3.2 The families of engineering alloys
3.2.4 Copper alloys
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Temperature
RT
Strength
α
Mf
Ms
β
RT = room temperature
Aged α + β
Anneal
Rapid quench from β-field Alloy content
3.13 Schematic diagram for the heat-treatment of titanium alloys (after P. H. Morton).
then aged to cause decomposition of the retained metastable β, a moderate increase in strength is observed, as indicated in Fig. 3.13.
Table 3.5 lists typical mechanical properties of some titanium alloys.
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so, for a given component, a designer can select an alloy before deciding on the manufacturing process. We will consider this family in three groups: the grades of copper itself, then the high-copper alloys and, finally, the alloys containing larger quantities of alloying elements.
Grades of copper
Tough pitch copper contains residual oxygen from the refining process, including oxide particles, which makes it unsuitable for tube manufacture or for welding. Deoxidized copper is more appropriate for such applications and small additions of phosphorus or another deoxidizer is made for this purpose. If residual deoxidizer remains in solid solution, the electrical conductivity of the copper is impaired, so high conductivity copper is refined and deoxidized to a high degree of purity for use in electrical applications.
Copper alloys of low solute content
Arsenical copper contains up to 0.5% As, which has the effect of reducing the tendency of the metal to scale when heated and also gives a slight increase in high-temperature strength through solute hardening.
Free-cutting copper contains ~0.5% Te which gives rise to the presence of second-phase particles of a telluride phase. During machining, these particles cause the swarf and chippings to break up into small fragments allowing the cutting fluid access to the interface between the workpiece and the tool. This increases tool life for a given rate of machining; sulphur-bearing alloys are also available for the same purpose.
Copper–beryllium alloys are age-hardenable, as suggested by the phase diagram of Fig. 3.14. Alloys contain typically 1.9% Be and are subjected to the normal sequence of solution treatment at 800°C, quenched and artificially aged in the range 300–320°C to precipitate the intermetallic phase CuBe.
The alloys also normally contain <0.5% Co or Ni which segregates to the
Table 3.5 Typical mechanical properties of some titanium alloys
Alloy Condition Density Young’s Proof UTS Elongation (Mg m–3) modulus stress (MPa) (% in 50 mm)
(GPa) (MPa)
Commercially Annealed 4.51 ~105 540 640 24
pure Ti (α) (IMI155)
Ti–6Al–4V(α/β) Annealed 4.46 ~106 925 990 14 (IMI318)
Ti–13Mo–11Cr– Aged 4.87 ~106 1200 1280 8
3Al (β)
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grain boundaries and inhibits the precipitation of coarse particles of CuBe in those regions. Tensile strengths in the order of 1400 MPa can be achieved, but with elastic moduli only two-thirds that of steel. This allows a large deflection for a given stress in such applications as springs, diaphragms and flexible bellows.
Copper alloys of high solute content
Zinc, aluminium, tin and nickel are the most widely employed alloying elements in copper, so we will consider the commercial alloys in terms of these four groups.
The structure of the copper–zinc alloys or brasses can be understood from the phase diagrams of Fig. 3.15. The zinc content can vary between about 5 and 40%; up to about 35% Zn single-phase α-solid solutions are formed with
Temperature (°C)
1400 1300
1200
1100 1038 1000
900
800
700
600
500
400
300 200
0 10 20 At% Be
1 2 3 Wt% Be
L
16.4 868 14.8 α
12.4 10.0 605
6.6
2.75 α + β 1.35
3.14 The Cu-rich end of the Cu–Be phase diagram.
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progressively increasing tensile strength due to solute hardening. The zinc also confers resistance to atmospheric and marine corrosion, as well as increasing the work hardening rate. The α-brasses are thus particularly suited to cold working into wire, sheet and tube.
At higher Zn contents, the β-phase appears in the microstructure, which increases the tensile strength of the alloy but is associated with an appreciable loss in room temperature ductility. These effects are summarized in Fig.
3.16. At 800°C, the β-phase is readily worked, however, the high Zn α/β brasses, such as the 60% Cu 40% Zn alloy (‘60/40 brass’, or Muntz metal) are well suited for shaping by extrusion and hot stamping.
The ‘high tensile brasses’ are formed from the 60/40 brasses by the addition of solution-hardening elements such as Al, Fe, Mn, Sn and Ni. These may be used for forgings as well as castings, notable among the latter being marine propellers. The single-phase α-‘nickel-silvers’ contain no silver, but 18–
20% Ni has a decolorising effect on brass and confers good corrosion resistance.
‘Nickel-brass’ usually refers to the α/β alloy with about 45% Cu, 45% Zn and 10% Ni which is available as extruded sections.
Copper–nickel alloys form a continuous series of solid solutions, as shown in Fig. 1.11, so have essentially similar microstructures. The copper-rich alloys are used as condenser tubes: the higher the Ni-content the higher their
At% Zn
0 10 20 30 40 50 60 70 80 90 100
1100 1000 900 800
700 600 500 400 300 200
Temperature (°C)
0 10 20 30 40 50 60 70 80 90 100
Wt% Zn
Liquid 902
834
β
700 600
423 η ε γ δ
560 468 β′
454 α
3.15 The Cu–Zn phase diagram.
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mechanical strength and corrosion resistance. The addition of 1–2% Fe increases their resistance to impingement attack in moving sea-water.
Copper–tin alloys or bronzes are again essentially α-solid solutions of Sn in Cu. The relevant phase diagram is shown in Fig. 3.17 and cast alloys may have tin contents in the range 5–19%. Tin is a strong solution-hardening element, but cast alloys are far from equilibrium and show cored microstructures and increasing volume fractions of the hard (α + δ) eutectoid as the Sn content increases above about 7%. Typical applications include cast bearings and bushings, and phosphor-bronzes (containing 0.3–1% P) are employed when a bearing surface is required to bear heavy loads with a low coefficient of friction.
Copper-rich aluminium alloys are known as aluminium bronzes, the composition of the commercial alloys ranges from 5–11% Al, and the phase diagram of Fig. 3.18 enables the microstructures to be understood. The alloys are characterized by high strength coupled with a high resistance to corrosion and wear. Up to 7% Al, the alloys consist of single-phase α, which is easily worked. They find widespread application in heat-exchanger tubing. The α/β alloys contain ~10% Al, and are either cast or hot-worked in the β-phase field. Slow cooling from β produces a eutectoid (α + γ2) mixture, and quenching produces a hardened martensitic structure of β′. Although these changes are analogous to those found in steel (see later), such heat treatments are not widely applied to aluminium bronzes in practice.
The machining properties of brasses and bronzes are enhanced by the addition of a few % of lead which, being insoluble in both solid and liquid phases, appears as globules in the microstructure. These allow turnings to break up during machining.
Table 3.6 gives the properties of some copper alloys.
Elongation
Tensile strength
60
40
20
Elongation
0 10 20 30 40 50 60
Wt% zinc 400
200
0 Tensile strength (N mm–2)
3.16 Showing the change in tensile strength and ductility with Zn content of brasses.
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