At the temperature where the stress-strain curves showed serrations, the dislocations observed are wavy, whereas in many of the structures MoSi 2 , WSi 2 the dislocations are straight. Andrade was the first to formulate creep phenomena indicating its stress, time and temperature dependence.
Disregarding instant elongation, creep is known to occur at three stages: transient creep, steady state creep and accelerated creep. Grain size has an important effect on the creep behavior and in polycrystals large grain size is essential, but single crystalline components are preferential. Basic relations for creep are presented for the creep rate showing the importance of d the grain size, p the grain size exponent and n the stress exponent. There is a transition from Newtonian viscous flow which is self diffusion dependent to a power-law creep associated with dislocation climb.
Dislocation creep involves glide and climb and both are associated with diffusion and the slowest of them controls the creep rate. A threshold stress and temperature exist below which creep will not occur. Repeated loading cyclically or a fluctuating stress may induce fatigue in a component. Plots of S-N curves define an endurance limit which in Fe and Ti is a horizontal line.
At the stress level defined by the horizontal line or below it a material endures a very large number of stress cycles without failure. However, in most other materials no definite endurance limit exists.
Chemical Vapor Deposition of Tungsten and Tungsten Silicides for VLSI/ ULSI Applications
Avoiding catastrophic and unexpected failure requires the best possible design and choice of material, therefore various combinations of components rather than the pure components are used to avoid fatigue faiure. These reinforcements may be other silicides, aluminides, carbides, nitrides or elements.
MoSi 2 reinforced with Nb in various forms is a most commonly used composite silicide and the largest improvement occurs when it is added as fibers. Basic fatigue equations are presented. Fracture is orientation and temperature dependent and occurs at some orientations even before the yield stress. An important design parameter is the fracture toughness determined by the stress intensity factor which can be evaluated from hardness measurements.
Fracture toughness describes the ability of a material to resist fracture. These silicides are generally brittle but ductility sets in at some elevated temperature depending on the type of the silicide and its orientation.
Catastrophic fracture should be eliminated by choosing the proper silicide, orientation and conditions of use. Strength properties are size dependent and increase with decreasing dimensions. The mechanical properties sharply deviate from those of macro-scale structures.maisonducalvet.com/la-cabrera-citas-online.php
Selective CVD tungsten silicide for VLSI applications - Semantic Scholar
It is expected to obtain improved strength properties in nano structures. Because the dislocation motion—if present at all—is restricted theoretical strength is anticipated. Contrary to macrostructures increase in strength is accompanied by increase in elongation. The theoretical tensile strength of MoSi 2 and WSi 2 were calculated and hardness measurements were measures.
Selective CVD tungsten silicide for VLSI applications
Segregation of B to grain boundaries strengthen them. The outstanding feature of B is on inducing ductility in brittle material. Also the oxidation resistance is improved as exemplified for MoSi 2 and pest formation is eliminated. Hardness and fracture toughness are higher in B added silicides as illustrated for MoSi 2. B added TiSi 2 coatings are an example of the effect of B additions, but no direct evidence of the effect of B on the mechanical properties exists, probably because efforts were directed to evaluate its use in VLSI.
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