Abstract New materials in the Al-Zn-Mg-Cu-La alloy system have been proposed for improving corrosion resistance in a study by Kishi et al, because Lanthanum, La, has a storage capability for hydrogen. Hydrogen is a problem in the stress corrosion cracking (SCC) growth mechanism. Zinc, Zn, Magnesium, Mg, and Copper, Cu, in the alloy have influenced the extrusion, strength and other properties; however, in the case of putting this material to use, it is also necessary to evaluate it from the viewpoint of fracture mechanics. The stress intensity factor, K, obtained from fracture mechanics is a necessary parameter for the evaluation of various materials. The first part of this study deals with the measurement of fatigue crack growth in the Al-Zn-Mg-Cu-La alloy system. The second part evaluates the fracture surface obtained from tests of these materials by the application of x-ray fractography.
It is well known that FePd alloys are effective as a magneto-thermoelastic actuator material, because they have large magnetostriction and shape memory effect. In order to use the alloys for a micro-actuator, magnetic properties and microstructures have been examined as for rapidly solidified Fe-29.6 at% Pd alloy ribbons. The ribbons exhibit a large magnetostriction at room temperature and good shape memory effect. Magnetostriction and coercive force of the ribbons markedly depend on the direction of the applied magnetic field. Maximum values of magnetostriction and coercive force are obtained at θ = 85 degree (θ is the angle between the magnetic field and the ribbon plane). Relief effects corresponding to the formation of FCT martensite variants are observed on the grains. X-ray diffraction profile at room temperature shows that FCT martensitic phase and FCC parent phase coexist in the ribbon. Dense striations are observed in the TEM bright field images of FCT martensite plates. Selected area electron diffraction patterns revealed the striations to be thin twins.
Relation between tensile deformation behavior and microstructure in variously aged Ti–49.7 at%Ni–1.3 at%Co shape memory alloys has been investigated. Stress-strain curves for the alloys aged at 623 K and 723 K for 1.8 ks were of a work-hardening type, but those for the alloys aged at the two temperatures over 28.8 ks were nearly of a constant-stress type, as for binary Ti–51 at%Ni aged alloys. Lenticular precipitates were observed in both the 623 K and 723 K aged alloys, and the distance among the precipitates increased with increasing aging temperature and period, as well as the size of the precipitates. The lenticular precipitates were identified to be of the composition of Ti3(Ni, Co)4 from an EDX analysis. Based on these observations, the constant-stress type stress-strain behavior for the alloys aged over 28.8 ks was attributed to some composition change accompanied with the aging progress by which Ti:(Ni+Co) composition ratio in the matrix of the Ti–Ni–Co alloys approached 1:1, as in the equi-atomic Ti–Ni binary alloys.
The relationship between the crack nucleation and stress-induced martensitic transformation in the retained massive austenites (RM-γ’s) of austempered ductile irons (ADIs) was examined in detail by carrying out tensile tests and scanning electron microscope (SEM) observations for an ADI material. The SEM observations revealed that cracks were not nucleated in the peripheral regions of graphite nodules but were nucleated in the RM-γ’s. Surface relief due to stress-induced martensitic transformation was observed near the cracks in the RM-γ’s, and it was also observed in the RM-γ’s in which cracks were not visible. For this reason, the cracks were concluded to be nucleated mainly in the RM-γ’s subjected to stress-induced martensitic transformation.
In this paper, the origin of micro shrinkage pores in gray cast irons is revealed, and a solidification process for the iron is proposed. Gray cast irons, casted under various casting conditions, were examined in terms of density and optical microscopy microstructures of the irons. The amount of the added inoculation agent affects the size and the number of graphite particles per unit area, but does not affect the volume fraction of the graphite. The density of the iron was affected by the cross-sectional area of the ingate, regardless of the amount of the added inoculation agent. When the cross-sectional area of the ingate is larger than the appropriate size, the density of the iron decrease. The reason is the formation of micro shrinkage pores in the iron. The relationship between the solidification process and micro shrinkage pores of the gray cast iron was discussed based on the above experimental results. When the completion of the solidification in the ingate occurs before than that in the cavity, the molten metal cannot flow between the cavity and the runner by passing through the ingate. Therefore, the expansion due to the crystallization of the graphite is equal to the shrinkage due to the crystallization of the austenite at the eutectic solidification in the cavity. As a result, gray cast iron without micro shrinkage pores was obtained.
