# 機械材料相關

Definitions of solids most always contain the phrase (definite volume and shape) to-gether with the fact that solids maintain their shape to a varying extent when subjected to figure 3-19

How atoms are arranged in (a) a crystalline solid,(b) a liquid,(c) a gas.

(D.C.Giancoli,Physics, Prentice-Hall,Inc,Englewood Cliffs,N.J,1980)

External mechanical forces.

Solids occur in two basic forms:crystalline and amorphous.

A true crystalline solid possesses an ordered,three-dimensional,geometric arrangement that repeats itself.

A metal would be representative of a crystalline solid (though not perfectly crystalline).

An amorphous solid,on the contrary,contains no repetitious pattern of atom ferred to as a supercooled liquid because of the randon nature of its atomic arrangement of matter,emphasizing the different degrees of disorder and closeness of their positions in relation to esch other.

Another unit cell (Fighre 3-27) is known as the body-centered cubic(bcc).

It is sim-ilar to the simple cubic unit cell,but contains an additional atom located in the center of the cube.

The third type of unit cell formed from the cubic axis system is the face-centered cubic (fcc).

One atom at each corner and one in the center of each of the cube faces make up the complement of atoms.

There is no atom at the center of the cube(Figure 3-28).

The hexagonal crystal system (Figure 3-30)3-30 can best be described using three axes(a1'a2'a3)in the x-y plane 120° apart and a fourth axis at 90° to x-y plane.

but the fourth intercept,labeled c,is of a different length.

This unit cell is made up basi-cally of two parallel planes (top and bottom basal)separated by a distance equal to the di-mension c.

The atoms shown in the figure trace out a right hexagonal prism.

Each of these two places can be divided into six equilateral triangles,with each side equal to the inter-cept a (Figure 3-30(b))

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but the fourth intercept,labeled c,is of a different length.

This unit cell is made up basi-cally of two parallel planes (top and bottom basal)separated by a distance equal to the di-mension c.

The atoms shown in the figure trace out a right hexagonal prism.

Each of these two places can be divided into six equilateral triangles,with each side equal to the inter-cept a (Figure 3-30(b))