Yield precipitation hardening

There are limits to the precipitation hardening that can be produced by direct cooling if the cooling rate is too liigh we will miss the nose of the C-curve for the precipitation reaction and will not get any precipitates at all But large increases in yield strength are possible if we age harden the alloy. 

Alloy 718 sheets (3.2 mm thick) were butt welded using a tool with a 16 mm diameter shoulder. Spindle speed was 500 rpm, and travel speed was 50 mm/min (2.0 in./min). The weld was fully consolidated and exhibited substantial grain refinement as compared with the base material. Yield and ultimate strengths of the transverse weld specimens were 670 and 985 MPa (97 and 143 ksi), respectively. There was not enough material available to make a base-metal measurement. However, for comparison purposes, typical yield and tensile strengths are 460 and 895 MPa (67 and 130 ksi) for alloy 718 in the annealed condition and 1170 and 1390 MPa (170 and 202 ksi) in the precipitation-hardened condition. 

The introduction of small particles into a ductile material can substantially increase the yield strength, even if the volume fraction is low (< 10 vol.%). The particles can be introduced by precipitation (precipitation hardening) or by physical addition (dispersion strengthening). For example. Fig. 6.28 shows the effect of precipitation of Mg0.Fe203 on the stress-strain behavior of MgO. The extent of the strengthening is determined by several factors, including volume fraction. 

The precipitate hardening efficiency iOyp/fp) vs fp for solute cluster hardening within the IVAR database, compared with trends derived from the Russell-Brown (RB) and Bacon and Osetsky (B-O) models. o ,p, irradiation-induced yield stress increment associated with precipitation fp, volume fraction of precipitates fp, average precipitate radius derived from SANS measurements. 

Reactions in metallic systems in which a new phase precipitates from a supersaturated solution have very broad technological applications. For instance, such processes cause the precipitation hardening of alloys. The yield point and the strength of the alloy are increased as a result of the interaction of the dislocations with the precipitated particles. Accordingly, the quantitative treatment of a hardening problem requires that the number, size, distribution, and shape of the precipitated particles be known. In the following sections, certain kinetic problems which play an important role in this regard will be discussed [8]. 

Yield Strength of Precipitation-Hardening Austenitic Stainless Steels... 

The yield strength of an aluminium-copper alloy is to be increased by precipitation hardening by Ai po,2 = 600 MPa. 

Alloy 718 is a precipitation-hardened, nickel-based alloy, designed to display exceptionally high yield, tensile, and creep rupture properties up to 1300°F... 

D15 A solution heat-treated 2014 aluminum alloy is to be precipitation hardened to have a minimum yield strength of 345 MPa (50,000 psi) and a ductility of at least 12%EL. Specify a practical precipitation heat treatment in terms of temperature and time that would give these mechanical characteristics. Justify your answer. 

For dispersion-strengthened composites, particles are normally much smaller, with diameters between 0.01 and 0.1 xm (10 and 100 nm). Particle-matrix interactions that lead to strengthening occur on the atomic or molecular level. The mechanism of strengthening is similar to that for precipitation hardening discussed in Section 11.9. Whereas the matrix bears the major portion of an applied load, the small dispersed particles hinder or impede the motion of dislocations. Thus, plastic deformation is restricted such that yield and tensile strengths, as well as hardness, improve. 

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