Solid-state Transformation Kinetics Research Articles

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel

Solid-state transformation kinetics of a 350 grade commercial maraging steel were investigated using a nonisothermal dilatometric technique. Two solid-state reactions—namely, precipitation of intermetallic phases from supersaturated martensite and reversion of martensite to austenite—were identified. Determination was made of the temperatures at which the rates of these reactions reached a maximum at different heating rates. The kinetics of the individual reactions in terms of activation energy were analyzed by simplified procedures based on the Kissinger equation. An estimated activation energy of 145 ± 4 kJ/mol for the precipitation of intermetallic phases was in good agreement with reported results based on the isothermal hardness measurement technique. Martensite to austenite reversion was associated with an activation energy of 224 ± 4 kJ/mol, which is very close to the activation energy for diffusion of substitutional elements in ferrite. Results were supplemented with microstructural analysis.

The thermodynamics and competitive kinetics of metastable τ phase development in MnAl-base alloys

Abstract The solidification of a metastable ferromagnetic τ phase in undercooled MnAlC alloys is examined in terms of the relative thermodynamic phase stability and competitive kinetics. The heat of transformation ΔH t τ → ϵ of the metastable τ phase to the equilibrium h.c.p. ϵ phase and the heat of fusion ΔH f ϵ → l of the h.c.p. ϵ phase in a Mn 0.55 Al 0.433 C 0.017 alloy were measured to determine the Gibbs free energy differences between the metastable and stable phases as a function of temperature. The results indicate that a minimum amount of undercooling of ΔT = 87 K is required to form the metastable ferromagnetic τ phase over the equilibrium phases in a Mn 0.55 Al 0.433 C 0.017 alloy. The thermodynamic evaluation was applied further to develop a complete temperature-time-transformation (TTT) diagram of the Mn 0.55 Al 0.433 C 0.017 alloy based on a solid-state transformation kinetics analysis of the ϵ → τ reaction during isothermal treatments. This analysis reveals the critical cooling rate of the order of 20 K s −1 to bypass the metastable τ phase nucleation during solid state transformation in agreement with empirical observations. The combined thermodynamic and competitive kinetics analysis provide a unified guide for the processing of near equiatomic MnAl alloys.

Solidification and solid-state transformation mechanisms in Si alloyed high-chromium white cast irons

Chromium white cast irons are widely used in environments where severe abrasion resistance is a dominant requirement. To improve the wear resistance of these commercially important irons, the United States Bureau of Mines and CSIRO Australia are studying their solidification and solid-state transformation kinetics. A ternary Fe-Cr-C iron with 17.8 wt pct (pct) Cr and 3.0 pct C was compared with commercially available irons of similar Cr and C contents with Si contents between 1.6 and 2.2 pct. The irons were solidified and cooled at rates of 0.03 and 0.17 K · s-1 to 873 K. Differential thermal analysis (DTA) showed that Si depresses the eutectic reaction temperature and suggests that is has no effect upon the volume of eutectic carbides formed during solidification. Microprobe analysis revealed that austenite dendrites within the Si alloyed irons cooled at 0.03 and 0.17 K·s-1 had C and Cr contents that were lower than those of dendrites within the ternary alloy cooled at the same cooling rate and a Si alloyed iron that was water quenched from the eutectic temperature. These lower values were shown by image analysis to be the result of both solid-state growth (coarsening) of the eutectic carbides and some secondary carbide formation. Hardness measurements in the as-cast condition and after soaking in liquid nitrogen suggest an increase in the martensite start temperature as the Si content was increased. It is concluded that Si’s effect on increasing the size and volume fraction of eutectic carbides and increasing the matrix hardness should lead to improved wear resistance over regular high-chromium white cast irons.

Analysis of nonisothermal transformation kinetics; tempering of iron-carbon and iron-nitrogen martensites

A powerful method for the analysis of solid-state transformation kinetics can be based on non-isothermal analysis where the temperature corresponding to a fixed stage of transformation is measured as a function of heating rate. Adoption of a specific kinetic model is not required for valid application of this method. This removes constraints imposed unnecessarily on so-called Kissinger-like procedures. The method was applied to study and to compare the tempering kinetics of iron-carbon martensite and iron-nitrogen martensite (about 1.1 wt pct interstitials). Dilatometry and differential scanning calorimetry were employed. This combination of experimental techniques allowed distinction between preprecipitation processes as segregation and clustering of interstitials. In contrast with iron-carbon martensite, no indications were obtained for clustering of interstitials in iron-nitrogen martensite. The preprecipitation stages have activation energies of 75 to 85 kJ/mole, which are ascribed to volume diffusion of interstitials. The precipitation of the transition carbide/ nitride (first stage of tempering) occurs with an activation energy of 110 to 125 kJ/mole, which is ascribed to pipe diffusion of iron. The precipitation of the equilibrium carbide/nitride (third stage of tempering) is associated with an activation energy of 195 to 205 kJ/mole which is ascribed to combined pipe and volume diffusion of iron.

Analysis of transformation kinetics by nonisothermal dilatometry

Nonisothermal dilatometry was shown to provide a powerful method for the analysis of solid-state transformation kinetics by measuring the shifts of the temperatures of inflection points on dilatometric curves as a function of heating rate. The method was applied to iron-nitrogen martensite specimens. To that end iron was nitrided in a NH3/H2 gas mixture and subsequently quenched. The experiments allowed the assessment of residual terms in the theoretical development. To demonstrate the high sensitivity of the method, the activation energy was determined for the initial stage of tempering iron-nitrogen martensite, involving a minute decrease of specific volume. The method developed is equally applicable to methods where other physical properties of the material under investigation(e.g., electrical resistivity, magnetization) are measured on isochronal annealing.

Critical examination of the DTA techniques used to study the kinetics of solid-state transformations

The use of the different equations proposed to study the kinetics of the glass-to-crystal transition from DTA curves has been extended to include the evaluation of the kinetic parameters of polymorphic transitionS. The transformation kinetics of CaCO3 (aragonite to calcite) and PbO (litharge to massicot) are the examples studied. The kinetic results of both transformations, obtained by the analysis of DTA curves, differ widely from those revealed by the analysis of the isothermal kinetic data. This discrepancy is probably due to erroneous simplifications used in deriving the equations applied in non-isothermal conditions.