Grain-boundary hardening

Figure 11. Grain-boundary hardening in AgMg, iUustrating hardening only with a stoichiometric excess of the more dectropositive element (Mg) (From Westbrook and Wood, 1%3)...

The addition of boron to austenitic Fe-Ni alloys causes grain boundary hardening flffough grain boundary segregation [1991 She]. 

Fig. 17. Structuie of U-700 after piecipitation hardening temperature of 1168 C/4 h + 1079" C/4 h + 843 C/24 h + TGO C/IG h with air cooling from each temperature. A grain boundary with precipitated carbides is passing through the center of the electron micrograph. Matrix precipitates are y -Nij(TiAl).

Mott played a major part, with his collaborator Frank Nabarro (b. 1917) and in consultation with Orowan, in working out the dynamics of dislocations in stressed crystals. A particularly important early paper was by Mott and Nabarro (1941), on the flow stress of a crystal hardened by solid solution or a coherent precipitate, followed by other key papers by Koehler (1941) and by Seitz and Read (1941). Nabarro has published a lively sequential account of their collaboration in the early days (Nabarro 1980). Nabarro originated many of the important concepts in dislocation theory, such as the idea that the contribution of grain boundaries to the flow stress is inversely proportional to the square root of the grain diameter, which was later experimentally confirmed by Norman Fetch and Eric Hall. 

Al-Mg (5000 Series) and Al-Mg-Si (6000 Series) In the binary alloy system strength is obtained mainly by strain hardening. Stress corrosion is thought to be associated with a continuous grain boundary film of Mg,Alg which is anodic to the matrix . Air cooling prevents the immediate formation of such precipitates, but they form slowly at ambient temperatures. Thus only low Mg alloys are non-susceptible (Al-3% Mg). Widespread precipitation arising from plastic deformation with carefully controlled heat-treatment conditions can lower susceptibility. Al-5Mg alloys of relatively low susceptibility are subjected to such treatments. Mn and Cr... 

Fig. 20.SS Transmission electronmicrograph showing intergranular and imragranular precipitation and the precipitate-free zone adjacent to the grain boundary in a high-strength precipitation-hardening Al-Zn-Mg alloy (x24 000, courtesy G. Lorimer)...

Tritium and its decay product, helium, change the structural properties of stainless steels and make them more susceptible to cracking. Tritium embrittlement is an enhanced form of hydrogen embrittlement because of the presence of He from tritium decay which nucleates as nanometer-sized bubbles on dislocations, grain boundaries, and other microstructural defects. Steels with decay helium bubble microstructures are hardened and less able to deform plastically and become more susceptible to embrittlement by hydrogen and its isotopes (1-7). 

The presence of grain boundaries also affect slip in the material even after plastic deformation has commenced. A phenomenon known as strain hardening, or work hardening or cold working, is the result of constrained dislocation mobility and increased... 

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