Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
stresses are tensile in character, then cracks may arise, since concrete is so
weak in tension.
In addition to shrinkage upon drying, concrete undergoes carbonation
shrinkage. Carbon dioxide (CO 2 ) is of course present in the atmosphere, and
in the presence of moisture it forms carbonic acid which reacts with CH to
form CaCO 3 ; other cement compounds are also decomposed. Carbonization
proceeds extremely slowly from the surface of the concrete inwards, at a rate
depending on the permeability of the concrete and its moisture content.
Carbonization neutralizes the alkaline nature of the hydrated cement paste
and thus removes the protection of steel reinforcement from corrosion. Thus,
if the full depth of concrete covering the reinforcement is carbonated and
moisture and oxygen can penetrate, corrosion of the steel and possibly cracking
will result. However, by specifying a suitable concrete mix for a given
environment, it is possible to ensure that the rate of advance of carbonization
declines within a short time to a value of less than 1 mm per year. Therefore,
provided an adequate depth of cover is present (as specified in British Standard
BS 8110:1985), the passivity of the steel reinforcement can be preserved for
the design life of the structure.
4.5 Bulk metallic glasses
In 1960, it was discovered by Duwez that if certain molten metal alloys were
undercooled rapidly enough (e.g. at 10 6 K s –1 ) the atoms did not have enough
time or energy to nucleate as a crystal. The liquid metal reached the glass
transition temperature and solidified as a metallic glass with remarkable
mechanical properties. The material had high strength and hardness, yet was
tougher than ceramics; the absence of grain boundaries means that it was
resistant to corrosion and wear. Heat conduction is low in these materials, so
that the requisite cooling rate can only be achieved for a very small critical
casting thickness. This precludes bulk casting and, therefore, their employment
in structural applications.
A range of glass-forming alloy systems is shown in Table 4.4.
Table 4.4 Glass forming alloy systems
Type
Metal–metalloid
Metal–metal
A group metal based
Typical alloy systems
Au–Si; Ni–P; Pd–Si; Fe–B;
Pd–Cu–Si; Fe–Ni–B; Fe–B–Si;
Pt–Ni–P; Au–Ge–Si;
Ni–Nb; Ni–Ti; Ni–Ta; Cu–Zr; Ir–Ta;
La–Au; Gd–Co; Gd–Fe;
U–V; U–Cr; U–Co;
Mg–Zn; Ca–Mg; Ca–Al;
Be–Zr–Ti; Al–Y–Ni; Al–La–Fe