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The stacking-fault energy of α-Fe (as of the other bcc transition metals) is very high. Segments of a can form by the reaction ( a / 2 ) + ( a / 2 ) → a. The Burgers vector of the minimum energy dislocation in α-Fe is ( a/2). It is likely that these results are strongly influenced by interstitial elements. Vacancies probably anneal out during quenching of α-Fe ( Johnson ). There is no experimental evidence for the existence of thermal vacancies in quenched α-Fe, perhaps because of the high ratio of energy of formation, u F, to energy of motion, u M : u F / u M ≈ 4 as compared to about 1 for the fcc lattice. This term produces the curvature of Δ F α−γ( T) as determined by Johannson and the re-occurrence of the bcc structure above 1394☌.įig. The contribution due to magnetic uncoupling in the α-phase is shown as a dashed line. 2, Δ F α−γ = F α – F γ is plotted as a function of temperature. Without this effect, α-Fe should transform to γ-Fe at about 700☌ without another transformation at 1394☌. At higher temperatures (depending on T c) the bcc structure is increasingly stabilized due to the entropy of demagnetization. The magnetic term is negligible up to the temperatures at which magnetization of α-Fe is nearly constant.
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#Iron atomic number free
The re-occurrence of the bcc structure at high temperatures can be rationalized by dividing the difference in free energy of bcc and fcc iron, Δ F α − γ into a magnetic and a nonmagnetic term ( Zener ). It crystallizes in both the fcc (912° T α< 1538☌) and again at T α <912☌. Iron is found between these two groups of elements. Although the fcc pattern is more closely packed the spaces between the atoms are larger than in the bcc pattern which, we shall see later, is important when alloying elements are present. When iron transforms from ferrite (bcc) to austenite (fcc) the atoms become more closely packed and the volume per atom of iron changes which generates internal stresses during the transformation. The change from one lattice pattern to another as the temperature changes is called a transformation. Furthermore some important physical and metallurgical changes can be initiated at the boundaries of the grains. Individual crystals can be seen under a microscope as grains the size of which can have a strong effect on the mechanical properties of the steel.
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When atoms are packed in one of these regular patterns the structure is described as crystalline. Iron in this form is called austenite ( Fig. This new pattern is called face centred cubic, abbreviated to fcc. If iron is heated to 910☌, almost white hot, the layout of the atoms in the lattice changes and they adopt a pattern in which one atom sits in the middle of each planar square of the old bcc pattern. (a) Body centred cubic structure (b) face centred cubic structure. Iron is a shiny solid at standard temperature and pressure and has a high melting point. Iron is a metal element so it is a good thermal conductor and a good electrical conductor. Properties Iron forms ionic bonds with non-metals. Iron loses electrons to form positive metal ions. Iron is in Period 4 of the Periodic Table because it has 4 electron shells. Atomic Structure The most stable isotope of Iron has 30 neutrons in its nucleus giving it an atomic mass of 56. Iron atoms join together in a giant metallic structure. Iron is a transition metal element, on the Periodic Table, with 26 protons in the nucleus.Ībout Iron Molecular Structure Iron has the chemical formula Fe.