Mura-Mori’s linear elasticity theory for continuum body has been applied to interpret the incommensurate direction of the modulated structure in iron-carbon martensite. Based on the assumption that the strain caused by the invasion of carbon atoms is proportional to the carbon concentration, an equation for the strain energy is derived as a function of the modulation direction on the (010) plane. A numerical calculation is made for the limiting case of zero carbon content by the use of the elastic constant of α-iron, since any elastic constant data for the bet martensite are absent. The theoretical modulation angle given by the minimum strain energy is 19.1°, and this is close to the observed angle 22° determined by the extrapolation to zero carbon content. It is concluded that the incommensurate direction comes from such an anisotropic lattice distortion that the lattice parameter c increases with carbon content while a decreases simultaneously.
The isothermal transformation of ferrite to austenite in a carbon-free Fe-8%Cr alloy has been studied at temperatures above A3 point, and quantitative analyses of the kinetics of the transformation have been made. The main results are as follows: (1) Austenite preferentially nucleates at ferrite grain boundary corners and grows with a constant velocity in all directions. (2) The nucleation sites are saturated in the early stage of the transformation. (3) The isothermal transformation behaviour of austenite can be expressed by a Johnson-Mehl type equation, and the reaction exponents have a value about 3 for all transformation temperatures. (4) The kinetics is in good agreement with Cahn’s theory of grain boundary nucleated reactions.
The microstructure and the crystallographic coherency of chromium films plated on copper substrates have been investigated by transmission electron diffraction and microscopy. The orientation relation ships between the deposited chromium film and the copper substrate are (110)Cr//(001)Cu, Cr//[1\bar10]Cu or Cr//Cu. When the crystals have the relationship of Cr//[1\bar10]Cu, the difference between the lattice spacings of (112)Cr plane and (1\bar10)Cu plane is −8.54%. The arrangement of chromium atoms and copper atoms along this direction is not in good condition to fit each other. The misfit strain exists in the interface between the deposited chromium film and the substrate. In transmission electron micrographs of chromium film with this relationship, many microtwin crystals are observed along the Cu and [1\bar10]Cu directions. It has been shown that these microtwin crystals are misfit twins and contribute to relief of the large misfit strain.
The morphological change during tempering the Ni40Zn50Cu10 martensite (L10 structure) was studied by means of optical and electron microscopy. The morphology of the L10 phase changed from the martensitic structure to the massive one during tempering at 823 K. At the early stage of tempering, new interface boundaries appeared and polygonal grains were formed in a martensite plate due to the polygonization. During further tempering, many subgrains were formed. The volume fraction of the twin decreased gradually in a polygonal grain. At the late stage of tempering, large massive grains appeared and a few thin plates of (111)L10 twins existed as growth faults inside them. The surface relief of the L10 martensite disappeared gradually with the growth process of the massive grains. These phonomena were discussed briefly in relation to the recovery and the recrystallization.
By use of Fe–0.10 and 0.97 mass%Si alloys, the immersion time dependence of the reaction between the Fe–Si alloys and liquid zinc has been studied for the time ranging from 60 s to 6 ks at various temperatures in the range from 713 K to 873 K. The relations between the thickness of the alloy layer (d) and the immersion time (t), and those between the quantity of iron having reacted with zinc (Δw) and the immersion time are obtained in the form of d=atm and Δw=btn, respectively, where a, m, b and n are constants. To obtain information on the silicon reactivity in hot dip galvanizing, the reactivity of the Fe–Si alloys with liquid zinc is compared with that of pure iron under the same condition used in this study. The reaction mechanism of the Fe–Si alloys with liquid zinc is discussed, by considering the morphology and growth of the alloy layer and the immersion time dependence of the quantity of iron having reacted with zinc. The rapid reaction of the Fe-0.10 mass%Si alloy with liquid zinc in the vicinity of 713 K and that of the Fe-0.97 mass%Si alloy in the vicinity of 753 K are caused by the effect of silicon that allows the formation of the (zeta+eta) mixture layer whose growth does not obey the diffusion-controlled process. The rapid reaction of the Fe-0.97 mass%Si alloy with liquid zinc in the vicinity of 793 K is caused by the effect of silicon on the rapid growth of the (delta 1+eta) layer.
A rapid straining electrode technique has been applied to investigate the initial process of anodic behavior on a newly created surface of pure iron in high temperature and high pressure water. A thin wire electrode was elongated rapidly to yield a newly created surface in 0.1 kmol·m−3 Na2SO4 solution at a constant potential at temperatures up to 573 K. The anodic current increases rapidly to a maximum and decays with time. Exept for the initial region of 0.01–0.1 s, the current density is found to decay following a power law; i=at−n. It is observed that at the passive potential region, the magnetite layer increases in thickness following the parabolic rate law. The rate constant for the growth, however, shows a negative temperature dependence. It is concluded that at higher temperature a more compact film suppressing diffusion through the film and active dissolution is formed. In a temperature range 423–463 K, however, pure iron showed no passive state and dissolved away rapidly.
To study the effect of tensile stress on high temperature oxidation of alloys, wire specimens of a Ni–20Cr–4Al alloy were oxidized by using a creep testing machine under constant loads of zero to 14.7 MPa at 1273 K. The results indicated that under the tensile stress of about 7.4 MPa a protective thin oxide film of Al2O3 formed firmly and the oxidation rate showed a minimum. However, an increase in the tensile stress beyond this stress range caused a severe oxidation penetrating into the alloy matrix. From the present study, two mechanisms for the stable formation of a protective thin film of Al2O3 under these tensile stresses are proposed as follows. (1) Tensile stress induced in a specimen by the constant load relieves the compressive stress induced in the growing oxide film, preventing the spalling of a thin oxide film of Al2O3 on the specimen surface. (2) The tensile stress causes a significantly enhanced diffusion rate of Al in the alloy matrix, and the protective oxide film of Al2O3 easily forms on the surface of the alloy.
Monotectic Al-17.5 mass%In alloys were solidified unidirectionally at various growth rates from 2.8×10−7 to 1.11×10−2 m/s and temperature gradients from 500 to 8500 K/m. When a temperature gradient is constant, the microstructure changes in the sequence with increasing growth rates; fibrous composite structure→periodical and regular array of L2(In) droplets→random dispersion of L2 droplets in the aluminum matrix. The critical growth rate to form regular composite structures increases, as the temperature gradient increases. The monotectic composite structure, i.e., the regular arrangement of L2 fibers or L2 droplets in the aluminum matrix is obtained at G⁄R more than 109 K·s/m2. The inter-fiber spacing λ of L2 is related to growth rates R; λ=KR−1⁄2, K being 2.8×10−8 m1.5 s−0.5.
Monotectic Al-17.5 mass%In alloys were solidified unidirectionally at various growth rates and temperature gradients, and were quenched during growth to reveal the solid-liquid interface morphology. The alloys having fibrous composite structures were held near the monotectic temperature for various time durations to examine the thermal stability of L2 fibers. When fibrous composite structures are forming, the planar solid-liquid interface proceeds with some projections of L2 phase toward L1. Periodical fluctuation of the growth front and depressions form at about 109 K·s/m2 of G⁄R, and funnel-like L2 droplets are incorporated into the aluminum matrix intermittently. These L2 droplets spheroidize during cooling to form regular arrays of spherical L2 droplets. The planar solid-liquid interface no longer lies at G⁄R below 109 K·s/m2, and regular monotectic composite structures can not be formed. Droplet composite structures are also formed by necking down, pinching off and spheroidizing of L2 fibers during cooling at high temperatures below the monotectic. The structural transition during cooling is enhanced, as the diameter of L2 fibers decreases, i.e., as growth rates increase.