After annealing in the two-phase region (non-isothermal heating) and quenching, the retained austenite with volume fraction of ~ 33 vol. % is observed at room temperature in nanostructured Fe91Ni8Zr1 alloy. EDS analysis in TEM shows that the Ni content in retained austenite is about 11 at. %, which is far below the value (at least 28 at. %) in retained austenite according to extensive studies in Fe-Ni alloys (for example, Lattice Parameters of Martensite and Austenite in Fe–Ni Alloys in Journal of Applied Physics 40, 3453 (1969)). On the other hand, if the Ni content in austenite for my alloy has at least 28 at. %, the total value 0.33X28%=0.0924 will bigger than the nominated content (~ 0.08). Two points are highlighted here: (i) Zr has no direct effect on the phase transition in Fe-Ni alloys, and it is not believed that Zr is responsible for the formation of austenite at room temperature; and (ii) TEM image shows that austenite is still in nanoscale. It is claimed that size effect may explain the presence of austenite, for example, martensite is hindered due to the nanoscale-size effect. However, quantitative analysis remains lack. So, what is the true reason for the presence of austenite at room temperature in my system?
In nano-scale, strain energy is more important than chemical energy. In addition, TEM sample prepare (cutting, grinding..) will make surface deformation of metal materials.
The local strain energy around the ppt. is 3 dimensions and very larger than the applied stress, which is unaxial. This local strain energy has important effect on the phase transformation.
Hi Rafael, thanks for your suggestion. ICP-MS or XPS may be not effective for my alloy, since both ferrite and austenite are very fine(generally, < 100nm). Though, TEM is a very local method; the EDS data from several points in the TEM samples are attained, and the results are therefore resonable.
Acutally, by increasing the annealling temperture and then quenching, the volume fraction of retained austenite at room temperture is about 44 vol %, if the "theoretical" value, 28 at % Ni is contained, the total content of Ni will exceed than 12 at %.
Thus, the Ni content, I argue, is partially responsible for the formation of austenite. The most possible explaination is size effect of austenite.
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