Precipitation hardening

The precipitation hardening is a heat treatment to increase the hardness and strength of alloys. The process is also referred to as curing. It is based on the precipitation of metastable phases in finely divided form, so that they constitute an effective barrier to the movement of dislocations. The yield strength of metals can be increased by up to 300 MPa.

Basics

Upon curing is used, that the solubility of one or more alloying elements is decreased with the lowering of the temperature. Therefore, the curing is not possible for all alloys, but only if certain conditions are met.

Requirements

• The alloy forms an alloy element at elevated temperature elementary mixed crystals.
• This alloy element has to have a decreasing solubility with decreasing temperature in the base metal.
• Driving force and diffusion rate must be sufficiently large in the precipitation temperature.
• The resulting precipitates dispersed in the material must be present and stable at operating temperatures against coagulation.

Treatment steps

We divide the curing in the three treatment steps solution annealing, quenching and aging ( leaving ).

Solution annealing ( diffusion annealing, homogenizing )

The alloy is heated until there are all the necessary elements for precipitation in solution. It should be a certain temperature can not be reached, since otherwise remain coarse particles that are detrimental to the mechanical properties of the material. On the other hand, must not be exceeded too high a temperature, otherwise melt individual structural constituents; , the alloy can not then be further processed.

Put off

By quenching the diffusion and thus an elimination of coarse particles can be prevented, and the solid solution remains in the metastable supersaturated phase state.

Outsourcing

By a subsequent annealing at 150 to 190 ° C (450 to 500 ° C in maraging steels ) can now be made up diffusion. The supersaturated single-phase solid solution is transformed into a two-phase alloy so. The volume -related and usually occurs with a higher proportion is mentioned phase matrix, the other precipitation. The type and rate of precipitation depends on the temperature, since the driving force of the diffusion is also temperature dependent.

As in the previous quenching many germs were formed, many small precipitates are formed which are homogeneously distributed in the microstructure. Thus, the properties of the workpiece can be tailored. Depending on the material and process, the precipitates concentrate in a certain way and interfere with its differing from the matrix crystal structure, the movement of dislocations and thus increase the strength of the metal. The precipitates can be coherent, partially coherent or incoherent to the matrix. Coherent precipitates are located inside a grain and join alloying elements on with similar lattice parameters. The highest increase in strength is usually achieved at particle sizes below 50 nm - the optimal particle radius is dependent on the physical properties of the matrix and precipitate phase. Alloying elements with different lattice parameters differ often incoherent on the grain boundaries. Incoherent precipitates may be spherical, when the precipitation has a relatively high surface energy, or dispersed, if the surface energy is very low.

Particles that are already eliminated during the diffusion annealing or earlier, dispersoids are called. They control the recrystallization by impeding grain boundary movements. Because of its low content in the alloy, their size and their incoherence to the matrix strength increase is usually negligible.

Similar operations as in the precipitation also occur during aging and the effect of BH steel.

In addition to the precipitation of further options relating to the increase in strength include warehousing of foreign atoms in the solid solution, the strengthening by cold forging, the grain refinement and the diffusionless transformation ( Umwandlungshärtung ).

Hardening aluminum alloys

The precipitation is the main way of increasing the strength of aluminum alloys, since they have no polymorphic conversion, and thus are not hardened by the formation of martensite.

A prominent example of the precipitation is the duralumin, an alloy of aluminum, 4% copper and 1% magnesium. Solution annealing is carried 495-505 ° C. After quenching, the material can be formed, in contrast to steel, is duralumin after quenching initially soft. The final strength is through natural aging ( at room temperature) or artificial aging reached ( a Ausscheidungsglühung ). Curing may be delayed by deep cooling ( min. -18 ° C). This is used for example, rivets of such alloys in order to achieve a longer working time. Almost all treatable aluminum alloys are susceptible to corrosion because the alloying elements hinder the formation of a closed oxide layer.