The role of intergranular precipitates in controlling creep cavitation

A satisfactory model for cavitational failure in creep must account for the fact that fracture can occur under a very low stress, for example, only 0.7 MPa for a solid solution magnesium alloy. A mechanism for growth based on the transfer of vacancies from high angle grain boundaries to intergranular cavities satisfies this low stress requirement for it converts a relatively high fraction of the work done by the applied load into surface energy of fracture. However, for such growth to proceed, cavity nuclei of radius greater than a critical value, r c , must exist on those grain boundaries which are approximately normal to the applied tensile stress axis. It can be shown quite simply that r c = 2y/o, where y is the surface energy per unit area and o the applied tensile stress. A typical value for r c is 1 pm which is far too large to occur spontaneously by chance accumulation of vacancies. It is in fact generally agreed that cohesion is lost owing to the concentration of stress at some small obstacle in a sliding grain boundary. These cavities are nucleated along the boundary under applied stresses which are lower than those needed to cause triple point cracking where the whole of the length of the boundary is available to concentrate stress. This was a puzzle until Smith & Barnby (1967) demonstrated that the stress concentrated at a small obstacle in a sliding boundary was far higher than that concentrated at a very large obstacle as incorporated in, for example, the Stroh derivation.

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Influence of Thermal and Thermo-Mechanical Processing on the Creep Resistance of Mg-10Gd-3Y-0,4Zr Alloy

Materials Science Forum ◽

10.4028/www.scientific.net/msf.675-677.487 ◽

2011 ◽

Vol 675-677 ◽

pp. 487-490 ◽

Cited By ~ 7

Author(s):

Vit Janik ◽

Qu Dong Wang ◽

Dong Di Yin ◽

Wen Jiang Ding

Keyword(s):

Grain Boundaries ◽

Creep Resistance ◽

Tensile Creep ◽

Secondary Phases ◽

Lower Cavity ◽

Creep Cavitation ◽

Peak Aged ◽

High Fraction ◽

Aged State ◽

Minimal Creep Rate

Alloy Mg-10Gd-3Y-0,4Zr in as-cast, as-extruded, cast-T6 (peak aged) and extruded-T5 (peak aged) state was tensile creep tested at 200, 250 and 300 °C and stress 50, 80 and 120 MPa. Comparison of minimal creep rate shows that alloy Mg-10Gd-3Y-0,4Zr in cast-T6 conditions is characterized by an excellent creep resistance, which is higher than that of commercially available Mg-alloys. Creep resistance of as-cast, as-extruded and extruded-T5 alloy Mg-10Gd-3Y-0,4Zr is lower. Cavity nucleation is heavily affected by the amount of secondary phases on the grain boundaries and also by the initial grain size of the microstructure. After extrusion and in the extruded-T5 conditions creep cavitation was not observed, whereas in the as-cast and cast-T6 conditions creep cavitation occurred on the high fraction of grain boundaries.

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Dynamic Embrittlement: Quasi-Static Interfacial Decohesion

MRS Proceedings ◽

10.1557/proc-586-303 ◽

1999 ◽

Vol 586 ◽

Author(s):

C. J. Mcmahon ◽

J. L. Bassani ◽

Y. Mishin

Keyword(s):

Grain Boundary ◽

Tensile Stress ◽

Grain Boundaries ◽

Boundary Structure ◽

Micromechanical Modeling ◽

Diffusion Creep ◽

Grain Boundary Structure ◽

Alloy Steels ◽

Constituent Elements ◽

Work Done

ABSTRACTIn a surprising variety of cases, interfaces in normally ductile materials can undergo timedependent brittle cracking under the influence of a tensile stress, either applied externally or existing as an internal residual stress. The connecting feature in all these cases is the presence of a surface-adsorbed element that is highly mobile in comparison to the constituent elements of the material. As in the phenomena of diffusion creep and diffusive growth of cavities at high temperatures, the driving force for this cracking is the work done by the tensile stress when a surface atom enters the solid. At temperatures below about 0.5 Tm of the solid, this occurs mainly along grain boundaries. Examples of systems that have been studied in some detail include cracking of alloy steels by sulfur, Cu-Sn alloys by tin, and nickel-based alloys by oxygen. Because the cracking involves diffusive penetration along grain boundaries, the rate of cracking is highly sensitive to grain-boundary structure and composition, and these variables offer opportunities to control the problem. We are aiming at a quantitative understanding of the effects of grain-boundary structure, stress, and temperature on this phenomenon by crack-growth experiments on bicrystals, by atomistic modeling of the stress-driven diffusion, and by micromechanical modeling of the events occurring at the tip of a growing crack.

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Observation of secondary slip systems in tungsten bicrystals

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100112361 ◽

1984 ◽

Vol 42 ◽

pp. 478-479

Author(s):

J. R. Fekete ◽

R. Gibala

Keyword(s):

Stress Field ◽

Grain Boundaries ◽

Deformation Behavior ◽

Stress Axis ◽

Polycrystalline Materials ◽

Slip Systems ◽

Metallic Materials ◽

Twist Boundary ◽

Secondary Slip ◽

Plane Normal

The deformation behavior of metallic materials is modified by the presence of grain boundaries. When polycrystalline materials are deformed, additional stresses over and above those externally imposed on the material are induced. These stresses result from the constraint of the grain boundaries on the deformation of incompatible grains. This incompatibility can be elastic or plastic in nature. One of the mechanisms by which these stresses can be relieved is the activation of secondary slip systems. Secondary slip systems have been shown to relieve elastic and plastic compatibility stresses. The deformation of tungsten bicrystals is interesting, due to the elastic isotropy of the material, which implies that the entire compatibility stress field will exist due to plastic incompatibility. The work described here shows TEM observations of the activation of secondary slip in tungsten bicrystals with a [110] twist boundary oriented with the plane normal parallel to the stress axis.

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The nucleation of cavities at grain boundaries

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100126299 ◽

1987 ◽

Vol 45 ◽

pp. 288-291

Author(s):

P. J. Goodhew

Keyword(s):

Surface Tension ◽

Surface Energy ◽

Grain Boundaries ◽

Radiation Damage ◽

Impurity Concentration ◽

Driving Force ◽

Nucleation And Growth ◽

Phase Boundaries ◽

Cavity Nucleation ◽

Spherical Bubble

Cavity nucleation and growth at grain and phase boundaries is of concern because it can lead to failure during creep and can lead to embrittlement as a result of radiation damage. Two major types of cavity are usually distinguished: The term bubble is applied to a cavity which contains gas at a pressure which is at least sufficient to support the surface tension (2g/r for a spherical bubble of radius r and surface energy g). The term void is generally applied to any cavity which contains less gas than this, but is not necessarily empty of gas. A void would therefore tend to shrink in the absence of any imposed driving force for growth, whereas a bubble would be stable or would tend to grow. It is widely considered that cavity nucleation always requires the presence of one or more gas atoms. However since it is extremely difficult to prepare experimental materials with a gas impurity concentration lower than their eventual cavity concentration there is little to be gained by debating this point.

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The Mechanics of Ozone Cracking

Rubber Chemistry and Technology ◽

10.5254/1.3539891 ◽

1962 ◽

Vol 35 (1) ◽

pp. 200-209 ◽

Cited By ~ 8

Author(s):

M. Braden ◽

A. N. Gent

Keyword(s):

Tensile Stress ◽

Crack Growth ◽

Energy Requirement ◽

Critical Energy ◽

Critical Value ◽

Experimental Measurements ◽

Rubber Sheet ◽

Polymer Molecules ◽

Two Factors ◽

Applied Tensile Stress

Abstract Experimental measurements are described of the growth of a cut in a stretched rubber sheet under the action of an atmosphere containing ozone. A well-defined rate of crack growth is obtained, substantially independent of the applied tensile stress when this exceeds a critical value necessary for growth to occur at all. The rate of growth is found to be similar for a number of polymers and principally determined by the ozone concentration when the mobility of the polymer molecules is sufficiently high. When the molecular mobility is inadequate, crack growth is retarded. The critical condition is found to be similar for all the polymers examined, and largely independent of the conditions of exposure; it appears to reflect an energy requirement for growth of about 40 ergs/cm2 of newly-formed surface. The effect of the degree of vulcanization and the presence of additives, including antiozonants, on these two factors has also been examined. The dialkyl-p-phenylene diamines are found to confer protection by raising the critical energy required for growth to occur, in contrast to other protective agents which affect only the rate of crack propagation.

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