Metals - Orthopaedic Biomaterials in Research and Practice - page 166

L

1000

S + L

S + L

T (˚C)

α

β

E

500

S (α+ β)

A B

Composition (%)

FIGUre 7.2 aronium-borium (binary eutectic) phase diagram.

α α + β

(A:B = 25:75)

β

FIGUre 7.3 Phase structures.

right or borium-rich region. Thus, the left S + L region might be
labeled α + L , the right one, β + L .
α, β. These are solids, with uniform, characteristic physical appear-
ances over a range of compositions as shown. When prepared for
metallographic examination, each would have a different appear-
ance. Figure 7.3 shows schematically what each of these phases
might look like. With care, materials with these compositions may
be prepared as single crystals,* without grain boundaries. †
α + β. This is also a solid region, with grains consisting of either α
or β structure and the ratio of grain volumes depending on the
distances to the α or β solid-solid phase boundaries. The size of
the grains depends on thermal history and impurities, whereas the
relative orientation is determined by other factors.

* A crystal structure of a material is defined by the arrangement of atoms or molecules.
† Grain boundaries are single-phase interfaces where crystals of different orientations
meet. It is important to note that grain boundaries only divide regions of differing crys-
tal orientation, where crystals on each side of the boundary are identical except in orien-
tation. Grain boundaries disrupt the motion of dislocations through a material and thus
tend to prevent slip. For this reason, decreasing grain size, and thus increasing the grain
boundary density in a given material, will tend to increase the strength of that material.