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Nmae:  Justin
Status: student
Grade: other
Country: Australia
Date: May 2, 2011


Question:
What makes a crystal brittle or malleable, and what how does its brittleness or malleability depend on the crystal's arrangement?



Replies:
Your question is also its answer, of sorts. Crystals (let us assume a single crystal) is an ordered array of atoms (and/or) molecules. Being ordered, it is possible for defects to propagate over long distances. This results in crystals being "brittle" -- for example, salt (NaCl). But life is not so simple. Real crystals also contain defects of various sorts. This is how a gem cutter is able to "carve out" a gem. The stronger the bonds forming the crystal, the more energy can be stored in the "stressed" crystal, but once a certain deformation point is reached, the crystals shatters. To make things even more complicated similar defects also occur in glassy (no long range order) materials. Think of what happens when a windshield (which is not a crystal) also shatters. To make things more messy, all of the above depends upon the time/temperature scale. Some substances can relax, to use the jargon, to absorb a stress (force) placed upon it. Some materials are brittle at "low" temperatures but can "flow" if the temperature is sufficiently high -- even if they are crystals.

This time/temperature behavior is illustrated by a very brittle crystal (LiF at room temperature) compared to a very pliable (malleable) crystal (CsI). LiF shatters at room temperature, it is very brittle. In contrast, CsI (at room temperature) is very malleable (flexible) at room temperature. It flows (deforms). There is a time/temperature play-off.

Yet another example is ice. We think of ice as brittle, but at temperatures not very far below the melting temperature you can "cut" ice with a strand of wire (no difference in temperature) if you are patient -- again an example of arranging the experimental conditions so that the desired outcome is determined by the time/temperature experimental conditions. The point of this rather detailed discussion is to illustrate that time and temperature are co-variables. Ice again is another example. Given a sufficiently long time and a temperature not too low, it "flows" without fracturing. Glaciers "move" down the side of a mountain. Your "simple" question turns out to have a not-so-simple response. Good Question!

Vince Calder


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