Crack grain boundary

Defects/impuritiss Point defect Dislocation/crack Crack/grain boundary... 

A very simplified model for such a fractured rock is demonstrated below (Schon, 1973,1996). Starting with a cube of solid material, it is assumed that the effect of aU defects (fractures, cracks, grain boundaries, intra-granular defects, etc.) can be described by one defect parameter D (Fig. 6.40). 

LiNi02 has two problems. We have just evidenced that Ni has a tendency to occupy Li sites and generate a deviation fi om stoichiometry, which alters the electrochemical properties. The second problem is due to the fact that Ni " is a Jahn-Teller ion, which means that the d-electrons in this configuration create a local lattice distortion. The accumulation of such distortions upon cycling favors the formations of cracks, grain boundaries and other extended defects that reduce the lifetime of the battery when this material is used as the active element of the cathode. 

A series of events can take place in response to the thermal stresses (/) plastic deformation of the ductile metal matrix (sHp, twinning, cavitation, grain boundary sliding, and/or migration) (2) cracking and failure of the brittle fiber (5) an adverse reaction at the interface and (4) failure of the fiber—matrix interface (17—20). 

Grain boundaries have different corrosion properties from the grain and may corrode preferentially, giving cracks that then propagate by stress corrosion or corrosion fatigue (Fig. 23.10). 

Henry Bessemer, the great Victorian ironmaster and the first person to mass-produce mild steel, was nearly bankrupted by this. When he changed his suppliers of iron ore, his steel began to crack in service. The new ore contained phosphorus, which we now know segregates badly to grain boundaries. Modern steels must contain less than =0.05% phosphorus as a result. 

Poor Weldability a. Underbead cracking, high hardness in heat-affected zone. b. Sensitization of nonstabilized austenitic stainless steels. a. Any welded structure. b. Same a. Steel with high carbon equivalents (3), sufficiently high alloy contents. b. Nonstabilized austenitic steels are subject to sensitization. a. High carbon equivalents (3), alloy contents, segregations of carbon and alloys. b. Precipitation of chromium carbides in grain boundaries and depletion of Cr in adjacent areas. a. Use steels with acceptable carbon equivalents (3) preheat and postheat when necessary stress relieve the unit b. Use stabilized austenitic or ELC stainless steels. 

Stress in crystalline solids produces small shifts, typically a few wavenumbers, in the Raman lines that sometimes are accompanied by a small amount of line broadening. Measurement of a series of Raman spectra in high-pressure equipment under static or uniaxial pressure allows the line shifts to be calibrated in terms of stress level. This information can be used to characterize built-in stress in thin films, along grain boundaries, and in thermally stressed materials. Microfocus spectra can be obtained from crack tips in ceramic material and by a careful spatial mapping along and across the crack estimates can be obtained of the stress fields around the crack. ... 

Grain-boundary cracking in the presence of liquid metal. 2ygi — 7a = 0-2 J/m ... 

It is seen that the presence of the liquid metal greatly lowers the surface-energy change for grain-boundary cracking ... 

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

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