High-Resolution Transmission Electron Microscope Investigation of Irrational Interfaces in Fe-Ni Alloys Undergoing Massive Transformation

Project Summary
Irrational (and incoherent) interfaces have played a central role in the theory of the massive transformation, where long-range diffusion is not required, and developing an understanding of these interfaces presents an opportunity to greatly expand our understanding of inter phase boundary (IPB) structure and properties. The major objective of this research is to determine the structures of massive transformation interfaces in Fe-3.3 at.% Ni alloy by HRTEM, and to develop a theoretical basis for understanding and predicting the structure of IPBs with irrational orientation relationships and habit planes, based on crystallographic theories such as those of Pond [1-3] and Sutton [4]. Particular attention will be paid to understanding the relationship between diffusional fluxes associated with high-index irrational interfaces, since this relationship is not well understood, but important in terms of the kinetic behavior of phase transformations such as in the massive transformation. Many important theoretical concepts related to these studies have been recently developed in collaboration with Aaronson and Hirth [5-6], and it is anticipated that this fruitful collaboration be extended in the proposed research.
In this project, we perform experimental HRTEM analyses on alloys undergoing massive transformation. Using the HRTEM images, we reveal how atoms match across irrational interfaces as well as the types of defects that are present at these interfaces. These observations will be compared with the types of interfacial structures that are predicted based on the crystallographic theories mentioned above. The Fe-Ni system is particularly interesting for study because Widmanstatten precipitate plates (as found in meteorites), massive grains and martensite plates all form in this alloy depending on the cooling rate from a high-temperature solutionizing treatment. Thus, this alloy provides the opportunity to observe the relationship among the transformation crystallography, interfacial structure, thermodynamics and kinetics for several distinctly different types of transformations, all in a single alloy. Such a study is likely to yield new insight into the behavior of interfaces in phase transformations.

Figure 1 shows an optical microstructure of a specimen ice brine quenched (IBQ) after solution treatment at 1300C. The microstructure consists of mix of martensite and massive ferrite grains, where the martensite phase was darkly imaged with smooth boundaries and the massive ferrite grains show irregular one. This result indicates that the nucleation and growth kinetics of massive transformation in this alloy with IBQ is comparable to that of martensitic transformation.

optical x200

A TEM micrograph shown in Fig. 2 much clearly demonstrate the difference in boundary structures of martensite and massive phases. The martensite has a lath structure with sharp smooth boundaries whereas the massive grains appear to be blocky with irregular boundaries. It should be noted that there are large number of dislocations observed interior of massive grains indicating that massive ferrite was partly deformed by the large transformation strain.
TEM x10k

The result of further study will be published and announced.

References
[1] R. C. Pond, "Chapter 38 - Line Defects at Interfaces", in Dislocations in Solids, F. R. N. Nabarro, Ed., Elsevier, Amsterdam (1989).
[2] R. C. Pond and J. P. Hirth, "Defects at Surfaces and Interfaces", in Solid State Physics, Advances in Research and Applications Vol. 47, H. Ehrenreich and D. Turnbull, Eds., Academic Press, San Diego, p. 288 (1994).
[3] J. P. Hirth and R. C. Pond, "Steps, Dislocations and Disconnections as Interface Defects Relating to Structure and Phase Transformations", Acta Mater., vol. 44, 4749 (1996).
[4] R. C. Pond and F. Sarrazit, "Diffusive Fluxes Associated with Interfacial Defect Motion and Interaction", Interface Science, vol. 4, 99 (1996).
[5] A. P. Sutton, "Irrational Interfaces", Prog. Mater. Sci., vol. 36, 167 (1992).
[6] J. M. Howe, H. I. Aaronson and J. P. Hirth, "Aspects of Interphase Boundary Structure", Acta Mater., .