Erland M. Schulson

A brittle to ductile transition in NiAl of a critical grain size

Tensile tests have been carried out on the strongly ordered B2 aluminide NiAl at 400 C to investigate the effect of the grain size on the ductility of the material. It is found that the ductility is very low and essentially independent of the grain size for aggregates of grains larger than about 20 microns; for finer-grained aggregates, the ductility increases sharply with decreasing grain size. Thus, NiAl exhibits a critical grain size below which polycrystalline aggregates are ductile in tension. For all grain sizes, fracture occurs in a brittle manner through a combination of intergranular decohesion and transgranular cleavage.

Brittle failure of ice

This paper reviews the brittle failure of polycrystalline ice at temperatures (>0.8Tmp) and strain rates (∼10−7–10−1 s−1) of engineering interest. The discussion addresses ice as a material and emphasizes structure–property relationships. Tensile behavior is discussed in terms of the nucleation and propagation of cracks. Brittle compressive failure, a more complicated process, is addressed in terms of frictional crack sliding, wing cracking and a new mechanism termed comb cracking. The ductile-to-brittle transition under compression is modeled in terms of the competition between crack-tip creep and crack propagation. The paper closes with a discussion of leads in the Arctic sea ice cover and with evidence that failure mechanisms may be independent of scale. They may also operate in rock and other brittle materials.

Grain boundary accommodation of slip in Ni3Al containing boron

Abstract Experiments at room temperature have established that the addition of 750 ppm by weight (0.35 at.%) of boron to stoichiometric Ni3Al reduces the effectiveness with which grain boundaries strengthen the alloy. This effect leads to boron-induced weakening of the most finely grained aggregates (d ≲ 10 μm). The effect of boron is explained in terms of an increase in the mobility of grain boundary dislocations, and is related to boron-induced ductility.

The brittle compressive fracture of ice

Abstract The brittle compressive fracture under uniaxial loading of fresh-water, granular ice Ih has been studied. Measurements are reported of the fracture stress at temperatures from −10 to −50°C at strain rates of 10 −3 and 10 −1 s −1 for grain sizes from approximately 1 to 10 mm. Also a summary is reported of measurements by Jones et al . (unpublished) of the kinetic coefficient of friction for ice on ice at temperatures from −10 to −40°C at sliding velocities from 5 × 10 −7 m s −1 to 5 × 10 −2 ms −1 . Observations via high speed photography of internal cracking during loading are included. The strength, albeit scattered, increases with decreasing grain size, with decreasing temperature and at −10°C with decreasing strain rate. Similarly, the coefficient of friction increases with decreasing temperature and at −10°C with decreasing sliding velocity. Wing cracks were observed on some inclined cracks nucleated during loading. The results are explained in terms of the frictional crack sliding-wing crack model [as developed by Ashby and Hallam, Acta metall. 34, 497 (1986)] of compressive fracture. Finally, a simple model is presented for the transition from ductile to brittle behavior. It is based upon the competition between the building up and the relaxation of internal stresses within the vicinity of the internal cracks, and it leads to a transition strain rate which can be expressed in terms of the fracture toughness, the creep rate, the kinetic coefficient of friction and the microstructural scale of the material.

The effect of grain size on the yield strength of Ni3Al

Abstract Experiments have established that the effect of grain size ( d = 2.9–1100 μm ) on the yield strength of Ni 3 Al at room temperature is given by the relationship σ y = σ 0 + kd − n where n = 0.80 ± 0.05, σ 0 = 93 ± 14 MPa and k = 2080 ± 105 MPa · μm 0.80±0.05 . The relationship is explained quantitatively in terms of work-hardening which occurs within the Luders bands which accompany yielding. The Luders strain, ϵ L , increases with decreasing grain size according to the relationship ϵ L = λd −0.55 where λ = 8.4 × 10 −2 μm 0.55 .

The ordering and disordering of solid solutions under irradiation

Abstract A review is presented of the long-range ordering and disordering of solid solutions under irradiation. The study is based upon research papers which have been published over the past thirty years, and focuses on the experimental evidence for the phenomena and the underlying mechanisms. Also, the work discusses the crystalline-to-amorphous transformation which occurs in certain systems. It is concluded that ordering and disordering occur under cascade-producing Irradiation (fastneutrons and heavy-ions) and under non-cascade-producing irradiation (energetic electrons); that the degree of long-range order decreases exponentially with increasing fluence; that cascade-producing irradiation is more effective than non-cascadeproducing irradiation in creating disorder; that irradiation at intermediate temperatures leads to an intermediate degree of order; that the thermally activated migration of vacancies is the dominant mechanism underlying irradiation-induced ordering; and that thermal spikes, replacement collision sequences and random vacancy/interstitial recombinations are probable disordering mechanisms of which the relative importance varies with the nature of the irradiation.

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.

Universal behaviour in compressive failure of brittle materials

Brittle failure limits the compressive strength of rock and ice when rapidly loaded under low to moderate confinement. Higher confinement or slower loading results in ductile failure once the brittle–ductile transition is crossed. Brittle failure begins when primary cracks initiate and slide, creating wing cracks at their tips. Under little to no confinement, wing cracks extend and link together, splitting the material into slender columns which then fail. Under low to moderate confinement, wing crack growth is restricted and terminal failure is controlled by the localization of damage along a narrow band. Early investigations proposed that localization results from either the linkage of wing cracks or the buckling of microcolumns created between adjacent wing cracks. Observations of compressive failure in ice suggest a mechanism whereby localization initiates owing to the bending-induced failure of slender microcolumns created between sets of secondary cracks emanating from one side of a primary crack. Here we analyse this mechanism, and show that it leads to a closed-form, quantitative model that depends only on independently measurable mechanical parameters. Our model predictions for both the brittle compressive strength and the brittle–ductile transition are consistent with data from a variety of crystalline materials, offering quantitative evidence for universal processes in brittle failure and for the broad applicability of the model.

The strength, hardness and ductility of Ni3Al with and without boron

Experiments were performed to investigate the effects of grain size (2–1100 μm) and temperature (77–1023 K) on the strength, hardness and ductility of stoichiometric Ni3Al with (0.35 at.%) and without boron. The results show that grain refinement strengthens and hardens the alloys at low temperatures (< 873 K), but weakens them at high temperatures; lowers the magnitude of the anomalous thermal strengthening, leading to thermal weakening; and effects a “ductile to more ductile” transition in Ni3 Al + B at room temperature and a brittle to ductile transition in both alloys at elevated temperatures (⩾ 673 K). Grain refinement has little effect on the room-temperature ductility of Ni3 Al and on the work hardening rate. The addition of boron hardens coarse-grained material but softens fine-grained aggregates. These results are explained in terms of deformation and fracture mechanisms.

The friction of ice on ice at low sliding velocities

Abstract The kinetic coefficient of friction μ has been measured for both freshwater, granular ice and saltwater, columnar ice sliding against itself. The variables were ambient temperature (−3°C to −40°C), sliding velocity (5 × 10−7 −5 × 10−2 ms−1), normal pressure (0.007–1.0 MPa) and grain size (2.5–6.0 mm). Generally, μ decreased with increasing velocity and with increasing temperature, but was relatively insensitive to both pressure and grain size over the ranges investigated. At the lower temperatures (−30°C and −40°C) the frictionvelocity curve exhibited a peak at intermediate velocities. The friction coefficients for freshwater and saltwater ice were almost indistinguishable at higher temperatures (−3°C and −10°C), but saline ice had lower friction at lower temperatures. The results are explained in terms of frictional heating, creep and fracture.