Kinetic Analysis Of Austenite â†' Ferrite Phase Transformation Subjected To External Minor Stress

The microstructural control during the solid-state phase transformation has become the most efficient method in the developing of a new generation of iron and steel materials. In recent years,due to the progress of TMCP (Thermo-Mechanical Control Process), theγ→αtransformation under stresses has attracted great attention in the field of materials. However, the study on this transformation is still insufficient, especially on the mechanism of theγ→αtransformation which subjected to tiny stresses.In this paper theγ→αtransformation behaviors of substitutional Fe-2.96at. %Ni alloys subjected to external transient tiny stresses in the austenite field were investigated by high resolution differential dilatometer. With the aid of the microstructural analysis and the measurement of mean grain size of the samples, the phase transformation model was developed and employed to analyze the kinetics and thermodynamics of the phase transformation. Finally, the influence of various stresses and loading temperatures to theγ→αtransformation was clarified. It was found out that:1. The improved isochronal transformation model had been modified successfully to suit the cooling condition.2. When different stresses were applied on the alloys at 1273K, the higher the applied external tiny stress was, the lower the onset temperature of theγ→αtransformation would be. The whole transformation time was shorter, and the average grain diameter was smaller, but the grain size distribution was less homogeneous, large grains accounting for a lower percentage.3. When same stress were applied on the alloys at different temperatures (1073K and 1048K), the lower the temperature was, the higher the onset temperature of theγ→αtransformation and the smaller the average grain diameter would be. The applied external stresses also decreased the time of the whole transformation, but a lower loading temperature lead to a longer transformation time.4. By fitting the experimental results of theγ→αphase transformation with the phase transformation model, it was found that: Generally, stress induced a raise of the nucleus density, but a decrease of the prefactor ( v₀) and the activation energy (Q_G ) in growth model. The higher the stress was, the higher the nucleus density would be, but both the prefactor and the activation energy were lower. The lower the loading temperature was, the higher the nucleus density, the prefactor and the growth activation energy would be.5. It is purposed that the grains will be greatly refined by choosing suitable quenching temperature in the austenite field and applied external tiny stress.