R. Gibala

Surface oxide softening of niobium single crystals

Abstract The effect of thin (〈 160 nm) anodic oxide films on the mechanical behavior of single crystals of niobium was investigated. At 295 K, oxide films cause hardening. At lower temperatures, the oxide films reduce the critical resolved shear stress and cause serrated flow. When oxide-coated niobium is prestrained into stage I at 295 K, the flow stress at lower temperatures is further reduced, the ductility is increased, the serrations disappear, and three-stage work hardening is observed. A model involving preferential generation and motion of edge dislocations from the oxide-metal interface is used to explain the results. Supporting evidence obtained from slip trace analysis, transmission electron microscopy of dislocation substructures, and etch pitting of near-surface dislocations is presented.

Dislocation relaxation in single crystals of molybdenum

Abstract The characteristics of the α dislocation relaxation peak in molybdenum have been examined in three orientations of high purity single crystals deformed in tension at 170, 300 and 450 K. For deformation at 450 K, the α peak is absent in crystals oriented for easy glide and deformed into stage 1 of work hardening. For crystals of the same orientation deformed into stages 2 and 3 and for crystals with [001] and [101] tensile orientations, the peak is observed superimposed on a step-like background; it grows rapidly, sharpens and moves to lower temperatures with increasing amounts of plastic deformation. Crystals with the easy glide orientation deformed at 170 and 300 K exhibit rapid work hardening and display small, broad, asymmetrical α peaks. Mild strain aging at 500 K does not greatly affect the α peak, but drastically reduces the step-like background damping. The overall response of the α peak to changes in dislocation substructure is very similar to that of the Bordoni peak in f.c.c. metals. Three widely divergent models are considered: double kink generation on non-screw dislocations, kink diffusion on screw dislocations, and breakaway of dislocations from immobile pinning points. It is concluded that the major features of both peaks are better explained by the breakaway models. For the α peak, a variation of the Hasiguti trapped kink model seems most appropriate.

An application of the Schoeck theory to the cold-work internal friction peak in iron

Abstract The metallurgical usefulness of the cold-work peak has been limited by an inadequate understanding of the mechanism of the peak. Our analysis examines the predictions of the Schoeck theory of the cold-work peak and compares these predictions to some experimental results obtained from α-iron. The distribution of interstitial atoms between lattice and dislocation sites is treated by Fermi-Dirac statistics. The most reasonable relaxation times are predicted by the Schoeck formulation when the mobility term is characterized by an activation energy which is the sum of the activation energy for lattice diffusion and the binding energy of the interstitial to the dislocation. An expression is derived which predicts the activation energy determined in a typical internal friction experiment. The predictions are consistent with experimental observations. Experimental peak position and relaxation strength data are analyzed and found to yield reasonable active (contributive) dislocation densities.

Internal friction in ferrous martensites

Internal friction measurements were made at 1 Hz in the temperature range of 25 to 500°C on quenched, tempered and cold-worked Fe-Ni-C martensites. The alloys, which contained 15 to 30 wt pct Ni and 0.1 to 1.0 wt pct C, andMs temperatures <0°C, transformed to martensite with a twinned (259)γ habit and exhibited a relaxation peak at ~160°C. These results could be contrasted with those for Fe-C martensites, which form above room temperature, have predominantly dislocated (111)γ or (225)γ habits, and exhibit an internal friction peak at about 250°C. The literature on substructures, tempering and internal friction of all ferrous martensites and cold-worked ferrites was utilized in the interpretation of the 160°C peak. Several dissimilarities between the 160°C peak and the 250°C peak in Fe-C martensites or the cold-work peak in ferrite were noted such that models of dislocation-interstitial interaction for these peaks could not explain the 160°C peak. It was concluded that the 160°C peak is associated with the stress-induced motion of twin boundaries containing mobile carbon atoms. Such a mechanism was shown to be consistent with the present experimental observations and all other available data.

Surface oxide softening in tungsten and molybdenum single crystals

Athermal-to-thermal transition defines the temperature range over which surface film softening occurs in bcc metals. Tungsten and molybdenum should exhibit substantial softening at temperatures in the vicinity of room temperature. For tungsten deformed at strain rates of the order 10/sup -4/ s/sup -1/ the transition temperature is above 600/sup 0/K (16); for molybdenum the corresponding temperature is approximately 450/sup 0/K (17). Mechanical testing of these metals, coated and uncoated, at room temperature should disclose if softening exists in these metals. Experiments on oxide-coated single crystals of tungsten and molybdenum were performed to test this idea. Surface film softening occurs for Mo and the effect is of the same order of magnitude (100 MN/m/sup 2/ out of 300 to 400 MN/m/sup 2/) as in W.

Structure of dislocations in tantalum and tantalum-nitrogen alloys

Abstract Weak-beam electron microscopy of Ta and Ta-N crystals deformed at 77 K shows that the image widths of 1 2 〈111〉 screw dislocations are approx. 10–20 A, regardless of the nitrogen concentration between ~ 5 and 15,000 at.ppm. Annealing treatments designed to decorate screw dislocations with nitrogen atmospheres have no effect on the widths. Furthermore, no dissociations are observed at the nodes of the dislocation reactions 1 2 〈111〉 + 1 2 〈1 1 1 〉 = 〈100〉 . It is concluded that the splitting of screw dislocation cores of ~50 A observed by field ion microscopy is enhanced by the local electric field; actual dissociations must be much smaller. Models of interstitial hardening in b.c.c. metals which rely on interstitial-enhanced core expansion of screw dislocations are also not supported by the present results.

Surface Oxide Softening of Body-centered Cubic Refractory Metals

The effects of surface oxide films on the mechanical behavior of single crystals of the body-centered cubic refractory metals Nb, Ta, Mo and W have been investigated. At low homologous temperatures T < 0.15 Tm, oxide films cause softening, i.e. reduced flow stresses and increased ductility, in all four metals. At higher temperatures, only film strengthening is observed. The results are well-explained by models that involve preferential generation and motion of edge dislocations from the film-substrate interface.

Dislocation substructure in Ta-Re-N alloys deformed at 77 K

High purity Ta, Ta-Re, and Ta-Re-N alloy single crystals were deformed in tension at 77 K, and the resulting dislocation arrangements were studied by transmission electron microscopy. Re and N have similar effects on the dislocation substructure. Alloying increases the fraction of primary screw dislocations at the expense of debris, tangling, and secondary dislocations. For a given increment in yield stress, Re causes much larger changes in the substructure than N. The substructures observed in Ta-Re-N alloys are similar to those in Ta-Re alloys, even though ternary alloys exhibit alloy softening and binary alloys exhibit alloy hardening. These observations can be explained in terms of the different intrinsic mobilities of edge and screw dislocations, the different interactions which substitutionals and interstitials have with edge and screw dislocations, and the large differences in concentration of substitutional and interstitial solutes.