Grain Boundary Stress Research Articles

Influence of grain size and degree of crystallization of intergranular glassy phase on the mechanical behaviour of a debased alumina

The influence of microstructure on the crack resistance (R-curve) behaviour of a commercial debased alumina containing large amounts of glassy phase (28 vol%) has been studied by strength measurements at controlled flaw sizes produced by indentation. Both the individual and combined effects of (a) grain size, and (b) intergranular second phase (glassy or crystalline) were evaluated. Enhancement of the R-curve behaviour was observed when the average grain size was increased from 3–18 μm by thermal treatment. However, no effect of the degree of crystallinity of the intergranular second phase on the R-curve behaviour, in either small or large-grained materials, was observed. These results are discussed with reference to the influence of grain-boundary residual stresses on grain bridging across the crack interface.

Glide twinning and pseudotwinning in peristerite: Si,Al diffusional stabilization and implications for the peristerite solvus

Albite glide pseudotwins related to grain-boundary stresses have been observed in an exsolved peristerite (Brown 1989). The glide operation transposes the pre-existing periodic oligoclase/albite lamellae and interfaces into a position rotated by only ∼0.5° in the pseudotwins, but transforms the indices from \((1\bar 80)\) outside to (081) inside the pseudotwin. The pseudotwin is anti-ordered with respect to Al and Si and both it and the transposed interface are unstable. They should revert to the initial state on stress removal. If however the stresses are maintained for a sufficiently long time, the pseudotwins are stabilized by inversion of Si,Al order and re-orientation of the interface by an angle of about 30° into a position close to \(\underline {(1\bar 80)} \). The continuous lamellae break up into a series of discs by diffusion of NaSi and CaAl, the minimum diffusion path being about the same as the thickness of the lamellae. On extrapolating available interdiffusion data in Ab-rich plagioclases to low temperatures, possible diffusion times may be calculated. The calculated times are long so that either the peristerite miscibility gap must be at a higher temperature than previously supposed or the low-temperature interdiffusion coefficients must be higher than the extrapolated experimental ones, or both. From recent data on ordering in albite, the crest of the gap is estimated to lie close to 650–625° C at low pressure and it is possible that interdiffusion under natural conditions is facilitated by hydrogen (protons) in feldspars.

Sintering stress of homogeneous and heterogeneous powder compacts

The thermodynamic meaning of the sintering stress is considered for some simple 2-dimensional porous bodies. The sintering stress acts as an effective mean grainboundary stress and can thus be meaningfully combined with applied stresses. The sintering stress can, in general, not be described in rigorous thermodynamic terms, especially for heterogeneous compacts. Simplifications do allow, however, to assess the effects of some microstructural heterogeneities, including the presence of large pores or of dispersed second phases, on the sintering stress.

Formation and Characteristics of Silicon Nitride-Lithium Aluminum Silicate Ceramics: Part II, Mechanical and Thermal Properties

Pressureless sintered Si{sub 3}N{sub 4} with devitrified grain-boundary phases was investigated for thermal expansion, fracture toughness, and strength behavior. An extended hysteresis of the heating and cooling dilatation curves was observed for the {alpha}ss/{beta}-eucryptite materials. Fracture toughness decreased in the {alpha}-ss/{beta}-eucryptite but increased from 4.2 to 5.6 MPa{center dot}m{sup 1/2} in the {beta}-ss/{beta}-spodumene microstructures. Fracture surfaces reveal extended crack deflection in the grain-boundary-toughened {beta}-ss/{beta}-spodumene materials compared with the glass-containing materials. Residual grain-boundary stresses resulting from thermal expansion mismatch are supposed to account for the deflection of crack path and a substantial toughness increment in the {beta}-ss/{beta}-spodumene microstructures.

Grain-size dependence of the slow-crack growth behavior in noncubic ceramics

The observed grain size ( G) dependence of the slow crack growth resistance of noncubic ceramics is related to residual grain-boundary stresses introduced by differential contraction across the boundaries by cooling after processing. In this model, the subcritical crack velocity is assumed to be a unique function of the local stress intensity factor which is determined by both the applied and residual stresses. The predicted crack growth parameters n and A decrease and increase respectively with increasing grain size. Stress-induced microcracking. which is activated when G exceeds a critical value G ∗ . leads to crack-tip shielding which lowers the crack-tip stress intensity by − K S l . Incorporating the latter additional term yields n and A parameters which become essentially independent of G and G > G ∗ . The model predictions are consistent with the observed behavior in noncubic ceramics such as alumina and TiB 2.

Sintering and Properties of Titanium Diboride Made from Powder Synthesized in a Plasma‐Arc Heater

The pressureless sintering to near theoretical density of high‐purity titanium diboride powder, produced in a plasma arc from halogen vapor reactants, is described. Physical and mechanical property measurements between 25° and 1400°C are reported for materials densified at T=1800° to 2275°C. Titanium diboride exhibits several unusual and interesting properties, which include high electronic conductivity, a large positive strength‐vs‐temperature coefficient, and an intrinsic resistance to high‐temperature slow crack growth (negative N) Property trends are explained in terms of microstructural development and residual grain‐boundary stresses.

A model for the effect of grain size on the yield stress of metals

Abstract It is proposed that the grain-size dependence of the yield stress of metals is due to the elastic incompatibility stresses at the grain boundaries. For clarity, the process of yielding is divided into three stages. In the first stage (prior to microyielding), the differences in elastic properties arising from the elastic anisotropy of adjacent grains establish localized stress concentrations at the grain boundaries. In the second stage, the stress concentrations at the grain boundaries result in localized plastic flow; this is the microyield region. The generation and motion of these 'geometrically necessary' dislocations attenuates the stress concentrations; a work-hardened grain-boundary layer is formed. As the stress is increased, the polycrystalline metal acts as a composite material consisting of a continuous network of a work-hardened grain-boundary material and discontinuous bulk material. The bulk is prevented from flowing plastically because the continuous network of work-hardened grain-boundary film acts as a reinforcement. Macro-yielding occurs when the applied stress is such that the grain-boundary film reaches its flow stress level; this is the third stage. Qualitatively, a polycrystal with a greater grain size has a proportionately smaller amount of grain-boundary reinforcement network and, consequently, a lower macroyield stress. This concept is developed into quantitative relationships between the grain diameter and the macroyield stress. Finite-element analysis is used to establish the value of the grain-boundary stresses caused by elastic compatibility conditions for a few simple cases. Theoretical predictions are compared with experimental observations made for nickel over a wide range of grain sizes.

A Diffusive Crack-Growth Model for Creep Fracture

A grain-boundary creep-crack-growth model is presented based on the assumptions that a crack propagates along the grain boundary by a coupled process of surface and grain-boundary self-diffusion, the adjoining grains on either side of the boundary behave elastically, and steady state conditions prevail. Under the action of the applied stress, atoms on the crack surfaces are driven by surface diffusion toward the crack tip, from where they are deposited nonuniformly by grain-boundary diffusion along the grain interface so that the grain boundary opens up hi a wedge shape ahead of the advancing tip which, hi turn, produces a misfit residual stress field. The total grain-boundary normal stresses which are the sum of this misfit stress field and that due to applied stress as well as the boundary opening displacements due to materials deposition are solved from a singular integrodifferential equation to give the following equation relating K to v: where K is the applied mode I crack-tip stress intensity factor,Kmin=1.69 kg is the minimum K below which no crack growth is predicted, KG being the critical K based on the Griffith theory; ν is the fixed crack-tip velocity, and νmin is the minimum v for which K=Kmia. In terms of the conventional expression of ν∝Kn, the present model predicts the values of n varying from 12 to infinity. A comparison with a set of creep crack-growth data on Si-Al-O-N at 1400°C shows good agreement between theory and experiment. A detailed analysis of the energy balance for the present model is also presented which indicates that / or (1–ν2)K2/E is indeed the correct energy release rate during the crack growth, as is true in the theory of elastic fracture mechanics. However, the major portion of the energy released in the diffusion processes comes from work done by the normal stress in opening up the grain boundary to accommodate the diffused material rather than from strain energy released by the adjoining grains.

Mechanical properties of the alloy 36NKhTYu in relation to the mechanisms underlying the precipitation of the ?? and ? phases. III

The kinetic characteristics of the changes in elastic limit, ductility, and stress relaxation taking place in samples of the alloy 36NKhTYu on aging at 700°C are considered in relation to aging time, the measurements being made at 20 and 600°C. The samples are initially taken in five different states induced by various forms of preliminary treatment, being distinguished from one another with respect to grain size, mechanism of γ phase precipitation, and state of the grain boundaries. In order to obtain material with a high elastic limit, working at moderately low temperatures, the discontinuous precipitation of the strengthening phase has certain clear advantages over continuous precipitation. For high working temperatures, in which grain-boundary stress relaxation starts playing an appreciable role, however, it is undesirable that discontinuous decomposition should undergo any major development in the aging of the material.

Al-Zn-Mg合金の粒界腐食と応力腐食割れの関係について

Al-Zn-Mg合金の粒界腐食と応力腐食割れの関係について

Some observations of grain-boundary sliding in aluminium bicrystals tested at constant strain rate and constant rate of stress increase

Bicrystals of pure aluminium, aluminium/0.05 wt% and aluminium/0.30 wt% copper have been deformed in shear within the temperature range 350 to 600°C at a constant rate of grip displacement (300 μm/h) and constant rates of increase of grain-boundary shear stress (0.30 to 1.04 g/mm−2 min−1). The stress/time, sliding/time and sliding/stress curves for these tests are presented together with metallographic observations. The stress/time curves exhibited changes in the strain-hardening rates which were accompanied by the occurrence of extensive crystal deformation. In many cases, following the change in strain-hardening rate and onset of extensive crystal slip, the extent of grain-boundary sliding was proportional to the shear stress on the boundary. The ratio of the extent of sliding to the grain-boundary shear stress increased with increasing test temperature. The temperature-dependence of the sliding behaviour, as reflected by the sliding/stress results, yielded apparent activation energies of ∼ 31 Kcal/mole in the temperature range 600 to 475°C and ∼ 8 Kcal/mole in the range 475 to 350°C.