Intermetallic compounds hardnesses

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

Intermetallic compounds derive their great usefulness by blending metallic and covalent bonds. The former generate toughness, while the latter provide strength and hardness. In many of them dislocations move with great difficulty. 

The vast array of intermetallic compounds seems limitless, although it is finite. The number of binary compounds alone is in the thousands, and the number of ternary compounds is much larger. Still larger is the number of quaternaries, quin ternaries, and so on, ad infinitum. Not all of them are useful, of course, and there are many compounds having the same hardnesses as others. These overlapping cases are usually separated when other properties—such as corrosion resistance, toughness, or cost—are taken into account. 

Data are insufficient for calculation of the hardnesses of most intermetallic compounds. Also, many are too complex for realistic calculations to be made. In these cases empirical correlations with shear moduli are most likely to give... 

In general, there are insufficient data available for quantitative estimates to be made of the hardnesses of intermetallic compounds. However, in some cases trends can be verified. Figure 8.11 illustrates one of these. It indicates that hardnesses and heats of formation tend to be related. In this case for a set of transition metal aluminides. The correlation in this case might have been improved if the heats per molecular volume couls have been plotted, but thr molecular volumes were not available. Nevertheless, the correlation is moderately good indicating that hardness and chemical bond strengths are related as in other compounds. 

E. R. Petty, Hot Hardness and Other Properties of Some Binary Intermetallic Compounds of Aluminum, Jour. Inst. Metals, 89,343 (1960-61). 

There are a number of intermetallic compounds that are hard at high temperatures (Fleischer and McKee, 1993). A few examples are AlNb2, AlZr2,... 

Figure 15.1 Comparison of the hot hardnesses of three strong oxides and the strong intermetallic compound, Ni3Al. Yttrium aluminum garnet is the hardest of all oxides at high temperatures.

Many alloys possess properties that would hardly be predicted from the known properties of the constituent components. For example, some of these alloys are magnetic, though neither of the components is markedly so (Heussler alloys). An example is the manganese-antimony alloy (intermetallic compound). 

The beryllides, being intermetallic compounds, are hard, strong materials which exhibit litde ductility at room temperature. Strength properties increase gradually as a function of temperature up to about 870°C, above which a sharp increase in strength occurs, peaking in the region of 1260°C the modulus of rupture values exceed 280 MPa (40,000 psi) at this latter temperature. 

A new system which might be extremely interesting in this context is the interme-tallic phase ZrW2. ZrW2 is a high-density, very hard intermetallic compound that reacts/burns in a highly exothermic reaction with air at high temperatures. This intermetallic phase should provide a very suitable reactive material for warhead applications. 

The difference in the chemistry of the light and heavy actinides may be rationalized in this way. The early members beyond thorium have unpaired d and / electrons available for forming covalent bonds and hence, for example, they readily form many complex ions and intermetallic compounds. Such ions are soft acids. Beyond americium, the 5/ electrons are not competitive and the closed shell of six 5/5/2 electrons will not be readily available for bonding, so that only those / electrons with /=7/2 are available. These tend to become buried radially as the atomic number increases and hence their divalent ions become relatively hard Lewis acids. These considerations are especially helpful in the region of superactinides because these elements do not have analogs in the known periodic table, where we have deeply buried but loosely bound 5g electrons. 

As discussed in Section 4.5, the desired properties in materials for permanent magnet applications are essentially a high remanent induction, Bf, a large coercive field (either gH or jifc) and a large energy product A brief discussion of the various mechanisms leading to increased coercivity is presented first. Since most of the new hard magnets are intermetallic compounds, a short account of their anisotropies is included. 

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