THE DISPERSED PHASES PRESENT IN METALS

In most theoretical discussions of strengthening mechanisms in particle-hardened alloys, attention is typically confined to the interaction of glide dislocations with finely dispersed precipitates typically 10nm in size.

In real alloys of this sort, however, dispersions of coarser particles also exist, which can play a crucial role within the deformation behavior and particularly within the fracture behavior of the alloy. The dispersed phases that will be present are often conveniently classified into three families, the hardening precipitates themselves, the coarse ‘residual’ particles, and also the so-called ‘dispersoids’ whose size range is intermediate between that of the opposite groups.

Hardening precipitates These may zero in size from, say, 1 to 100nm.

In steels and in age hardening non-ferrous alloys the particles are formed by precipitation from supersaturated solution, and this constitutes far and away the foremost commonly used technique for producing a dispersed second phase.

Other methods include diffusion reaction techniques, like nitriding of steel or internal oxidation of, for instance, copper alloys, and powder-metallurgical techniques like ‘mechanical alloying’, and that we will briefly consider at this time the assembly of oxide dispersion strengthened (ODS) materials.

INTERNAL OXIDATION

In this technique an alloy consisting of a dilute primary solid solution of a base metal during a more metallic element is heated under oxidizing conditions, when oxygen diffuses into the alloy and a dispersion is produced of the oxide of the bottom metal during a matrix of the metal.

Silver, copper and nickel have commonly been employed as solvent metals, and suitable solutes include silicon, aluminum, magnesium and beryllium.

After internal oxidation, dispersions of silica, alumina, magnesia or beryllia are formed during a matrix of Ag, Cu or Ni, and these oxides are stable at temperatures up to it of their formation, which might be near the freezing point of the alloy.

The time required for the whole internal oxidation of huge cross-sections is incredibly long, since the speed of formation of an indoor oxide layer follows a parabolic law.

Furthermore, the oxide particle size isn’t uniform, but becomes coarser because the depth from the specimen surface increases.

A limited amount of internally oxidized material with useful engineering properties has been produced industrially by employing a powder-metallurgical approach: if the alloy is in affinely divided form, diffusion times are short for the entire internal oxidation of every powder granule.

The oxidized (two-phase) product can then be compacted and densified into a useful product. a way more flexible powder-metallurgical technique for the assembly of ODS materials is that of mechanical alloying.

MECHANICALLY ALLOYED ODS MATERIALS

Conventional metallurgy involving the mechanical mixing of metal and oxide powders cannot achieve a sufficiently uniform dispersion of oxide particles within the final product, which is crucial permanently mechanical properties.

In 1970 J.S. Benjamin announced the invention of mechanical alloying, during which elemental or alloyed powders, along with the oxide are charged into a dry, high-energy, high-speed ball mill, referred to as a Sevara attritor grinding mill.

Eventually the method establishes an identical fine oxide dispersion (of sizes of the order of tens of nanometers) within relatively coarse (60-100 J.Lm) powder particles.

The mechanically alloyed powders are finally consolidated, usually by placing them in an exceedingly steel can followed by degassing under vacuum, and hot extrusion.

COARSE RESIDUAL PARTICLES

When considering such particles, which are larger than 1 J.Lm in size, it’s convenient to debate separately the occurrence of those coarse inclusions in steels and people encountered in non-ferrous alloys.

INCLUSIONS IN STEELS

The origin and constitution of non-metallic inclusions in steels are the topic of intense study over a few years, because it has long been recognized that they’re a possible source of weakness.

For this reason, strenuous efforts are now made within the production of ‘clean’ steels for several applications. The three main sources of non-metallic inclusions are: (i) deoxidation, and also the segregation of the products of deoxidation; (ii) the presence of Sulphur and phosphorous, and therefore the segregation of their compounds; (iii) extraneous sources, including the trapping of slag and eroded refractory materials within the molten steel.

INCLUSIONS IN NON-FERROUS ALLOYS

Coarse insoluble particles are formed during casting of non-ferrous alloys, and although these could also be choppy and distributed more uniformly through the structure by hot-working of the ingot, they’re again recognized as a possible source of weakness within the material.

Commercial aluminum alloys contain from about 1% to five by volume of huge iron or silicon-rich inclusions, and should also contain copper-bearing particles arising from non-equilibrium micro segregation during solidification.

Iron is, of course, the principal impurity in bauxite, so its presence isn’t unexpected within the final product. These particles in commercial aluminum alloys are often stated as constituent particles.

INTERMEDIATE-SIZED DISPERSOIDS

These may zero in size from, say 0.1 to 1 urn, and that we will again take as our example aluminum alloys (in which such particles are commonly found), although they’ll occur in many materials. Chromium, zirconium, or manganese is added to several commercial wrought aluminum alloys. The element usually remains in solution during casting, but during fabrication these alloys are normally given a so-called ‘homogenization’ heat-treatment at relatively warm temperature.

The heat-treatment leads to the formation of particles of inter metallic compounds containing chromium, zirconium or manganese, whose size and spacing rely upon the temperature and time of homogenization.