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The Structure of Iron.
A. Westgren und A. Lindh. Zeitschrift für Phys. Chemie, 98, 781, 1921.
As early as 1917, Hull showed that ordinary iron has the structure of a centered cube with edge \(= 2.86 \cdot 10^{-8}\) cm. Later, in 1921, Westgren and Lindh studied various modifications of iron by the Debye method. In
with a low carbon content, three allotropic forms of iron are known up to a temperature of 768° C: α-iron is present; in the interval from 768° to 800°, β-iron; and above 800°, γ-iron.
Three X-ray photographs of iron in \(K\alpha\) with wavelength \(=1932\) Å were obtained: at room temperature, at a temperature of 800°, and at 1000°. As the object to be photographed, iron with a small admixture of carbon (0.15%) was taken.
The first photograph gave (at room temperature) the following table of sines of the glancing angles from the corresponding faces:
| \(\operatorname{Sin}\dfrac{\theta}{2}\) | \(h_1\ h_2\ h_3\) — indices of the face |
|---|---|
| 0.488 | 113 |
| 0.686 | 200 |
| 0.835 | 211 |
| 0.959 | 220 |
\[ \operatorname{Sin}^{2}\frac{\theta}{2} \]
are in the ratio \(1:2:3:4\).
Such a ratio is characteristic either of a simple cube or of a centered one. The first lattice contains one node, whereas the second contains 2 nodes in the unit cube.
Hence
\[ d_{100}^{3}=\frac{n.M}{\rho}=\frac{1.55.8.\ 1.6.\ 10^{-24}}{7.86} \]
or
\[ d_{100}^{3}=\frac{2.55.8.\ 1.6\ 10^{-24}}{7.86} \]
where \(n\) is the number of nodes, \(M\) is the molecular weight, and \(\rho\) is the density. The first gives \(2.29\cdot 10^{-8}\) cm, the second \(2.87\cdot 10^{-8}\) cm. The second value agrees with those found by Hull (\(2.87\cdot 10^{-8}\) cm). Consequently, “α-iron” has the structure of a centered cube.
The second X-ray photograph was obtained at a temperature of 800° C.
| \(\operatorname{Sin}\dfrac{\theta}{2}\) | \(h_1\ h_2\ h_3\) |
|---|---|
| 0.464 | 110 |
| 0.663 | 200 |
| 0.815 | 211 |
| 0.942 | 220 |
That is, here there is the same structure of a centered cube, but only the cube edge \(=2.92\cdot 10^{-8}\) cm. Taking into account the mean coefficient of expansion \(15\cdot 10^{-6}\), we find that the edge of the cube of “α-iron,” as a result of temperature expansion alone at 800°, should have length \(2.90\cdot 10^{-8}\) cm. Hence it is obvious that β-iron and α-iron are identical.
In the third photograph the temperature of the iron was 1000° C.
| \(\operatorname{Sin}\dfrac{\theta}{2}\) | \(h_1\ h_2\ h_3\) |
|---|---|
| 0.467 | 111 |
| 0.512 | 200 |
| 0.754 | 220 |
| 0.883 | 311 |
| 0.917 | 222 |
Among the reflecting faces there are either only those with even or only those with odd indices. This is a feature characteristic of a cube with centered faces.
The calculation of the cube edge gave \(3.60\cdot 10^{-8}\) cm. Thus, γ-iron has a structure different from α and β, and it must be assigned the structure of a cube with centered faces.
Furthermore, Westgren and Lind investigated such important varieties of iron as austenite and martensite.
Austenite was obtained by quenching in water nickel or manganese steel heated to \(1{,}000^\circ\) C.
Composition of the nickel steel:
\(Ni—25.2\%\), \(C—0.24\%\), \(Mn—0.5\%\), \(Si—0.1\%\), \(P—0.047\%\), and \(S \pm 0.01\%\). The manganese steel had the composition \(Mn—12.1\%\), \(C—1.34\%\), \(S—0.52\%\), and \(P—0.1\%\).
The X-ray photographs gave in both cases the structure of iron with a cube edge of \(3.58 \cdot 10^{-8}\) cm (nickel steel) and \(3.61 \cdot 10^{-8}\) cm (manganese steel). Thus austenite contains only \(\gamma\)-iron.
Martensite was obtained by cooling austenite in liquid air.
The manganese steel gave, as before, only \(\gamma\)-iron; the X-ray photograph of the nickel steel has a more complex pattern, analysis of which shows that martensite contains both \(\alpha\)-iron and \(\gamma\)-iron. In addition, at small glancing angles there were lines that could not be assigned either to the \(\alpha\)- or to the \(\gamma\)-modification.
N. Selyakov.