Materials for engineering, 3rd Edition - (Malestrom)

Structure of engineering materials 27
solid state, and dependent upon atomic migration to proceed, they may well
be suppressed if the material is cooled quickly.
Quenched structures
Alloys may deliberately have their microstructure controlled by cooling the
material very rapidly (‘quenching’) to room temperature from a hightemperature,
single-phase region of the phase diagram, such as the α field in
Fig. 1.20. This may involve plunging the piece straight from a furnace into
a bath of oil or water and, in general, two possibilities exist:
1. A metastable supersaturated α solid solution may form. If the temperature
is then progressively raised again until solid state diffusion may proceed
at a measurable rate, the supersaturation will be relieved by the formation
of a second phase. These are the processes underlying ‘age hardening’,
which we will discuss more fully later.
2. In some systems, the instability of the α phase is so high at low temperature,
that it undergoes a diffusionless phase transformation to a different (again,
metastable) structure we will call α′. This type of phase change is called
a martensitic phase change and its most important occurrence is in steel,
to which we will return and consider in some detail.
Some alloys have been designed to transform to a martensitic phase when
they are plastically deformed. If they are then heated, they may revert
completely to their original crystal structure: this is accompanied by a reversal
of the original plastic deformation, a process known as the shape-memory
effect. A number of shape-memory alloys (SMAs) exist: the earliest devices
were of Ni–Ti (‘Nitinol’), but more recently Cu- and Fe-based SMAs have
been developed in which an element ‘remembers’ the shape it had prior to
deformation. During this shape recovery, the element can produce a
displacement as a function of temperature or, if constrained, a force and
displacement. The SMAs have been employed in circuit breaker actuators as
well as in certain prosthetic devices for the fixation of artificial teeth.
1.3 Molecular structure of organic polymers and
glasses
The highly regular crystalline structures we have considered so far will not
be formed if the interatomic binding requirements are satisfied simply by
adding new units to the end of a chain. Instead, this can lead to the formation
of high molecular weight, long chain structures known as polymers.
Although their properties differ widely, all polymers are made up of long
molecules with a covalently bonded chain of atoms forming a ‘backbone’. In
organic polymers the covalent chain is of carbon atoms, but in other polymers