Materials for engineering, 3rd Edition - (Malestrom)

Metals and alloys 91
Table 3.4 Typical mechanical properties of some magnesium alloys
Alloy Condition Density Young’s Proof UTS Elongation
(Mg m –3 ) modulus stress (MPa) (% in 50
(GPa) (MPa) mm)
Mg–6Al–3Zn– Sand cast ~1.8 ~45 75 180 4
0.3Mn (AZ63) Peak aged 110 230 3
Mg–9.5Al– Cast and ~1.83 ~45 127 239 2
0.5Zn–0.3Mn peak aged
(AZ91)
Mg–6.5Al–1Zn– Extruded ~1.8 ~45 180 260 7
0.3Mn (AZ61)
At room temperature, titanium has a hexagonal crystal structure (α) and
this changes to a body-centred cubic structure (β) at 882 °C, staying the
same up to the melting point. There are, thus, three types of Ti alloy
microstructure, namely those with α, β and mixed α/β structures.
α-alloys
α-alloys can be divided into three subgroups:
(i) single-phase α-alloys, which owe their strength to solute hardening,
(ii) near-α alloys which contain up to 2% of some β-stabilizing elements
and may thus be forged and/or heat-treated in other phase fields, and
(iii) alloys which respond to conventional age-hardening treatment.
Single-phase α-alloys
The most widely used single-phase α material is in fact Commercially Pure
(CP) titanium, which is essentially a Ti–O alloy, solution-hardened by adding
a controlled amount of oxygen, which dissolves interstitially in the metal.
Higher strengths are achieved by substitutional solid solution hardening, of
which Ti–5Al–2.5Sn is an example.
Near-α alloys
The near-α alloys show the greatest creep resistance above 400°C of all Ti
alloys and have attractive properties for use as a compressor disc alloy in gas
turbines. A typical example is Ti–6Al–5Zr–0.5Mo–0.25Si, known as IMI 685.
Single-phase α- and near-α-alloys have phase diagrams of the form shown
in Fig. 3.11.
Age-hardening alloys
Ti–2.5Cu may be strengthened by a classical age-hardening heat-treatment,