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commercially pure metal and its alloys. Perhaps it can be said
that
the an
more
deliberately alloying
addition has been made and the larger the amount of the addition, the more likely it is that
the
product
will
specifically be called an alloy. In any event, the chemical composition of a metal or an alloy must be known and controlled within certain limits
if
consistent
performance is to be achieved in service. Thus chemical composition has to be taken into account when developing an understanding of the factors which determine the properties of metals and their alloys.
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Of the 50 or so metallic elements, only a few are produced and used in large quantities in engineering practice. The most important by far is iron, on which are based the ubiquitous steels and cast irons (basically alloys of iron and carbon). They account for about 98% by weight of all metals produced. Next
in
importance
for
structural uses (that is, for structures that are expected to carry loads) are aluminum, copper, nickel, and titanium. Aluminum accounts for about 0.8% by weight of all metals produced, and copper about 0.7%, leaving only 0.5% for all other metals. As might be expected, the remainders are
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all used in rather special applications. For example, nickel
alloys
are
used
principally in corrosion-and
heat-resistant applications, while
titanium
is
used
extensively in the aerospace
industry because its alloys have good combinations of high strength and low density. Both nickel and titanium are used in high-cost,
high-quality
applications, and, indeed, it is their high cost that tends to restrict their application. We cannot discuss these more esoteric properties here.
Suffice it to say that a whole
complex of properties in addition
to
structural
strength is required of an alloy before it will be
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accepted into, and survive in, engineering practice. It may, for example, have to be strong and yet have reasonable corrosion resistance; it may have to be able to be fabricated by a particular process such as deep drawing, machining, or welding; it may have to be readily recyclable; and its cost and availability may be of critical importance.
Unit 2 Selection of Construction Materials
工程材料的选择
There is not a great difference
between
“this”
steel and “that” steel; all are very similar in mechanical properties. Selection must be made
on
factors
such
as and
在钢之间没有太大的区别；所有的钢在机械性能方面都是近似的。它们的选取标准是诸如脆硬性，价格，和可用性等。不仅仅是因为这种钢含有2％的合金元素另一种钢含有1％而使前者具有了后者没有的某些能力，或者是某种钢具有神奇的名字。经过热处理后，任何一种钢都具有大范围的特性；这种性质同样在合金钢中存在。
hardenability, price,
availability, and not with the
idea that “this” steel can do
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something no other can do because it contains 2 percent instead of 1 percent of a certain alloying element, or because it has a
mysterious(神秘的，不可思议的)
name. A tremendous range of properties is available in any steel after heat treatment; this is particularly true of alloy steels.
Considerations in
fabrication(制造)
The properties of the final part (hardness, strength, and
machinability), rather than
properties required by forging, govern the selection of material. The properties required for forging have very little relation to the final properties of the material; therefore, not much can be
done
to
improve
its
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关于加工的考虑
最后零件的特性（硬度、强度和可加工性）而不是锻造特性决定了材料的选择。可锻性与材料的最后特性联系不大；因此，提高金属的可锻造性价值不大。高碳钢很难锻造。如果在随后的热处理过程进行细化，大尺寸晶粒是最好的。
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