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August, 1925
slip in the crystal. Fig. 117 represents a section perpendicular
to the polished surface, and illustrated how
slip has taken place and how the formerly smooth
surface has been broken up by slip, on various planes
of the crystals.
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FIG. 116—Slip in nearly pure iron. (Carbon 0.02%.)
All at same spot. (200 x)
(a) Unstressed. (Polishing marks horizontal.)
(b) Stressed to yield point.
(c) Stressed close to ultimate.
What Cements Crystalline Grains Together?
The question naturally arises why metals do not
fail by the separations of the grains along their boundaries—what
holds the grains together?
In discussing the crystalline nature of metals, and
the formation of crystalline grains, in Chapter III, it
was shown that crystals grew from small beginnings
by taking on atoms from the surrounding material and
arranging these atoms according to a regular pattern,
267
which is uniform throughout the crystal. When two
crystals of different orientation meet, there are three
possible conditions at the grain boundaries:
(1) There are voids (atomless spaces), between
the two crystals.
(2) There is a zone in which some of the atoms
are held in both crystal lattices (patterns), in
which case the lattices would be distorted at the
surface of contact.
(3) There is a zone of disorganized or "amorphous"
metal.f
There is no way known at present of determining
the actual structure at the grain boundaries of metals.
There are many indications, however, that the grains
are held together by a sort of "cement", composed of
atoms which are not arranged according to any regular
(crystalline) pattern.
The absence of slip or cleavage planes in this noncrystalline
material, gives it great hardness and
strength at ordinary temperatures. There being no
planes of weakness, the amorphous material more nearly
develops the absolute cohesion of the metal, and
is therefore harder and stronger than the crystalline
grains. At elevated temperatures, the amorphous ma-
FIG. 117 (left)—Formation of slip bands. Section perpendicular
to polished surface.
(a) Before slip.
(b) After slip.
FIG. 118 (center)—Key particles intefering with slip.
FIG. 119 (right)—Large key particles, leaving unkeyed planed.
Iterial takes on some of the properties of a fluid. It is
|then weaker than the crystalline material, and in this
state, failure may occur at the grain boundaries.
Hardening Effect of Grain Refinement.
Ordinarily, a piece of metal is composed of many
crystalline grains, differently oriented. As the slip
planes of adjacent grains run in different directions,
and seldom coincide in adjacent grains, there cannot
be any continuous plane of weakness through the piece
as in the case of a single grain. When slip takes
place, it cannot continue from one grain to the next,
without change of direction. This naturally hinders
its progress. The amorphous cement at the grain
boundaries, being free from planes of weakness, also
interferes with the progress of slip from grain to grain.
The smaller the grains, the shorter and more irregular
will be the possible planes of slip, and the greater will
be the quantity of amorphous grain boundary material.
Refinement of the grain structure of a metal, therefore
increases slip interference. The result is a greater
resistance to permanent deformation, and consequently
an increase in elastic limit, hardness and strength.
Hardening by Cold Working.
Permanent deformation below the recrystallization
temperature of a metal is called "cold working". This
(Continued on page 283)
-(•"Amorphous,, is the opposite of crystalline, that is, not arranged
according to any regular atomic pattern. Amorphous
metal would therefore have no slip or cleavage planes.