Showing papers in "Materials Transactions Jim in 1974"

Morphology and Crystallography of Thermoelastic Cu–Al–Ni Martensite Analyzed by the Phenomenological Theory

Abstract: The thermoelastic martensite transformation from theβ1 phase to the γ' phase in Cu-Al-Ni alloy has been analyzed by the phenomenological theory, special attention being given to the understanding of the unique morphology. It is shown that there are a pretty good agreement between the theory and experiment with respect to the habit plane and the orientationrelationship and a less good agreement with respect to the relative twin width,for δ=1. The unique \"spear-like\" mor-

Electron Microscopy and Diffraction Study of the Carbide Precipitated at the First Stage of Tempering of Martensitic Medium Carbon Steel

Abstract: Crystal structure and precipitation mode of the carbide in a martensitic 0.45 wt%C steel tempered at 120 and 200°C have been studied by means of electron microscopy and selected area diffraction. In both plate-like and lath martensites, the precipitated carbide is η-Fe2C like the previous case of high carbon steel(Acta Met.,20(1972),645). The crystal morphology and the orientation relationships between the carbide and the matrix are also the same. In general, the precipitation occurs along dislocations, but it has been found that grain boundary precipitation also takes place in the non-parallel lath region and that in fine grains in this region the carbide precipitation does not occur. Some discussions are given to the crystal structure and the precipitation mode of the carbide. (Received September 4,1973)

Spinodal Decomposition in Alnico 8 Magnet Alloy

Abstract: matrix. The high coercivity can be satisfactorily explained by the single-domain behavior of the particles. In 1961 Cahn proposed a theory of spinodal decomposition(3), which enabled him to give better account of various precipitation processes associated with the above two-phase separation transformation. Since then, Alnico-type magnet alloys have been frequently investigated from a viewpoint of the spinodal concept(4)(5). So it has been found that Alnico-type alloys exhibit many interesting aspects characteristic of the spinodal decomposition. Among these it is noteworthy that the precipitate particles are formed to align along the specified crystallographic orientations, i.e. along the <100> direction of each crystalite. Furthermore, considering the contribution of magnetic energy in the decomposition of a ferromagnetic alloy, Cahn suggested that an applied magnetic field might affect the decomposition to render the precipitates elongated along the field direction(6). This theoretical prediction has at least been qualitatively substantiated by the experimental results on some Alnico-type alloys. The structure resulting from the spinodal decomposition is generally too finely divided to quantitatively examine the spinodal mechanism. In the present work, an electron microscope is used to investigate details of the microstructures and to compare them with the theory. Particularly, in this alloy, the precipitate particles are so regularly aligned along a certain direction that a precise determination of the particle size is made possible. Most of the theoretical approaches have been concerned with the earlier stages of the decomposition, whereas the present work deals with a rather later stage of the decomposition. It is found, however, that the results obtained are consistent with the theory in many essential aspects. In addition, some interesting facts are observed which have not been predicted hitherto. These results would make important contributions to a further development of the theory as well as to an improvement in permanent magnet technology.

Analysis of the Grain Size Dependence of the Yield Stress in Copper-Aluminum and Copper-Nickel Alloys

Abstract: Grain size dependence of the yield stress in polycrystalline copper-aluminum and copper-nickel alloys is measured at 77,194 and 293° K. It is found that the Hall-Petch relation is valid for these materials. The Petch slope ky is nearly independent of the temperature, but increases with increasing solute concentration. In copper-aluminum alloys the increase in ky is nearly proportional to the square root of the solute concentration, while in copper-nickel alloys it is proportional to the concentration itself. The concentration dependence of ky in Cu-Al alloys is considerably larger than that in Cu-Ni alloys. None of previously proposed theories on the Petch slope can explain these results. The experimental results will be compared with the calculation in the following paper.

Twinning in γ′ Cu–Al–Ni Martensite with Cu 3 Ti Type Ordered Structure

Abstract: The{121}γ'and{101}γ'twinnings in γ'Cu-Al-Ni martensite with Cu3Ti type ordered structure, whose sublattice is HCP, have been analyzed by the theory of deformation twinning and the phenomenological theory of martensitic transformations. As a result the following conclusions have been reached: (1) The twinning elements of{121}γ'twinning consistent with the phenomenological theory are as follows. K1= (121)γ,η1=[1,0.7954,0.5907]γ, K2=(1,1.5036,0.5036)γ', ηe=[111]γ', s=0.26]. (2)In spite of the ordered structure, the original structure can be restored by the above twinning mode and simple and small shuffles, thus providing a theoretical background for experimental evidences such as the mobile nature of twin boundaries and the absence of extra spots in electron diffraction patterns. (3) {101}γ' twins are considered to be variants of martensite rather than a lattice invariant strain in a variant of martensite.

Effects of Retained Austenite on the Rolling Fatigue Life of Ball Bearing Steels

Abstract: (2) In the fatigue test, both the surface and the internal hardness of the rolling part of the ball bearing steel increase to a great extent when a large amount of retained austenite is contained. (3) A semicircular stress-affected zone is present at the rolling part in the fatigue test, where plastic flow and tempering effects are recognized in the matrix. It is also found that the retained austenite gradually decomposes during testing. (4) The presence of the retained austenite plays an auxiliary role in improvements of the fatigue life and reduces its irregularity. (Received May 7, 1973)

Coarsening of Lamellar Structures at Elevated Temperatures

Abstract: The coarsening of lamellar eutectic alloys is controlled by faults in the as-grown structure. A model, based on the use of In cosh cylinder co-ordinates, is proposed to calculate the rate of shrinkage of lamellar terminations. It is found that for high volume fractions and diffusion through the continuous phase around the fault, the fault shape remains approximately constant and a value can be obtained for the rate of shrinkage of an isolated fault. For the case of interface diffusion, the shape is not preserved and only the initial shape changes can be calculated. The problem of coarsening of an array of faulted lamellae is considered. The important variables in the coarsening rate are the lamellar spacing and fault density. These results are compared to the experimental observations of Graham and Kraft on unidirectionally solidified Al-CuAl2 alloys. (Received August 11, 1973)

Studies on the Structure Developed around Fatigue Cracks of Aluminium Using a High Voltage Electron Microscope

Abstract: The structures formed around fatigue cracks of aluminium were examined using a 500 kV electron microscope in order to confirm the previous results obtained by a 100 kV electron microscope and to find a clue to the elucidation of the mechanism of fatigue fracture involved. There was a well-defined substructure around a fatigue crack. A large number of dislocation loops and dislocation dipoles were found near sub-boundaries. A strain-concentrating region where many voids existed was observed in the vicinity of the fatigue crack. These results are in good agreement with those obtained in previous work by use of the 100 kV electron microscope and are also consistent with the previous X-ray observations. There was direct evidence that propagation of fatigue cracks occurs through growing-up and joining-up processes of the voids formed in the strain-concentrating regions. The mechanism of void formation due to the coalescence of vacancies is proposed on the basis of the present observations. The short-circuit diffusion along the sub-boundaries with the driving force caused by the stress gradient near the crack is suggested to explain the vacancy migration through the strain-concentrating regions.

Kinetics of Ordering in Fe–Al Alloys

Abstract: The kinetics of ordering in Fe-Al alloys with about 25 at %Al is investigated both theoretically and experimentally. The kinetic equations which express the time variation of the degrees of order in B2 and DO3 structures are obtained by taking into account the direct interchange of atoms between the first and the second nearest neighbors. The experimental results of X-ray diffraction for these alloys on isothermal annealing or slow cooling are reproduced satisfactorily by theoretical rate equations, and the characteristics of the ordering in the Fe-Al system is investigated. The ordering in DO3 type is influenced substantially by the atomic interchange between the second nearest neighbors. The B2 type ordering takes place so rapidly that the quenching from a disordered phase does not prevent the ordering. An oscillatory change of the degree of order in the B2 structure is expected from the calculation in the isothermal annealing. (Received August 27, 1973)

Stresses in Bi-Layered NiO Scales

Abstract: flexure of the specimen (99.9 % pure) is achieved during oxidation; the former is on the convex side. When removed electrochemically from the substrate of nickel (99 or 99.9% pure), the black/green bi-layer was elongated causing bending to the opposite side. On the contrary, the black/black bi-layer formed on the 99.9 % pure nickel remained straight with a negligibly small elongation as compared with the black/green bi-layer. It is found that high compressive stresses occur in the green layer, and negligibly small stresses in the black/black bi-layer. Macroscopic elastic stresses in the green layer at room temperature are estimated to be the order of magnitude 109 dynes/cm2. Stress gradients across the scale are roughly estimated. Discussions are given of the formation of bi-layered structures. (Received July 25, 1973)

X-Ray Diffraction and Electrical Resistivity Study of Ag 2 In and High Temperature Ag 3 In Phases

Abstract: The phase transition of Ag2In was investigated by means of X-ray diffraction, measurements of specific heat and electrical resistivity, and the composition dependences of lattice parameter and electrical resistivity of the C phase (high temperature form of Ag3In) were also examined over the range of 25-40 at %In. It was found that (1) the Ag2In phase transformed from the γ-brass type to hcp structure at 222°C with a heat of transition of 130 cal/g-atom, and that(2) the lattice parameter, and the electrical resistivity and its temperature coef5cient changed abruptly at 33.0 at%]n in the homogeneous region of ζ phase.