W.W. Milligan

Strength and tension/compression asymmetry in nanostructured and ultrafine-grain metals

Abstract The recent literature is reviewed with respect to the strength-limiting deformation mechanisms in nanocrystalline and ultrafine-grain metals. Based on these results, a deformation mechanism map is proposed for FCC metals with ultrafine-grain sizes. In the absence of flaw-controlled brittle fracture, it is concluded that the strength-limiting mechanism in metals with grain sizes between approximately 10 and 500–1000 nm is dislocation emission from grain boundary sources. A simple model for the strength in this regime of grain sizes is developed from classical dislocation theory, based on the bow-out of a dislocation from a grain boundary dislocation source. The model predicts not only the strength as a function of grain size, but also the observed tension/compression asymmetry of the yield strength. The tension/compression asymmetry arises from the pressure dependence of the dislocation self-energy during bow-out. The pressure dependence is a function of material and grain size, consistent with experimental observations. Finally, the model provides a physical basis for a pressure-dependent yield criterion.

Observation and measurement of grain rotation and plastic strain in nanostructured metal thin films

Abstract The deformation behavior of nanostructured gold thin films, with grain diameters of 10 nm and film thicknesses of 10–20 nm, has been studied by means of in situ high resolution transmission electron microscopy. Grain rotation was observed by measuring the changes in the angular relationships between the lattice fringes of different grains during deformation at low strain rates. The strain tensor was calculated by measuring the relative displacements of three material points, and using an analysis similar to that for strain gage rosettes. Relative grain rotations of up to 15 degrees, along with effective plastic strains on the order of 30%, were measured. No evidence of dislocation activity was detected during or after straining. Identical experiments on coarser-grained silver thin films, with grain diameters around 110 nm, yielded clear evidence of dislocation activity. These results indicate that grain rotation and grain boundary sliding can make significant contributions to the deformation of nanostructured thin films at low homologous temperatures.

A simple, mixtures-based model for the grain size dependence of strength in nanophase metals

Abstract A model is presented for the strength of nanophase metals. The model assumes that polycrystalline metals consist of two phases: the “bulk” intragranular regions, and the “grain boundaries”. The boundary phase is treated as a glassy, but not highly rate-dependent material with a constant strength equal to that of the amorphous metal. The crystalline phase is assumed to follow a Hall-Petch equation for the grain-size dependence of strength. Treating the material as a composite, with a rule of mixtures approach, predicts a change in the Hall-Petch slope at small grain sizes, as has been observed. Grain size softening is predicted, but not until sizes below 5 nm. The model is compared to data in the literature.

IN SITU STUDIES OF DEFORMATION AND FRACTURE IN NANOPHASE MATERIALS

Abstract Nanocrystalline gold (8–25 nm grain size) and gold/silicon nanocomposites were prepared by sputtering and then strained to fracture in a transmission electron microscope. In situ and post mortem analyses revealed that the nanophase gold films were ductile, and significant plasticity was associated with fracture. Observations of pore formation, as well as a strain-rate effect on deformation behavior and direct lattice imaging of deformation, all indicated that the deformation occurred by diffusion-based mechanisms. Fracture was intergranular, but not brittle. Gold/silicon nanocomposites containing large volume fractions of brittle, amorphous Si and nanocrystalline gold precipitates exhibited behavior indicating significant toughness.

Glasslike behavior in a nanostructured Fe/Cu alloy

In this article, the authors report the observation of shear bands and perfectly plastic behavior, typical of metallic glasses, in an Fe-Cu alloy with grain sizes in the 100 nm range. A strengthening effect with decreasing grain size was also observed, and the shear bands occurred on planes oriented at about 40 deg from the tensile axis instead of the maximum shear plane of 45 deg, as has been observed in amorphous polymers.

The mechanisms and temperature dependence of superlattice stacking fault formation in the single-crystal superalloy PWA 1480

Deformation microstructures in PWA 1480 nickel-base superalloy single crystals were studied in the range of 20 °C to 1100 °C. Similar to previous investigations, superlattice stacking faults were observed after slow strain rate deformation at temperatures between 700 °C and 950 °C. Unlike previous studies, a high density of superlattice stacking faults was observed after deformation at 200 °C and below. The mechanisms of fault formation in the two temperature regimes were different. In the range of 700 °C to 950 °C, single isolated superlattice-intrinsic stacking faults (SISFs) were produced by the decomposition of an a/2(110) matrix dislocation in the γ/γ′ interface. The a/3(112) partial shears the particle, while the a/6(112) Shockley remains in the interface. At 200 °C and below, a high density of faults was produced on closely spaced parallel planes. The most common feature after deformation in this range is the faulted loop, which is most often observed to be a superlattice-extrinsic stacking fault (SESF). These low-temperature faults, along with their temperature dependence, were quite similar to those observed in single-phase Ll22 materials. The available evidence suggests that the low-temperature faults were produced by the dissociation of an a<11> unit superdislocation into a pair of a/3<112> partials. The temperature dependence of the faulting (at low temperatures) was modeled by linear isotropic elasticity, and the results suggest that the SISF energy increases significantly from 20 °C to 400 °C. Multiplanar, overlapping superlattice faults were analyzed with respect to bond violations. This analysis suggested that an antiphase boundary (APB) on top of an SISF has a very high fault energy, similar to that of the complex stacking fault. Therefore, the presence of SISF loops on glide planes promotes further dissociation by the SISF scheme instead of the APB scheme and explains the high density of SESFs and microtwins observed in the deformation structures.

On the failure of pressure-sensitive plastic materials part II: Comparisons with experiments on ultra fine grained Fe-10% Cu alloys

In Part 1 of this article the authors presented theoretical results pertaining to the yield behavior of pressure-sensitive plastic materials with emphasis on shear band width and orientation. These results are applied here to model the experimental data obtained for compression and tension tests of ultra fine grained Fe-10% Cu alloys. As will be reported in detail elsewhere these alloys exhibit almost perfectly plastic behavior and deform by intense shear banding as the only mode of plastic deformation. Due to the fine grain structure, it may be speculated that grain rotation and void formation can contribute significantly to the aforementioned shear band mechanism for plastic flow, in addition to the traditional dislocation motion. It follows that the pressure-sensitive plastic models elaborated upon in Part 1 may be suitable candidates for the description of the shear band behavior for these alloys. In fact, it is shown in Part 2 that the mechanical property data obtained for the strengths in tension and compression, the shear band angles and the shear band widths verify the theoretical expressions derived in Part 1 for the yield condition, the shear band orientation, and the shear band width.

On the failure of pressure-sensitive plastic materials part I. Models of yield & shear band behavior

The purpose of this two-part article is to theoretically establish and experimentally support a simple method for calculating shear band angles and shear band widths in tension or compression samples commonly used to investigate mechanical properties. The method is illustrated in conjunction with experimental results on the yielding behavior and shear band formation in tension and compression of an ultra fine grained Fe-10% Cu alloy, properly milled and consolidated. The experimental techniques for preparing the samples, their microstructural characteristics and the details of the performed mechanical tests will be reported elsewhere, but a brief summary is contained in Part 2 of the present article. In Part 2 comparisons are made between theoretical predictions for pressure-sensitive plastic materials established in Part 1 and experiments on the yield behavior of an ultra fine grained Fe-10% Cu alloy along with observations on shear band orientations and widths.

Effects of alloy composition on environmental embrittlement of B2 ordered iron aluminides

The mechanical properties of B2 ordered iron aluminides have been found to be very sensitive to test environment. Recent studies have revealed that these alloys exhibit significantly higher ductilities when tested in oxygen or vacuum that in air. Moisture in the air is believed to be responsible for the loss in ductility. The purpose of this paper is to further investigate the effects of the environment on the mechanical properties of iron aluminides by observing how these effects change with alloy composition. Aluminides with compositions ranging from 23 to 38 at. % Al have been tensile tested in environments of air, vacuum and oxygen.

Phase stability in (Ni, Pt)3Al alloys

The primary goal of this study was to produce pseudobinary alloys based on (Ni,Pt)[sub 3]Al, and then test them mechanically as a function of temperature and observe the resulting dislocation structures. The ternary phase diagram has not been established with confidence, but it was hoped that extended solid solubility would exist and that the transition from one type of behavior to another could be captured and understood. These plans were based on the following observations: Ni[sub 3]Al and Pt[sub 3]Al are both L1[sub 2] materials, with similar lattice parameters; Ni and Pt are isomorphous above about 900 K; and a previous study established a preference for Pt to occupy Ni sites in Ni[sub 3]Al. In this paper, the authors present the results of the phase stability study; the mechanical behavior and dislocation structures will be presented later.