Mechanisms of phase transformations within the miscibility gap of Fe-rich Fe-Al alloys

The structure and properties of age hardened Cu-Ti alloys

The decomposition of alloys roughly based on the composition Ti3Al with ternary additions of Nb has been studied through the use of transmission electron microscopy and selected area electron diffraction. It has been shown that a wide variety of transformation behavior in the metastable high temperature β phase can be produced through varying composition, cooling rates, and heat treatment. Transformations observed during quenching from the β phase field include the formation of hcp martensite in alloys with low Nb contents, the occurrence of B2(CsCl)-type ordering over a wide composition range in alloys richer in Nb, and the formation of an “ω-type≓ phase in the parent matrix subsequent to B2 ordering. In many of the as-quenched alloys a “tweed-like≓ or modulated micro-structure is observed with accompanying elaborate networks of rel rods and local diffuse maxima in electron diffraction patterns. During aging the decomposition proceeds through a complex sequence of reactions the nature of which will be discussed below. Many of the anomalies observed in these alloys are quite similar to behavior observed during decomposition of other ordered β(B2, DO3)-type phases in a wide variety of other alloy systems.

Phase transformations and modulated microstructures in Ti-Al-Nb alloys

We describe an electron microscopic investigation of an as-quenched 63 at. % Ni-Al alloy. This material exhibits strong tweed contrast and associated diffraction effects, commonly called ‘premartensitic effects’. Anomalous diffraction effects in martensite-forming β3 Hume-Rothery phases have been studied extensively, but the mottled microstructures have received little attention. This paper describes an attempt to characterize the microstructure systematically. In addition to some well-known characteristics, we have discovered that in many ways tweed contrast resembles the fringe contrast observed on stacking faults. This includes the variation of fringe spacing with foil orientation, with the extinction distance of the diffraction vector and with the deviation from the Bragg condition. Such behaviour provides a basis for understanding the origin of premartensitic effects in Hume-Rothery phases, and tweed microstructures in general.

Tweed microstructures I. Characterization in β-NiAl

The thin foil transmission electron microscope technique has been used to study the ordering of CuAu I. At low annealing temperatures ordered plate-like CuAu I nuclei are formed which are coherent with the disordered matrix and parallel to the (101) planes. At higher annealing temperatures short range strain accommodations in the form of microtwinning occur at first and later long range accommodations in the form of coarse twin lamellae. The latter result from the accommodations to strains set up by the growth of ordered lattice domains whose nucleation centers had different c-axis orientations. The orientation of the twin planes is (101) and the twins appear to be nucleated by other twins or on free surfaces and not by dislocations. The ordered domains become disordered at the twin interfaces but remain intact within the lamellae. The increase in mechanical strength upon ordering is related to the coherency strains between ordered nuclei and disordered matrix and the decline of the mechanical strength is due to the elimination of coherency strains by twinning.

L.E. Tanner

Papers

Comments on “the early stages of the transformation in dilute alloys of titanium in nickel”

Experimental observations of continuous transformations in binary alloys