Precipitate Hardening

Observations on precipitation hardening make it evident that in certain cases the key microstructural features in this context are the mean particle radius and the 

11 Points, Lines and Walls Defect Interactions and Material Response 

At this point our task is to relate the areal density pArea to the volume fraction of second-phase particles, denoted by /, and to the particle radius R. 

This result may be used to compute the area per obstacle Aobstacie)- Note that the area per obstacle is related to the areal density by pArea = 1/ Aobstacie- The area per obstacle is found by dividing the total area of the slip plane by the number of such particles and yields the estimate 

In light of these arguments, the critical stress for particle cutting is of the form 

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The enhanced strength and corrosion properties of duplex stainless steels depend on maintaining equal amounts of the austenite and ferrite phases. The welding thermal cycle can dismpt this balance therefore, proper weld-parameter and filler metal selection is essential. Precipitation-hardened stainless steels derive their additional strength from alloy precipitates in an austenitic or martensitic stainless steel matrix. To obtain weld properties neat those of the base metal, these steels are heat treated after welding. 

Both sohd-solution hardening and precipitation hardening can be accounted for by internal strains generated by inserting either solute atoms or particles in an elastic matrix (11). The degree of elastic misfit, 5, produced by the difference, Ai , between the lattice parameter, of the pure matrix and a, the lattice parameter of the solute atom is given by... 

Precipitation Hardening. With the exception of ferritic steels, which can be hardened either by the martensitic transformation or by eutectoid decomposition, most heat-treatable alloys are of the precipitation-hardening type. During heat treatment of these alloys, a controlled dispersion of submicroscopic particles is formed in the microstmeture. The final properties depend on the manner in which particles are dispersed, and on particle size and stabiUty. Because precipitation-hardening alloys can retain strength at temperatures above those at which martensitic steels become unstable, these alloys become an important, in fact pre-eminent, class of high temperature materials. 

Precipitation hardening consists of solutioning, quenching, and aging. Solutioning entails heating above the solvus temperature in order to form a homogeneous soHd solution. 

Fig. 1. Development of wrought nickel alloys (see Tables 5 and 6), where ( ) represent soHd solution material and (C3) represent precipitation-hardenable...

Several of the Al—Li alloys developed in the 1980s contain both magnesium and copper. No quaternary Al—Cu—Li—Mg phase has been found in the alloys. The S -phase in addition to 5 and provides precipitation hardening. 

In Ni—P electroless deposits, there can be as much as 10% by weight of phosphoms. The amount depends on the added complexing agents and the pH. The Ni—P deposits are fine-grained supersaturated soHd solutions, which may be precipitation hardened by heat treatment to form dispersed Ni P particles in a nickel matrix. 

Because these alloys are precipitation hardenable, they can be customized for specific requirements across a wide range of property combinations. Advances in composition control, processing, and recycling technology have broadened the capabiUties and expanded the range of appHcation. Data sheets pubhshed by the manufacturers and others (41) give compositions, properties, and typical appHcations. 

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