| The wide use of light weighted Fe-Mn-Al-C alloys in automobile industry and cryogenic environment is due to not only their ultra-high product of strength and elongation and excellent work hardening capability, but also their low ductile-brittle trans ition temperature and cheap alloy addictions, which can replace the role of austenitic stainless steel in a wide range. The study found that after aging treatment at medium temperature, dispersed(FeMn)3AlC carbide will foremed in the austenite, thereby changing the mechanical properties of this material. The possible mechanism for the precipitation of carbides, whether is spinodal or nucleation-growth, is still in argument. Therefore, the present work attempts to reveal the features of carbide precipitation from the viewpoint of thermodynamics.In this work, the first step was to modify the criterion of thermodynamic spinodal decomposition for ternary alloys and proposed a simpler mathematical criterion for application with c lear expression. Meanwihle, the thermodynamic expression concerning composition and temperature relationship of Fe-Mn-Al-C alloys, on the basis of the central atom model, was established. In orde to analyze the role of Al and C elements in the carbide precipitation, a pseudo ternary vers ion was obtained by setting Mn content as constant. This also fits the requirement of ternary criterion for spinodal decomposition.According to the calculation, in the Al added twinning induced plasticity steel Fe-Mn-Al-C alloys, the spinodal decomposition zone is mainly determined by the concentration of Al and C and aging temperature simultaneously. The ordered carbide precipitation(FeMn)3AlC, namely K phase, can form with spinodal decomposition manner only when both C and Al concentration are high and aging temperature is low. Even for this case, spinodal decomposition only occurs in the early aging stage. With the depletion of Al and C in the surrounding parent phase, further formation of K phase will gradually turn to nucleation-growth manner.In addition, the main conclusions are as follows:1) When the matrix was in austenite state, the content of Mn had no significant influence on the formation mechanism of K phase, indicating that the simplification of alloy from quaternary to pseudo-ternary was reasonable. The increase of aging temperature will reduce the spinodal composition zone of Al and C. When the strain energy was considered, the composition zone will shrink further. Therefore, it is unreasonable to attribute the formation of K phse to spinodal decomposition only. When the content of Al and C were low in the alloy, the formation of K phase may be dominated by the traditional nuc leation-growth mechanism. The K phase with different formation mechanisms may exhibit different mechanical properties and corrosion resistance.2) In order to reveal the role of Al and C further in the precipitation of K phase, the as-quenced Fe-26Mn-8Al-1C alloys was aged isothermally at 823 K for different time. The relationship between the size of K phase and agint time was obtained. The growth rate was vefiried by diffusion equation. It was found that it is Al, but not C, determines the grwoth of K phase. The main effect of C is to garrantee K phase as the ordered face centered cubic structure. |
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