much slower. The liquid phase remained at a hypoeutectic Sn-Bi composition estimated at 80 wt pct Sn, while the mixture maintained a melting point of 139 °C.