Thermodynamics of martensite transformation in low alloy TRIP steel was evaluated.It follows from the calculation that ε phase might be formed at above room temperature in the low Mn steel.Tested steel was therefore prepared according to the estimation result.High volume fraction of retained austenite(above 20%) containing 1.37%C was obtained in the steel after continuous annealing treatment.Tensile test showed that the ultimate strength of the steel was 1248 MPa,elongation rate 25%,the product of strength and elongation rate 31200 MPa%,reaching the grade of the third generation automobile steel.XRD detection indicated that after tensile test there existed ' martensite while TEM revealed that ε martensite also existed in the steel.Strengthening-toughening of the steel was thus raised due to the effect of transformation induced plasticity of two kinds of martensite.In addition,the phase stability of carbonitrates containing V was estimated and the complex micro-alloying effect was applied to the steel to control the size of austenite(about 1 μm) and enhance the stability of austenite.
LI LinGAO YiZHU NaQiongHE YanLinLIU RenDongHE ZhongPingSHI WenZHANG Mei
New experimental measurements of the phase diagram and the mixing enthalpy of liquid phase along with the previous experimental data were used in a reassessment of the Ni-Ga system.A set of self-consistent thermodynamic parameters of the Ni-Ga system is obtained using the calculation of phase diagram(CALPHAD) technique,and the available phase diagram and thermodynamic data are reproduced well within experimental error limits.Some noticeable improvements are obtained in the present work compared with the previous assessment:(1) the calculated Ga-rich liquidus is more reasonable;(2) Ni_3Ga_7 is adopted as the most Ga-rich compound rather than NiGa_4;(3) the Ni_5Ga_3 phase is treated as the nonstoichiometric compound with consideration of its narrow homogeneity range;(4) the phase transformation between B8_(1.5)_Ni_3Ga_2 and Ni_(13)Ga_9 is considered instead of treating them as Ni_3Ga_2 phase simply;(5) the latest experimental data of mixing enthalpy for the liquid phase are adopted.The present study can be used as a basis for the development of a thermodynamic database of the Ni-based semiconductor alloy systems.
