Microhardness indentation markers

Table 1.3. Typical changes in distance between the microhardness indentation markers onto the surface of a Ni-Bi cross-section (see also Figs 1.17-1.19). Temperature 250°C. The time of the first anneal is 18xl04 s (50 h), while that of the second is 9xl04 s (25 h), 27xl04 s or 75 h in total150...

Before the third anneal, microhardness indentation markers were put onto the electropolished cross-section surface in the Ni phase (five markers, about 50 pm from each other), in the middle of the NiBi3 layer (one marker) and in the Bi phase (five markers, about 75 pm from each other). Only part of them are shown in Fig. 1.19. 

The specimen, most suitable for such measurements, is shown schematically in Fig. 1.8. The upper part of the specimen is used for comparison. To prevent the interaction of components A and B in this part, a thin barrier layer of some substance which does not react with both A and B under chosen experimental conditions is deposited. The position of the layer interfaces is measured at certain moments of time relative to the inert markers located at the initial interface between substances A and B and/or inside the ApBq layer. Microhardness indentations onto the specimen cross-section surface, thin wires and strips of chemically inert materials, bubbles of inert gases, etc., can serve as the markers (for more detail, see for example Refs 35, 124). 

The compound layer thickness was determined using a conventional microhardness tester and an optical microscope. The microhardness tester was also used to put inert markers onto the surfaces of the phases involved in the interaction before each anneal of the Ni-Bi couples, except the first one when the intermetallic layer was still too thin. The microhardness indentations are known to possess advantages over other markers (inert... 

Note that the distances between the markers located in the Ni phase did not change in the course of annealing, indicative of no formation of a solid solution of bismuth in nickel. The distances between the markers located in the Bi phase remained unchanged as well. Also, no changes in size or configuration of the microhardness indentations were observed in either phases. Hence, the components are indeed mutually insoluble at 150-250 °C. 

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