Precipitation or Age Hardening

This process is used to strengthen Al-Cu alloys. The initial precipitates are thin discshaped clusters of Cu atoms called GP zones after Guinier and Preston who first identified these structures. The GP-1 zones are 1-2 atoms thick and 25 atoms in diameter that form platelets parallel to the (100) planes. These zones maintain the same lattice structure as the A1 matrix, but the smaller Cu atoms produce lattice strain, which provides some strengthening of the material. 

Why do the GP zones and then the 0 phase form before the 9 and the 9 phase Recall from Section 11.4 that the barrier to nucleation is directly proportional to the interfacial energy. The small clusters of Cu atoms and coherent 9 phase have much lower interfacial energy than the incoherent 9 phase. However, the 9 phase is the stable phase so that eventually the metastable 9 and 9 will proceed to the stable 9 phase. 

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Most wrought alloys are provided in conditions that have been strengthened by various amounts of cold work or heat treatment. Cold worked tempers are the result of cold rolling or drawing by prescribed amounts of plastic deformation from the annealed condition. Alloys that respond to strengthening by heat treatment are referred to as precipitation or age hardenable. Cold worked conditions can also be thermally treated at relatively low temperatures to affect a slight decrease in strength (stress rehef annealed) to benefit other properties, such as corrosion resistance and formabiUty. 

Precipitation (Age) Hardening Alloys. Only a few copper alloys are capable of responding to precipitation or age hardening (7). Those that do have the constitutional characteristics of beiag siagle-phase (soHd solution) at elevated temperatures and are able to develop iato two or more phases at lower temperatures that are capable of resisting plastic deformation. The copper alloy systems of commercial importance are based on iadividual additions of Be, Cr, or Ni + X where X = Al, Sn, Si, and Zr. 

In other instances, heat treatments involving quenching, tempering, or holding at some temperature to precipitate an age-hardening compound are employed to secure some desired level of hardness or other mechanical properties. It is obviously necessary to explore what effects such heat treatments may have on the corrosion resistance of the material in the condition, or conditions, of heat treatment in which it is to be used. 

Tin additions to lead-calcium alloys change dramatically the method of precipitation and age-hardening from discontinuous precipitation of PbsCa to a mixed discontinuous and continuous precipitation of PbsCa and (PbSn)3Ca and, finally, to a continuous precipitation of SnsCa. Such precipitation reactions have been described for alloys that contain low tin contents [45,48,68-70]. The reactions are not influenced or modifled by impurities in the lead alloys [71,72]. A ternary phase diagram has been proposed [41], which sets the areas of stability of PbsCa, SusCa, and mixed (PbSn)3Ca precipitates, and this is shown in Fig. 2.5. The phase diagram has been conflrmed [73]. 

Second phase particles can be made to precipitate from a supersaturated solute. By carefully controlling the time and temperature, these precipitates can ripen by solid-state diffusion until they reach the optimiun size in which their lattice nearly matches the host lattice, but their mismatch puts enough strain in the lattice to block the motion of dislocations. This process is called precipitation hardening or age hardening because the size of the precipitate is controlled by the time the alloy is held at a temperature the precipitate can grow by solid-state diffusion. The methods for forming such precipitates will be discussed in Chapter 14. 

Al—Cu—Mg. The first precipitation hardenable alloy was an Al—Cu—Mg alloy. There is a ternary eutectic at 508 0, and there are nine binary and five ternary intermetallic phases. For aluminum-rich alloys, only four phases are encountered in addition to the aluminum soUd solution (Table 18). Several commercial alloys are based on the age hardening characteristics of the metastable precursors of 0 or S-phase, principally 0 or S. Hardening by T- and p-phases is not very effective. Alloys of greatest age hardenabiUty have compositions near the Cu Mg ratio of the S-phase. Additions of about 0.12% Mg to alloys containing as much as 6% Cu, however, significantly increase strength by refining the 0 precipitate. 

IGC is observed in heat-treatable, precipitation age-hardened Al alloys, particularly after slow cooling of thick sections or after certain isothermal heat treatments. In fact, quench rate over the temperature range from 400 to 315 °C is a very strong factor deterrnining both IGC and IGSCC suscep-tihihty in Al-Cu and Al-Cu-Mg alloys that are, subsequently, naturally aged [75]. 

It seems that only alloys with particular properties are suitable for casting grids for lead—acid batteries. Lead—calcium alloys, similar to lead—antimony alloys, belong to the age-hardening or precipitation-hardening group of alloys. With decrease of temperature, the solubility of calcium in the a-Pb solid—solution decreases and it precipitates in the form of small Pb3Ca particles. A similar picture was described earlier for the Pb—Sb alloys. 

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