G.C. Weatherly

Hydride precipitation in α/β zirconium alloys

Abstract The precipitation of δ-hydride (ZrH 1.5 ) in fine-grained α/β zirconium alloys has been studied. Both interphase and transgranular hydrides are observed, the mechanism of growth being dependent on the nature of the α/β interface. Macroscopic hydrides observed in the optical microscope are always composed of stacks of smaller hydride plates with habit planes close to the basal plane. The apparent habit plane of the macroscopic hydride plate is determined by the geometry of the stack. The stability of different hydride stacks is considered and the role of the texture of the alloy and of a stress (external or internal) acting on the basal planes in determining the orientation of the stacks is explained.

The invariant line and precipitation in a Ni-45 wt% Cr alloy

Abstract A three-dimensional invariant line strain model for f.c.c./b.c.c. phase transformations is developed and used to predict the growth direction and habit plane of lath-shaped precipitates. The results of this approach are shown to be essentially identical to those that can be derived from Bollmanns 0-lattice theory for a Kurdjumov-Sachs orientation relationship. Both theories are in excellent agreement with the experimental observations of Cr-rich (b.c.c.) precipitates in a Ni-Cr alloy. The growth direction of the lath precipitates lies ~5 1 2 ° from the common close-packed directions in the two lattices along an invariant (or near-invariant) line. The observed habit plane, {112}f is the unrolated plane of the phase transformation. The second facet plane of the precipitates, lying close to {313}f, is also predicted from the 0-lattice model.

Precipitation in a Cu-0,55 wt.% Cr alloy

Abstract A detailed study was made of the crystallography and strain field contrast at chromium needles precipitated in a Cu-0,55 wt.% Cr alloy. The orientation relationship between the Cu matrix and Cr needles was found to be Kurdjumov-Sachs, with a 〈651〉Cu growth axis being preferred. The strain field of the needles was shown (to a first approximation) to have the same character as that of a dislocation dipole: using this analogy a reasonable match between experimental and computed images was obtained. The magnitude and direction of the Burgers vector of the dislocation dipole suggest complete coherence between the b.c.c. Cr and f.c.c. Cu phases. Experimental evidence for the nature of the precipitate in the early stages of ageing is presented, and the implications of the results for nucleation in this system are considered.

The precipitation of γ-hydride plates in zirconium

Abstract On slow cooling high purity zirconium containing 40 ppm hydrogen, γ-hydride (ZrH) precipitates as acicular plates. This paper considers the possibility that such plates form by a displacive or shear transformation. The habit planes and orientation relationships of hydrides have been determined and related to possible dislocation mechanisms for the transformation. It is shown that habit planes close to {10–17} Zr are predicted for hydrides having a [l10] γ ∥[1210] Zr , (111) γ ~ ∥(0001) Zr orientation relationship. Hydrides with {10-10} habit planes have a [110] γ or [101] γ ∥[1210] Zr and [001] γ or [010] γ ∥[0001] Zr orientation relationship. In this case, the observed habit plane can only be invariant if the pure lattice strain is succeeded by a pure shear shape change. Experimentally, internal twinning in the plates and dislocation arrays at the interface are observed and it is suggested that these accomplish the required shape change. In the final section of the paper, these results are related to the problem of hydride reorientation and hydride cracking in zirconium alloys.

The structure of ledges at plate-shaped precipitates

Abstract The structure and nature of ledges at three faceted precipitates, θ 1 and θ in an aluminum-4 wt. % copper alloy, and β(Mg 2 Si) in an aluminum-1.5 wt.% Mg 2 Si alloy, have been determined by electron microscopy. The nucleation of ledges usually occurs heterogeneously, at precipitate impingements or at APBs. Macroscopic ledges of varying height and morphology, and irregularly spaced, were observed at all three precipitates.

On the nucleation of martensite in iron precipitates

Abstract Copper-rich copper-iron alloys containing small defect-free iron precipitates, which in the austenitic condition have practically the same lattice parameter as the surrounding copper matrix, transform to martensite only when the alloy is cold worked. The transformation occurs by the formation of laths with a Kurdjumov-Sachs orientation relationship. Strain energy calculations indicate that the driving force for the transformation in these precipitates, once having introduced the necessary defect, is large enough to accommodate a lath-shaped martensite in agreement with observation. Energy calculations show that the spontaneous transition of an austenite sphere to a martensite ellipsoid is unlikely; furthermore dislocations moving close to precipitates have no chance of triggering off the transformation. The critical step to bring about the transformation is for the precipitate to be cut by the moving matrix dislocation. However, precipitates smaller than about 200 A in diameter do not transform even although they are cut by dislocations and on this basis it is shown that there is a critical size of martensite nucleus which is only a few atoms in thickness.

The precipitation of γ-deuterides (hydrides) in titanium

Abstract The crystal structure of titanium deutende precipitates in specimens of titanium containing up to 0.64 at.% deuterium was found to be face-centred-tetragonal TiD, c a = 1.12 , isomorphous with γ-ZrH, in agreement with recent findings of Numakura and Koiwa. This deutende phase was observed by transmission electron microscopy to precipitate on prism habit planes, and near-basal habit planes {2027}, about 27.5° from the basal plane. These habit planes can be related to dislocation mechanisms for the formation of γ by a martensitic transformation. In particular, the calculated near-basal habit plane is in reasonable agreement with observation, while the prism habit plane is the result of a plane strain transformation. Complex precipitation was found to be rather common, an example of a prism habit γ-plate consisting of sub-units of near-basal γ-laths is shown. Experimentally, the presence of γ Ti interfacial ledges with effective displacement vectors in the 〈a〉-direction is consistent with the proposed mechanism of formation of γ-plates with prism habit planes.

The β to α transformation in a Zr-2.5 wt% Nb alloy

Abstract The β (b.c.c.) to α. (h.c.p.) phase transformation in a Zr-2.5 wt% Nb alloy has been studied by transmission electron microscopy. The complex arrays of dislocations observed at the α/β interface were associated with 〈a+c〉 slip on (1011)H planes and interactions of 〈a〉 dislocations with the 〈a+c〉 array. The 〈a + c〉 dislocations lay parallel to the invariant line of the phase transformation which corresponded closely to the intersections of (1011)H, (101)B with the cone of unextended lines of the transformation. The interactions between 〈a〉 and 〈a +c〉 dislocations at the interface were analyzed by considering the similarities between step formation at tilt boundaries in single phase alloys and those found at the α β interface in the present alloy.

The temperature dependence of the mechanical properties of aluminum alloys containing low-melting-point inclusions

Abstract The energy absorbed during high temperature impact testing of binary aluminum alloys containing low-melting-point inclusions of Bi, Cd or Pb has been found to drop suddenly in a narrow temperature range; below the transition temperature, the specimens bend without breaking, while above it they fracture intergranularly. This embrittlement is clearly associated with the presence of the liquid inclusions. Classical liquid metal embrittlement (LME) of a 6061 aluminum alloy where its external surface was wetted by the ternary BiCdPb eutectic confirmed that the brittle intergranular failures observed in AlBi, AICd and AlPb are a manifestation of LME. It was found that a fracture surface energy model for LME is able to account for the result that embrittlement is most severe in alloys containing liquid Bi inclusions and least severe in alloys containing liquid Pb inclusions. The reduction in the fracture surface energy when the inclusions melt facilitates crack initiation and crack propagation. However, the finite amount of embrittling liquid produced by each inclusion is capable of facilitating crack propagation over short distances only, which may account for the observation that a minimum concentration of grain boundary inclusions are necessary to cause embrittlement in these alloys.

The response of carbo-nitride particles in hsla steels to weld thermal cycles

Abstract The behaviour of carbo-nitride particles in HSLA steels when subjected to a weld thermal cycle corresponding to heat inputs of 2, 5, 10 or 20 kJ/mm has been studied by analytical electron microscopy. Three types of response were observed: 1. (a) complete dissolution followed by reprecipitation on cooling (in a Nb-V containing steel), 2. (b) partial dissolution accompanied by precipitate coarsening (in two Ti bearing steels) 3. (c) dissolution followed by reprecipitation at the peak temperature of the thermal cycle (in Ti-Nb bearing steels). The latter behaviour was shown to be dependent upon the reheat temperature used during manufacture and was only found in a steel reheated to 950°C; this treatment promoted the formation of Nb-rich particles having a narrow size range in the base metal. The rates of dissolution observed in the experiments were supported by numerical calculations, which gave dissolution times of the order of 1 s or less at the peak temperature (1350°C) of the weld thermal cycle. The changes in the particle size distributions in the Ti and Ti-Nb bearing steels and the composition changes observed for (Ti, Nb) carbo-nitride particles were explained by modelling the precipitation behaviour in the austenite phase field.