This means that precipitates — combinations of selected elements — come out solution and appear as small particles distributed throughout the alloys microstructure. This increases the yield strength of the alloy. Depending upon the combination of temperature and time used for the ageing heat treatment, both the number, size and distribution of precipitates can vary. Therefore, it is possible to produce a range of different strength levels within the same alloy composition, purely by altering the temperature and time of the ageing heat treatment.
The ageing heat treatment can involve holding the bar or plate at a single temperature for a period of time, or holding it at different temperatures for different times. The cooling rate after removal from the furnace can also influence the nature of the precipitation process, so some alloys will be slowly air cooled whilst others are quenched in oil or water.
The precipitation process relies upon complex thermodynamics — previously alloys were developed through experimentation and testing, but can now be optimised by the ability to model different alloy compositions and how precipitates form during different thermal histories. High strength alloys are an attractive proposition for many applications.
The ability to design components that fully exploit higher-strength alloys allows customers to reduce the overall material requirement, saving cost and time. In addition, lighter components can contribute savings in other areas, for instance suspended loads are reduced, thereby reducing the impact upon ancillary systems. Langley Alloys stocks a number of high strength alloys that rely upon precipitation hardening to achieve their increased strength levels. These include:. Considering that dislocations can glide only on specific atomic planes, the most probable scenario is probably that dislocations cannot cut through the precipitates as the slip plane of precipitate interior is not parallel to the slip plane of the matrix.
Although the bcc Nb is softer than the hcp Zr in terms of shear modulus, the bcc Nb precipitates in the hcp Zr matrix are actually nonshearable, strong obstacles against gliding dislocations. The excess Nb atoms formed nanoprecipitates when the alloy is subjected to high-energy particle irradiation.
This analysis is based on an assumption that the irradiation-induced hardening occurred solely due to nanoprecipitates. In reality, however, the irradiated samples may also have contained defect clusters such as dislocation loops at high density.
This hypothesis is still open for further investigation. Soft precipitates can become non-shearable obstacles against dislocations due to the effect of crystallography. Likewise, hard precipitates can become shearable if crystallography allows, i. An example shown here is a coherent fcc Co precipitate particle embedded in fcc Cu matrix [ 23 ] Figure The shear modulus of the fcc Co is about two times greater than that of the fcc Cu [ 24 ]; nevertheless, the Co precipitates are actually shearable Figure It still remains unclear how much hard particles are shearable.
It appears that this process occurs only in a limited circumstance. Otherwise, dislocations bypassed the Co precipitates via the Hirsch mechanism [ 25 ] Figure The Hirsch mechanism is similar to the Orowan mechanism but distinct in terms of the type of dislocation loop remained after the interaction. The Orowan loop is a shear dislocation loop whose Burgers vector is parallel to the loop plane, whereas the Hirsch mechanism produces a prismatic loop [ 26 ] whose Burgers vector is not parallel.
When the Burgers vector is perpendicular to the loop plane, the prismatic often exhibit one-dimensional back and forth motion along the Burgers vector [ 27 ]. The Hirsch mechanism is frequently observed in TEM in situ straining experiments using thin foil specimens, which have a less constraint for deformation in the thickness direction i. In such a thin foil geometry, screw dislocations that compensate the out-of-plane shear displacements are dominant over edge dislocations [ 28 ].
Screw dislocations exhibit cross slip slip transfer from the slip plane to another slip plane on non-shearable obstacles.
The Hirsch mechanism is induced by the cross slip of screw dislocations on the surface of obstacles [ 25 ]. Hence, the Hirsch mechanism is probably dominant over the Orowan mechanism in the deformation of thin foil samples. Coherent fcc Co precipitates embedded in fcc Cu matrix of a Cu-3 wt. The strain contrast around the precipitate particles in undeformed samples is lost in deformed samples.
Cutting of strong obstacles by dislocations: fcc Co precipitates in fcc Cu matrix [ 25 ]. The shear modulus of the fcc Co is two times larger than the fcc Cu [ 24 ]. The Hirsch mechanism [ 26 ] observed by TEM in situ straining experiments: fcc Co precipitates in fcc Cu matrix [ 25 ]. Precipitation hardening is a key research subject not only for developing new, strong materials but also for estimating the engineering lifetime of existing materials.
For instance, engineering lifetime of reactor pressure vessels RPVs of light-water nuclear reactors is determined by embrittlement due to precipitation of minor alloying elements such as Cu, Ni, Mn, and Si rather than accumulation of irradiation damages.
Since the RPVs are practically non-replaceable due to economic reasons, their engineering lifetime determines the useful lifetime of entire power plants.
Establishing a predictive model of material embrittlement loss of ductility is a long-standing challenge in fundamental physical metallurgy. Although the theory of dislocations is well established for quantitatively describing the strength of materials, the dislocation theory is incapable of directly describing the ductility. Hence, the loss of ductility has often been indirectly scaled by the degree of hardening, based on a generally accepted empirical rule that stronger materials exhibit less ductility.
The size of irradiation-induced precipitates in the RPV steels is typically a few nm. In the early stage of precipitation, they may be solute clusters rather than second-phase particles crystallographically distinct from the matrix. In order to evaluate hardening due to solute clusters, the Orowan model needs a modification as follows. The simulation by Foreman and Makin was performed not only on strong obstacles but also on weak obstacles. From Eqs. In practice, however, applying Eq.
The Russell-Brown model [ 22 ] is an alternative model, more practically useful than the previous model for this purpose Figure In this model the obstacle strength is scaled by the ratio of the energy of dislocation segments in precipitates and in matrix. The energy of dislocations is dependent on the shear modulus. The shear modulus of fcc Cu is lower than bcc Fe.
According to the results of ab initio calculations, the shear modulus of bcc Cu is even smaller. The energy of dislocation segment inside the Cu precipitate is lower than that in the matrix Fe. The Russell-Brown model for weak obstacles [ 23 ]. This model scales the obstacle strength of soft precipitates by the ratio of shear modulus between precipitates and matrix.
Ryan Wojes. Ryan Wojes wrote about commodities and metals for The Balance and worked as a metallurgist for more than 13 years. LinkedIn LinkedIn. Updated January 25, Featured Video. Cite this Article Format. Wojes, Ryan.
Learn About Precipitation Hardening. Metallic Character: Properties and Trends. An Introduction to Cryogenic Hardening of Metal. Palladium Facts Pd or Atomic Number Chemical Element Pictures - Photo Gallery. Physical Properties of Beryllium Copper. Magnesium Facts Mg or Atomic Number Sodium Element Na or Atomic Number
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