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the same Anst&iMr- a ll the intermetallic |
bonds in |
the crystal |
plane along which a particular shearing |
process |
is taking |
place* |
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When a crystal undergoes a permanent ajnount of plastic
defermation, microscopically this can be observed' as the outcome of a process of slipping or gliding of parts of
the crystal past one another on a number of crystal planes*
However, in reality, and on the atomic scale of observat ion, the slipping mechanism its e lf is the net effect of
succeeding displacement of consecutive dislocations of the two kinds referred to above. A large number of such displ acements produce the microscppic effect of crystal slip .
As the stress required to start a dislocation moving is very
much smaller that that which would produce the same effect
in a dislocation-free crystal(fche difference of atoms mov ing one by one rather than a large number at once), the pre sence of dislocations in most crystals explains why natural crystals deforms plastically much more readily than disloc ation-free orystale.
Micro-and macro-imperfections of the structure are constituent crystals and defects visible by optical micro scopic methods* It is convenient to discuss these stru ctural variables in three main groups:
Another kind of anisotropy arises through the uneven
distribution of other crystals(dispersed phases and impur
ities |
) or of cavities in the main or matrix crystals. |
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If , |
for example, any one of these is distributed preferent |
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ia lly |
along certain |
crystallographic planes or at the |
grain of the matrix |
arystals then the properties of such |
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erystals w ill not be |
isotropic (fig.2.34i*b-3^* Such |
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anisotropy of crystals can affect properties of the struc ture of a casting as a whole*
The term of homogeneity of cast structure is used to
deseribe the regularity of distributions of dissolved elemehts, or of dispersed phases or impurities or cavities, in the structure on either microscpic or macroscopic scale.
(The origin of such various forms of non-homogeneity or |
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segr^ition has been considered in the previous section). |
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Consequently |
the structure of a casting may not be homo- |
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geneeeous at |
a given cross-section, and the type |
or dggree |
of homogeneity may vary from one cross-section to |
another. |
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This is a particularly d ifficu lt problem when analysing
the properties of a casting as a whole, as the properties at various section may differ widely depending on the types of segregation present*
The fundamental concepts of nucleation and crystal growth apply to all metals and alloys irrespective of their composition.
Before pouring castings, liquid alloys are heated to
100° to 300°C above their melting points (superheating)* Some impurities, originally in the charge or formed in the
melting |
process or introduced by the melt treatments, may |
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s t ill be |
solid at these temperatures , but others may |
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dissolve completely in the liquid metal. |
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Acad. Baikov A*A. found that unsoluble impurities being |
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contained in an alloy ploy the role of solidification |
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nucleis |
i f |
parameters |
of their |
crystalline |
lattice are near |
to those |
of |
a metallic |
base of |
the alloy* |
Due to this* |
the alloj^jstructure gets fine grained and its properties are
improved* However, when the |
superheating temperature is |
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higher than the |
‘proper 1&ийЛ, the active layer on the |
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boundary of the impuhity and ihetal is destroyed and its |
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desactivation is |
observed* |
The rate of heating, the length |
of time for which the higher temperature is maintained» |
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and the rate of |
cooling to the crystallisation temperatec*e |
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may affect the solution of some passible heterogeneous |
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nuclei among the |
impurities |
present* These factors may |
also affect the size of the nuclei and the way in which they separate out from the liquid alloy solution on cool ing, guch diverse changes in impurities, which cannot readily
be |
studied quantitatively, |
are |
knofon as |
the superheating |
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effect* |
This implies |
that |
the |
cast structure of an alloy |
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is |
often |
sensitive to |
its |
history in the |
liquid state* |
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of separate grains in the structure and the type of cast
structure obtained. The effect |
of |
increasing |
the rate |
of |
/ |
|
|
|
|
cooling during crystallisation |
has |
been found |
to |
to |
change the shape, number and-in some cases-the constitution
and structure of the |
crystals |
grown, |
The magnitude |
of the |
effect varies mainly |
with the |
alloy |
composition, as |
shown |
in Pig.2.35, and is |
generally |
least |
with pure metals |
and |
greatest with the eutectic alloys. Hucleation and growth of both phases in an eutectic constituent are so strongly affected by the concentration gradients in the liquid imme diately $he growth has started that both the micro-and macrostructure of an eutectic can be completely altered. The best known examples are Al-Si and Fe-C eutectics.
Changes in the rate of cooling can affect the nuclea-
tion, the growth' mechanisms, or both, and ary of these
effects may produce similar changes in the cast structure. For example, commercially pure aluminium and many other
metals and alloys poured into a metal mould |
(a ll |
other |
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factors |
being kept constant) |
give coarse columnar crystals |
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i f the pouring |
temperature is |
high |
and very |
fine |
equiaxed |
|
crystals |
i f the |
pouring temperature |
is near |
the |
melting |
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point. |
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From the point of view of the ce.st structure formation, chemical composition of alloys is the most important.
This question has been discussed in more detail in the previous paragraph. Also the effect of chemical compos ition on the cast structure is shown in fig .2 .31»
