Age hardening

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. 

Lead—tin (1.8—2.5 wt %) is used both as a cable sheathing ahoy (BS 801 ahoy A and DIN 17640) and as a battery connector ahoy ia sealed lead—calcium—tin batteries (15). Tia is generahy added to lead—arsenic cable ahoys ia smah amounts. The arsenic ahoys have excehent creep resistance and mechanical properties, but are unstable and lose arsenic readily by oxidation. The addition of smah amounts of tin (0.10—0.20 wt %) eliminates arsenic loss. Lead ahoys having 0.4 wt % tin and 0.15 % cadmium, which are used for cable sheathing, do not age harden, show excehent corrosion and creep resistance, and are very ductile. 

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). 

Demands for improved efficiency in aircraft gas turbines led to the use of a family of age hardenable, controlled expansion superaHoys for engine seals and casings. INCOLOY aHoys 903 [61107-16-2] (UNS N19903), 907 [107652-23-3] (UNS N19907), and 909 evolved from a continuing effort to improve the environmental resistance of this Cr-free, Fe—Ni—Co based system. 

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. 

All property data for berylhum—copper are for material after age-hardening heat treatment. 

AJ—Zn. Aluminum-rich binary ahoys (Fig. 18) are not age hardenable to any commercial significance, and 2inc [7440-66-6] Zn, additions do not significantly increase the abhity of aluminum to strain harden. Al—Zn ahoys find commercial use as sacrificial claddings on high strength Al—Cu—Mg—Zn aircraft ahoy sheet. The eutectoid composition near 78% Zn has found use as a superplastic sheet ahoy. 

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. 

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