Page 227 - Materials Chemistry, Second Edition

214 3 Metals

Among the refractory metals, perhaps none are as widely exploited for commer-
cial applications as titanium. From golf clubs to shavers, titanium is now pervasive
throughout our modern world. Although some of the proposed applications may be
suitably classified as “hype,” the broad appeal for titanium alloys is due to its
favorable properties such as high strength/weight ratio and superior corrosion
resistance. Titanium is also readily available from a number of mineral sources; it
is the sixth most abundant metal, behind Al, Fe, Cu, Zn, and Mg.
As you will note from Table 3.6, the density of titanium is significantly less than
the other refractories (midway between aluminum and iron); even so, the yield
strength ranges up to 1,800 MPa. To put this in perspective, this strength is of the
same magnitude as Ni-doped ultrahigh strength stainless steels – at a fraction of
the weight. The low density of Ti (and other metals such as Be and Mg) is due to the
hexagonal close-packed crystal structure, which is much less dense than bcc or fcc
arrays (as discussed in Chapter 2).

Shape-memory alloys
As their name implies, shape-memory alloys are able to revert back to their original
shape, even if significantly deformed (Figure 3.33). [18] This effect was discovered in
1932 for Au–Cd alloys. However, there were no applications for these materials
until the discovery of Ni–Ti alloys (e.g., NiTi, nitinol) in the late 1960s.
As significant research has been devoted to the study of these materials, there are
now over 15 different binary, ternary, and quaternary alloys that also exhibit this
property. Other than the most common Ni–Ti system, other classes include Au–Cu–
Zn, Cu–Al–Ni, Cu–Zn–Al, and Fe–Mn–Si alloys. [19]
The shape-memory effect is observed when the temperature of a piece of alloy is
cooled to below that required to form the martensite phase: M s (initial martensite
formation) until M f (martensite formation complete), as seen in Figure 3.34. Upon
heating the martensitic material, a reformation of austenite begins to occur at A s until

Figure 3.33. Photographs of the shape-memory effect at varying temperatures for a Ni–Ti wire.
Reproduced with permission from the real-time video clip made by Rolf Gotthardt (rolf.gotthardt@epfl.
ch) – found online at http://www.msm.cam.ac.uk/phasetrans/2002/memory.gif.