J.A. Horton

Effect of boron on grain-boundaries in Ni3Al†☆

Abstract The effects of boron additions (up to 0.4 wt% B) on grain-boundary chemistry and tensile properties of Ni3Al containing 24–26 at.% Al were studied. Room-temperature ductility and fracture behavior of B-doped Ni3Al depended critically on deviation from alloy stoichiometry. As the aluminum content of B-doped Ni3Al is decreased below 25 at.%, the ductility increases dramatically and the fracture mode changes from intergranular to transgranular. Auger studies indicate that the intensity of boron segregation to grain boundaries increases and the concentration of grain-boundary aluminium decreases significantly with decreasing bulk aluminum concentration. These results suggest that alloy stoichiometry strongly influences grain-boundary chemistry which in turn, affects the grain-boundary cohesion. Boron exhibits an unusual segregation behavior in Ni3Al, i.e. it has a strong tendency to segregate to the grain boundaries but not to cavity (free) surfaces. On the other hand, sulfur, an embrittling impurity, tends to segregate more strongly to free surfaces than to grain boundaries. The beneficial effect of boron is in agreement with existing theories of solute segregation effects on grain-boundary cohesion. The yield stress of B-doped Ni3Al decreases with increasing grain size produced by long-term annealing at 1000°C The yield stress obeys the Hall-Petch relation: σ y = α o,y + k y d 1 2 with σy = 163 MPa and ky = 8.2 MPa cm 1 2 . The tensile elongation was initially independent of grain size, and showed only a moderate decrease from about 50–40% with grain diameters larger than 110μm.

In-situ straining of Ni3Al in a transmission electron microscope

Abstract TEM in-situ straining experiments have been performed on thin foils of Ni 3 Al and Ni 3 Al containing 750 ppm (0.35 at.%) boron. During the straining, gliding APB-coupled dislocations were observed to leave superlattice intrinsic stacking faults (S-ISFs) as debris in their wake confirming a mechanism for S-ISF formation first suggested by Pak et al . [ Scripta metall. 19 , 1081 (1976)]. The APB-coupled dislocations (i) pile-up at grain boundaries, (ii) become extrinsic grain boundary dislocations and (iii) initiate slip in adjacent grains. The characteristics of the crack propagation process indicated that plastic flow, albeit localised, precedes fracture.

Atom probe analysis of grain boundaries in rapidly-solidified Ni3Al☆

Abstract The distribution of boron in a rapidly solidified Ni-24 at.% Al alloy containing 0.24 at.% B was investigated by atom probe field-ion microscopy. Boron was found to segregate to both APBs and grain boundaries. Field-ion micrographs revealed that some of the APBs and most of the grain boundaries were decorated with bright spots identified by single atom analysis as boron atoms. A 0.4–1.2 nm thick boron-enriched phase was observed on most of the grain boundaries. The distribution of this phase was not uniform either along a boundary or from boundary to boundary. The degree of long-range order as measured by the atom probe was greater than 0.97. This state of order continued to the interface between the matrix and grain boundary phase.

Micromechanisms of yield and flow in ordered intermetallic alloys

Abstract The stability and mobility of active slip systems in superlattice structures, for both cubic and noncubic crystals, are theoretically investigated based on the energetics and kinetics of dislocation dissociations. The main concept of the force couplet model for the positive temperature dependence of yield and flow stress is introduced. Two sources of the glide resistance in ordered lattices are the fault dragging mechanism and the cross-slip pinning mechanism. The effective fault energy consists of two terms related to the chemical and mechanical instability of a shear fault (APB or SISF). Dependence of the yield stress on the orientation and the sense of applied stress stems from the signs and magnitudes of the glide and non-glide stresses. As the effective fault energy is altered by solute segregation and/or high non-glide stress, the two glide resistance mechanisms are affected differently. In Ni 3 Al and β-CuZn, the anomalous yield strength, strian rate sensitivity, in situ deformation TEM observations, and nonstoichiometry effect are discussed in view of the present model.

TEM observations of dislocation emission at crack tips in aluminium

Direct observations were made of the propagation of ductile cracks and associated dislocation behaviour at crack tips in aluminium during tensile deformation in an electron microscope. In the electropolished area, the cracks propagated as a Mode III shear-type by emitting screw dislocations on a plane coplanar to the crack plane. A zone free of dislocations was observed between the crack tip and the plastic zone. As the cracks propagated into thicker areas, the fracture mode changed from Mode III to predominantly Mode I. The crack top of the Mode I cracks was blunted by emitting edge dislocations on planes inclined to the crack plane. The blunted cracks did not propagate until the area ahead of the crack tip was sufficiently thinned by plastic deformation. The cracks then propagated abruptly, apparently without emitting dislocations. The stress intensity factor was measured from the crack tip geometry of Mode III cracks and it was found to be in good agreement with the critical value of the stress intensity factor required for dislocation generation.

Anisotropic antiphase boundaries in rapidly solidified Ni3Al

Abstract Rapid solidification of Ni 3 Al produces a network of curved antiphase boundaries (APB). Annealing, which allows a closer to equilibrium structure to develop, results in the APBs preferentially aligned on {100} planes; thus indicating that the APB energy is less on {100} planes than on other planes. This provides the first direct experimental evidence of an APB energy anisotropy which forms the basis of the Kear-Wilsdorf cross slip model used to explain the increase in yield stress with an increase in temperature.

APB dragging in Ni3Al deformed at intermediate temperature

Abstract In a Ni3Al alloy deformed at 650°C, evidence is provided for glide of KearWilsdorflocks on a {111} plane, accompanied by dragging of antiphase boundaries.

Interfaces in nickel aluminide/alumina fibre composites

Metal matrix composites based on the intermetallic alloy Ni3Al and fibres of Al2O3 were fabricated by hot-pressing nickel aluminide powders and alumina fibres. Two matrix alloys were used in this investigation: Ni3Al microalloyed with boron and Ni3Al alloyed with 8 at% chromium and smaller amounts of zirconium and boron. The materials were studied using optical and transmission electron microscopy with particular emphasis placed on the characteristics of the matrix-fibre interface. The base Ni3Al/Al3O3 composite displayed no evidence of chemical reaction at the interface, an intimate bond between matrix and fibre was observed, and the material exhibited 10% ductility at room temperature. Composites with the more complex matrix alloy were brittle, a phenomenon attributed to the formation of zirconia particles at the interface.

Transmission electron microscopy of rapidly solidified powders of Ni3Al

Rapidly solidified powders (<100 μm diameter) of stoichiometric Ni3Al and Ni3Al(B,Ti) were examined using transmission electron microscopy. The powders consisted of martensitic dendrites in a Ll2 matrix that contained numerous dislocations. The stoichiometric Ni3Al powder also contained martensitic lamellae between the dendrites. The martensite plates were internally twinned and enriched in aluminum, and probably formed by the transformation of β-NiAl.

Some comments on dislocation bowing and partial separation during in-situ straining of γ′Ni3Al

Abstract Some results of transmission electron microscopy in-situ straining of single-phase boron-doped Ni3Al are presented which suggest that superdislocation partials can become rather widely separated (50 nm) and bowed (scalloped) along their length. The significance of these observations to in-situ straining experiments on superalloys of γ/γ′ is discussed.