Age-Hardenable Alloys

Haynes 242 Alloy. [Haynes Int l.] Ni-Mo-Cr alloy age-hardenable alloy widi high temp, strengdi, low diermal expansion, resistance to high-tenq>. fluorine and fluoride environments. 

Au-Cu-Ag alloys based on the inter-metallic phases CuAu and CujAu have found applications in dentistry because of their extremely high corrosion resistance, their advantageous mechanical properties such as high strength and ductility, and their decorative gold color (Yasuda, 1991). These alloys age-harden as a result of complex ordering and decomposition reactions by which the phases CujAu I, CuAu I, CuAu II, and an Ag-rich tXj phase are formed, depending on the composition. 

Criteria Strain-hardenable alloys Age-hardenable alloys ... 

Gold and gold-based alloys ate used for corrosion-resistant equipment. Gold—platinum alloys, 75 Au-25 Pt or 84 Au-15 Pt-1 Rh, ate used as cmcible material for many molten salts (98). Spinnerets for rayon manufacture ate based on the Au—Pt system which exhibits a broad miscibility gap in the soHd state so that the alloys can be age-hardened. Spinneret alloys contain 30—40% or mote platinum modified by small additions of usually rhodium (99). Either gold or gold—platinum alloys ate used in mpture disks for service with corrosive gases (100). 

Lead—silver alloys show significant age hardening when quenched from elevated temperature. Because of the pronounced hardening which occurs using small amounts of silver, the content of silver as an impurity in pure lead is restricted to less than 0.0025 wt % in most specifications. Small additions of silver to lead produces high resistance to recrystaUization and grain growth. 

Fig. 9. Microhardness profiles across interface of explosion-clad age-hardenable aluminum alloy 2014-T3 where the initial hardness is shown as Q (a) low,...

Typical apphcations for the nickel—copper alloys are in iadustrial plumbing and valves, marine equipment, petrochemical equipment, and feedwater heat exchangers (see Piping systems). The age-hardened alloys are used as pump shafts and impellers, valves, drill parts, and fasteners (see Pumps). 

Other alloys have been developed for use in particular corrosive environments at high temperatures. Several of these are age-hardenable alloys which contain additions of aluminum and titanium. Eor example, INCONEL alloys 718 and X-750 [11145-80-5] (UNS N07750) have higher strength and better creep and stress mpture properties than alloy 600 and maintain the same good corrosion and oxidation resistance. AHoy 718 exhibits excellent stress mpture properties up to 705°C as well as good oxidation resistance up to 980°C and is widely used in gas turbines and other aerospace appHcations, and for pumps, nuclear reactor parts, and tooling. 

Table 25. Age Hardening of Aluminum Alloys 6061 and 2024 at Room Temperature after Heat Treatment and Quench...

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. 

Table 24. Conductivity and Age Hardened Tensile Properties of Chromium—Copper Alloys...

Iron [7439-89-6] Fe, is used to produce age-hardening ia Au—Pd—Pt alloys at temperatures iavolved ia ceramo—metallic techniques (141). 

Alloys based on Ag—Pd have been used for a number of years and are available from most gold alloy manufacturers (148). The palladium content is 22—50 wt % silver content is from 35 to 66 wt %. Minor amounts of Zn, In, or Sn are often present to increase fluidity. Both In and Sn form intermetaUic compounds with both Pd and Ag and, therefore, some of the commercial alloys are susceptible to age hardening (149). These alloys are somewhat difficult to fabricate and require meticulous processing. They may also produce a greenish discoloration when they are fused with porcelain veneers. Nevertheless, clinical experience generally has been satisfactory, and cost is the primary criterion for use. 

Series major additive Cu Al -r 4 Cu -r Mg, Si, Mn Strong age-hardening alloy aircraft skins, spars, forgings, rivets. 

Series major additives Al + 0.5 Mg 0.5 Si Moderate-strength age-hardening alloy anodised extruded sections, e.g. window frames. 

Series major additives Al -r 6 Zn -r Mg, Cu, Mn Strong age-hardening alloy aircraft forgings, sports, lightweight railway carriage shells. 

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. 

To age harden our Al-4 wt% Cu alloy we use the following schedule of heat treatments. 

Finally, Table 10.4 shows that copper is not the only alloying element that can age-harden aluminium. Magnesium and titanium can be age hardened too, but not as much as aluminium. 

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