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Isothermal transformation from austenite in an Fe-9.14 pct Ni alloy has been studied by optical metallography and examination by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In the temperature range 565 ЊC and 545 ЊC, massive ferrite (␣ q ) forms first at prior austenite grain boundaries, followed by Widmanstätten ferrite (␣ W ) growing from this grain boundary ferrite. Between 495 ЊC and 535 ЊC, Widmanstätten ferrite is thought to grow directly from the austenite grain boundaries. Both these transformations do not go to completion and reasons for this are discussed. These composition invariant transformations occur below T 0 in the two-phase field (␣ ϩ ␥). Previous work on the same alloy showed that transformation occurred to ␣ q and ␣ W on furnace cooling, while analytical TEM showed an increase of Ni at the massive ferrite grain boundaries, indicating local partitioning of Ni at the transformation interface. An Fe-3.47 pct Ni alloy transformed to equiaxed ferrite at 707 ЊC Ϯ 5 ЊC inside the single-phase field on air cooling. This is in agreement with data from other sources, although equiaxed ferrite in Fe-C alloys forms in the two-phase region. The application of theories of growth of two types of massive transformation by Hillert and his colleagues are discussed.

In the middle of last century, some scientists discovered grain-boundary anelastic relaxation (GAR) peaks by means of torsional pendulum. Later, various models about the origin of GAR peaks are established through further research. However, its micro-mechanism is still unclear. Recently, according to the results of solute grain boundary segregation or dilution caused by elastic stress, a micro-mechanism of GAR which is grain-boundary absorbing or emitting vacancies has been proposed. Then, the equilibrium equations and the kinetic equations of GAR are established, and the process of GAR is expressed analytically. Furthermore, it has successfully elaborated the intermediate temperature embrittlement peak movement which exists widely in metals. Those developments of GAR theory are reviewed in the present paper.

Serrated grain boundaries in conventional cast nickel base superalloys can enhance high temperature mechanical properties of the alloy due to a large number of precipitates arranged in grain boundaries, which effectively inhibited the movement of dislocations. During service at elevated temperatures, the ripening, coalescence or degeneration process may occur in these precipitates, resulting in the coarsening of grain boundaries with time. This paper aims to investigate the coarsening kinetics and microstructure evolution of serrated grain boundaries in a conventional nickel base superalloy during long term aging. After solution and aging heat treatment, a long term aging treatment at 900 ¥ for 3000 h was carried out on a cast K480 nickel base superalloy. The microstructure of grain