Ductility alloy

Fracture Mechanics Methods These have proved very usebd for defining the minimum stress intensity K[scc. t which stress corrosion cracking of high-strength, low-ductihty alloys occurs. They have so far been less successful when apphed to high-ductility alloys, which are extensively used in the chemicm-process industries. 

Thermal Gradients may be measured or calculated by means of heat flow formulas, etc. After they are established it is likely to be found from the formula that for most cyclic heating conditions the tolerable temperature gradient is exceeded. This means that some plastic flow will result (for a ductile alloy) or that fracture will occur. Fortunately, most engineering alloys have some ductility. However, if the cycles are repeated and flow occurs on each cycle, the ductility can become exhausted and cracking will then result. At this point it should be recognized that conventional room temperature tensile properties may have little or no relation to the properties that control behavior at the higher temperatures. 

The addition of Cr to Ni-Al alloys results in a remarkable synergistic effect which is of great technological importance. For example, chromium additions of about 10 wt% can enable external AI2O3 formation on alloys having aluminium levels as low as 5 wt%. This phenomenon has allowed the design of more ductile alloys and coatings. 

HSC involves the brittle fracture of a nominally ductile alloy under a sustained load at tensile stresses below the yield strength. For HSC to occur, hydrogen must be absorbed into the alloy and then transported through it, along either a concentration gradient or a stress gradient. Some time is therefore required for cracks to nucleate. HSC is normally associated with steels but has been observed to a limited extent in other materials when hydrogen is concentrated near the surface. 

It is possible that CF, SCC, and HE could occur at the same time in some service conditions. The simultaneous operation of SCC and HE can occur in some systems. The interrelationship among stress corrosion, CE, and HE are discussed in the literature. The cross-hatched regions represent the most serious practical situations involving ductile alloy/enviromnent systems. These regions indicate the combination of any two failure mechanisms. In the center, all three phenomena interact, which is probably realistic in ductile alloy/aqueous environment systems (4). 

The next generation of fibre-reinforced materials were the metal matrix composites (MMC) of ductile alloys reinforced with hard carbon or ceramic fibres and whiskers. The MMC offer serious advantages over other types of composites, mainly in the following aspects ... 

Figure 7.11 shows the fracture surface of an alloy that is similar to the ductile alloy in Figure 7.9 but has a solute adsorbed at its grain boundaries and has undergone brittle intergranular fracture with a stress-strain curve also corresponding to Figure 7.8(b). The latter behavior is the... 

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