The precipitation hardening is a heat treatment for increasing the strength of alloys . The process is also known as curing . It is based on the excretion of metastable phases in a finely divided form, so that they represent an effective obstacle to dislocation movements. The yield strength of metals can be increased by up to 300 MPa.
When curing is used, that the solubility of one or more alloying elements decreases with the lowering of temperature. Therefore, hardening is not possible with all alloys, but only if certain requirements are met.
- The alloy forms mixed crystals with one or more alloying elements at elevated temperature .
- The alloy components of the hardenable alloy must show a decreasing solubility as the temperature falls.
- The alloying elements must have a solubility in the base metal that decreases with decreasing temperature.
- Driving force and diffusion speed must be sufficiently high at the precipitation temperature to do the work of nucleation.
- The resulting precipitates must be finely distributed in the material and be resistant to coagulation at operating temperatures .
Hardening is divided into three treatment steps: solution heat treatment , quenching and aging (precipitation).
Solution annealing (diffusion annealing, homogenizing)
The alloy is heated until all the elements required for precipitation are in solution. The temperature should not fall below a certain level, otherwise coarse particles will remain, which are detrimental to the mechanical properties of the material. On the other hand, the eutectic temperature of the alloy must not be exceeded, since otherwise areas with enrichment of alloy elements could melt due to segregation .
The solution annealing time can last from a few minutes to hours and depends on the structure formation (fine, coarse-grained), type of alloy, type of semi-finished product (rolled, forged) and dimensions of the component.
By quenching can diffusion and thus a formation to prevent secretions. The mixed crystal remains in the metastable, supersaturated single-phase state. This is achieved by cooling at at least the critical speed . Cold water, tempered water, oil or compressed air can serve as a quenching medium.
Subsequent aging at 150 ° C to 190 ° C (450 ° C to 500 ° C for maraging steel ) accelerates the diffusion . The aging temperature depends on the alloy. The supersaturated single-phase mixed crystal is transformed into a two-phase alloy by the formation of precipitates. The phase that is coherent in volume and usually occurs with a higher proportion is called the matrix , the newly formed excretion . The type and speed of excretion is temperature-dependent, since the driving force of diffusion is also temperature-dependent.
Since many nuclei were formed during the previous quenching, many small precipitates are formed which are homogeneously distributed in the structure . This allows the properties of the workpiece to be set in a targeted manner. The precipitates and the distortion fields they generate in the matrix lattice prevent the dislocations from sliding and thus increase the technological strength and resistance to plastic deformation. The excretions can be coherent , partially coherent or incoherent with the matrix. Coherent precipitates are located within a grain and occur with alloy elements with similar lattice parameters . The highest increase in strength is usually achieved with particle sizes below 50 nm - the optimal particle radius depends on the physical properties of the matrix and the precipitation phase. Alloy elements with different lattice parameters often precipitate incoherently on the grain boundaries. Incoherent precipitates can be spherical if the precipitate has a relatively high surface energy , or dispersed if the surface energy is very low. Due to their surface energy, incoherent precipitates have a tendency to grow. The large excretions grow through the dissolution of small excretions, and coagulation occurs. A decrease in strength due to aging is observed.
Particles that precipitate during diffusion annealing or earlier are called dispersoids . They control recrystallization by hindering grain boundary movements. Because of their low content in the alloy, their size and their incoherence with the matrix, their increase in strength is usually negligible.
Similar processes to precipitation hardening also occur with aging and the BH effect of steel .
In addition to precipitation hardening, other options for increasing strength are u. a. the incorporation of foreign atoms in the mixed crystal , solidification through cold forming, grain refinement and non-diffusion transformation ( transformation hardening ).
Hardening of aluminum alloys
Precipitation hardening is the most important way of increasing the strength of certain aluminum alloys ( aluminum-copper alloys and aluminum-magnesium-silicon alloys ), as these do not have a polymorphic transformation and therefore cannot be hardened through martensite formation.
A prominent example of precipitation hardening is duralumin , an alloy made from aluminum , 4% copper and 1% magnesium . The solution heat treatment takes place between 495 ° C and 505 ° C. After quenching, the material can be reshaped. The final strength is achieved through cold aging (at room temperature) or artificial aging (precipitation annealing ). A noticeable hardening phenomenon is already present after several minutes at room temperature. This reaches its maximum after about 4 days.
The curing processes can be inhibited by deep-freezing (min. −20 ° C). This is used, for example, for rivets made of such alloys in aircraft construction in order to achieve a longer processing time. The rivets are stored in a cooling container until they are further processed in the quenched, supersaturated state. Then cold curing takes place at room temperature.
Age-hardenable aluminum alloys are more susceptible to corrosion than pure aluminum, since the precipitates prevent the formation of a closed oxide layer.
- ^ T. Gladman: Precipitation hardening in metals . In: Materials Science and Technology . tape 15 , no. 1 , 1999, p. 30-36 , doi : 10.1179 / 026708399773002782 .
- ↑ a b c d Bergmann, Wolfgang: Material technology: Application: with 44 tables. 4th edition Munich: Hanser, 2009.
- ↑ Bergmann, Wolfgang: Material engineering 1: Structural structure of materials - Metallic materials - Polymer materials - Non-metallic-inorganic materials. M: Carl Hanser Verlag GmbH & Co KG, 2013.
- ↑ Manfred Riehle, Elke Simmchen: Fundamentals of material technology . 2nd Edition. German Verlag für Grundstofftindustrie Stuttgart, p. 250 .