Properties of Alloys

PROPERTIES OF ALLOYS Properties of Pure Metals.—Theproperties of an alloy de pend on the properties, the proportions and arrangement of the constituents. Pure metals differ greatly in specific properties, such as hardness, melting point, electrical conductivity, ductility, density, etc. Many of these properties are transmitted to alloys of which a pure metal is one constituent, and the effect can be approximately calculated by applying the rule of mixtures. One of the outstanding characteristics of metals is their ability to be plastically deformed without rupture. This plastic deformation is possible because a portion of a crystal of a metal will slip on one or more of the crystallographic planes without rupture. In general, slip takes place the more easily the purer the given metal, and the larger the grains. Slip takes place at a much lower unit load than that which would be required to separate one plane of atoms from the other simultaneously. The total attractive forces on one of these planes would be the sum of the attractive forces of all of the atoms participating, a value termed "absolute cohesion," which is large as compared with the force required to produce slip. An automobile, weighing two tons, standing on a smooth but level pavement exerts a pressure on the pavement equal to its weight. A force of 15o lb. applied horizontally, how ever, may start the car in motion. This latter force would be comparable to that required to produce slippage in a metal crystal, whereas the weight of the automobile would be compared with the absolute cohesion. That the absolute cohesion is markedly greater than the unit force required to produce slip in a plastic metal crystal is clearly demonstrated by the fact that the same atoms can be differently arranged, as by reduction in grain size or by cold working, so as to greatly increase not only the force required to produce slip but also that required to produce rupture. A single crystal of iron, for example, may have a tensile strength of only II tons per square inch. Iron of the same purity com posed of small grains may have a tensile strength of 20 tons per square inch. Another piece of iron of the same purity. drawn cold through a number of dies may have a tensile strength of 4o tons per square inch. In general, the strength and hardness of a given pure metal increase with decrease in grain size, with decrease in temperature, and with increase in degree of cold work. There are some exceptions to the rule in connection with temperature change, notably in iron which undergoes an allotropic transforma tion at about 900° C and has a higher tensile strength in the gamma condition slightly above 900° C than in the alpha condition slightly below this temperature. Also, commercial iron and most of the ordinary steels have higher tensile strength at about 200° C than at room temperature. Because the softness of a metal is dependent on ease of slip on the crystallographic planes, any condition which interferes with the ease or freedom of this slip movement is a hardening factor. The factors favouring high plasticity are generally those which favour softness, namely large grain size, freedom from cold work and high temperature. Properties of Solid Solutions.—Theproperties of very dilute solid solutions are usually quite similar to the properties of the commercially pure solvent metals themselves. The departure from similarity takes place gradually as the amount of solute in creases, and obtains to. a different degree for equal amounts of different solutes. Pure metals freeze or melt at a constant tem perature, whereas most solid solution alloys freeze or melt through out a range of temperature. Solid solution alloys are harder and in general less plastic than the solvent metal. The electrical conductivity is less than that of the solvent metal. In fact, this property changes to a much greater extent than would be indi cated by the law of mixtures. The hardness also disobeys the law of mixtures. For example, tin is much softer than copper, but a solid solution of tin in copper is considerably harder than the copper itself. The temperature coefficient of electrical resistivity of solid solution alloys is usually lower and sometimes markedly lower than in the pure metals. Certain solid solution alloys pos sess a zero or even slightly negative temperature coefficient. Also, the thermoelectric force of a solid solution is often markedly different from that of the solvent metal.

Properties of

Compounds.—Compounds, and particularly inter-metallic compounds, are characterized by relatively high hardness and low plasticity. Metallic compounds are very im portant constituents in alloys from the standpoint of increasing hardness, decreasing grain growth and particularly for the pur pose of maintaining strength of alloys at high temperatures. Each compound has its specific properties, some having high melt ing point, some low, some with high hardness, some with low hardness, a few with considerable plasticity, but many with practically no useful plasticity.

Proportions and Arrangements of Constituents.

Certain properties of alloys, such as density and resistance to penetration by X-rays, may be determined by the proportions and specific properties of the various constituents. Many of the properties, however, depend to a greater or less extent on the arrangement as well as the proporti In an alloy containing more than one type of constituent, the most important thing to be considered is which constituent possesses continuity. This is particularly im portant if the different constituents vary considerably in hard ness and plasticity.

The importance of the degree of refinement of the separate con stituents has been demonstrated forcibly in recent years. In many alloys the proportions of the constituents may be changed by heat treatment, and at the same time the manner of aggregation may be changed. Using the system as an example, the alloy containing 6% will consist largely of solid solution of nickel and silicon in copper after quenching from 900° C. The Brinell hardness is about 7o. By heating to 5 2 5 ° C a short time a considerable portion of the nickel and silicon precipitate in submicroscopic particles of increasing the hardness to above zoo. During this heating period the hardness rises for a certain length of time, reaches a maximum and eventually may decrease somewhat to approximately a constant value. These changes in hardness are associated with change in amount and size of the hard particles of There appears to be an average size of precipitated particle which produces maximum hardening effect for a given quantity of the constituent. In this condition, referred to as "critical dispersion" for maximum hardness, the particles are too small to be resolved with a micro scope.

The above is an example of what has become a common prac tice in commercial selection and heat treatment of a number of alloy types. The high temperature step in the heat treatment is referred to as the solution treatment, and the low temperature step as precipitation treatment. Many alloys have been found to exhibit changes in properties and structure when subjected to this type of heat treatment. The alloy compositions must be carefully selected with reference to solubility limits of solid solutions, change of . solubility with change in temperature, and the temperature and time of the heat treatment must be varied tc1 suit the particular alloy. Some alloys, for example, duralumin and lead-antimony alloys, need not be given an "artificial" precipitation heat treatment because the precipitation takes place spontaneously at room temperature. Other alloys must be heated above room temperature in order to cause precipitation. These changes in particle size and in the amount of solute in a solid solution are possible in alloys only because of diffusion in the solid state. Diffusion is supposed to occur by migration of solute atoms in the grain boundaries and in the space lattice of the solvent. Rapid diffusion is favoured by high temperature and at certain relatively low temperatures, different for different alloys, diffusion may, for practical purposes, be considered not to take place. Also diffusion is more rapid along grain boundaries than through the crystalline grains themselves.