R. Taggart

The influence of iron and aluminum on the precipitation of metastable Ni3Nb phases in the Ni-Nb system

The effect of additions of aluminum and iron on the formation of transitional phases has been examined for alloys of the Ni-Nb system. It has been established that metastable phases of approximate composition Ni3Nb will not form in binary Ni-Nb alloys under normal conditions of quenching and aging, but that iron or iron and aluminum additions promote their formation when certain size and electronic factors are satisfied. In Ni-Nb-Fe alloys containing more than ≈ 12 at. pct Nb, iron promotes the formation of a bct (DO22) Ni3Nb precipitate. Aluminum additions to these Ni-Nb-Fe alloys promote the precipitation of fcc (L12) Ni3Nb γ’, initially together with the bct phase, but at higher aluminum concentrations the tetragonal phase disappears. The appearance of both precipitates followed the Hagel-Beattie requirement for interfacial mismatch of < or ∼ 1 pct between the matrix and the precipitate. The presence of both precipitating phases can be predicted on the basis of the Engel-Brewer correlations. The preferrede/a ratio for the stability of the bct (DO22) phase is 2.50 to 2.62. In the present investigation this phase precipitates only in the presence of iron (bcce/a ratio = 1.5), and only when the lattice constant of the matrix is greater than 3.600Å. The preferrede/a ratio for the stability of the γ’ having a fcc L12 structure is between ∼2.75 and 3.0. Precipitation of this phase occurred only when aluminum(e/a) ratio = 3.0) was present, and when the matrix lattice constant was greater than 3.589Å.

Ni8Ta in nickel-rich Ni−Ta alloys

An ordering reaction has been observed in the nickel-rich binary alloys of the Ni−Ta system that are aged below 570°C following quenching from elevated temperatures. This reaction has been investigated by employing electron diffraction microscopy, X-ray diffraction, and electrical resistivity techniques. The ordered phase has been identified as Ni8Ta and its structure is identical to that of the Ni8Nb (Cb) reported earlier. In the stoichiometric Ni8Ta alloy (Ni-11.1 at. pct Ta) three variants of the Ni8Ta phase are nucleated apparently “homogeneously” throughout the matrix. The precipitate or domain morphology has been identified as a cuboid elongated in one direction such that the cube faces are parallel to the {100} planes of the matrix. The cuboid morphology of the precipitates tends to disappear as the precipitates coalesce and the particle size approaches 1000Å.

A phase separation reaction in a binary titanium-chromium alloy

Abstract Experimental evidence is presented to show that a phase separation reaction occurs in the beta phase of Ti-Cr binary alloys. The reaction has been detected prior to the precipitation of both the omega and the alpha phases, and it becomes a prominent mode of decomposition when omega precipitation is inhibited either by alloying additions or through appropriate heat-treatments. It is suggested that the β2 particles in Ti-Cr alloys are leaner in solute than the matrix. The precipitation of the alpha phase during prolonged aging has been observed to occur on the β2 particles formed during prior phase separation.

The morphology and substructure of Ti-Cu martensite

AbstractThe martensites in Ti-Cu alloys containing up to 8 wt pct Cu have been examined using transmission electron microscopy techniques. The martensite has a massive morphology in the alloys which contain 4 pct Cu or less, whereas the alloys containing 6 and 8 pct Cu exhibit acicular martensite. Experimental evidence is presented to show that the lattice invariant deformation in the massive martensite occurs by internal slip with a Burgers vector

Role of deformation bands in fatigue crack nucleation and propagation in copper crystals

1/3[2\bar 1\bar 13]

The effects of environment on surface pit formation in aluminium

and these results are discussed in terms of recent calculations. The transition from the massive to acicular martensite morphology is also discussed.

Ni8X phases in the systems Ni-V, Ni-V-Nb, and Ni-V-Ta

A new model is presented which makes it possible to predict the lattice parameters of metallic solid solutions as a function of composition. This method is based on the hypothesis that the measured lattice parameter of a solid solution alloy is the average of all the interatomic spacings within a selected region of the lattice.

The influence of constitution and microstructure on the temperature coefficient of resistivity in Ti-base alloys

Abstract The deformation behaviour of copper crystals of [ 1 22] orientation under low strain amplitude cyclic loading is reported. Specimens with square cross section have been cycled to failure and changes in surface topography have been studied using scanning electron microscopy and X-ray Laue techniques. Two sets of deformation bands are shown to form on all four sides of the specimens prior to crack propagation and to lead to the development of surface waviness. The bands form on 45° planes of maximum shear and their structure is discussed. The exact location of the crack nucleation site within a deformation band is shown to be influenced by the slip system with the greatest Schmid factor. The selection of the [ 1 22] crystal orientation, which provides two equally stressed slip systems on the primary plane, is believed to be one of the major factors influencing the deformation characteristics observed, the other being the macroscopic state of stress.