X-Ray Investigation on the Strength of Inhomogeneous Metallic Materials

Abstract

Cast iron is taken as a typical example of inhomogeneous metallic materials. It has graphite in its structure and its complex structure causes complicated deformation mechanism in microscopic scope during loading. However, the mechanical behavior of cast iron has not been sufficiently interpreted in terms of microscopic mechanism of deformation. Because of lack of usable information, the designing of machine parts made of cast iron is obliged to follow so-called experience, although various sorts of cast iron are widely used in engineering equipments and machines.In the present paper, it is intended to elucidate the microscopic deformation mechanism of cast iron by observing the relation of macroscopic deformation behavior, which is determined by tension and compression test, to the deformation characteristics of atomic planes of ferritic matrix observed by X-ray diffraction technique.The deformation behavior of spheroidal graphite cast iron with pearlitic matrix was discussed in the previous paper on the basis of lattice strain measured by Co-Kα radiation and the change in half-value breadth. In the previous investigation, the local plastic deformation in the matrix around graphite and the reduction of the effective cross sectional area caused by graphite were observed even under low tensile stress. It was reported, moreover, that the X-ray elastic modulus of cast iron was measured with confidence after 3 to 10 stress cycles for annealed specimens.This paper is to present the results of the further experiments, and it is a supplement to the previous investigations. The crystal plane dependence of lattice strain or elastic constants of spheroidal graphite cast iron with pearlitic matrix is observed by using Co-Kα and Cr-Kα radiations.The difference of deformation behaviors between the spheroidal and the flake graphite cast iron has been studied by employing the X-ray method as well as the optical microscope. The X-ray elastic constants for stress measurement on cast iron are also calculated from the obtained results.The following conclusions have been obtained:(1) The characteristic deformation behavior of spheroidal graphite cast iron reported in the previous paper is confirmed again by Cr-Kα measurements and by the observation with optical microscope.(2) The crystal plane dependence of lattice strain occurs under stress more than about 15kg/mm2 for spheroidal graphite cast iron with larger values of lattice strain on (310) planes than those on (211) planes.(3) Spheroidal graphite cast iron have almost the same deformation mechanisms as flake graphite cast iron in substance. That is, the formation of cracks is made in the matrix by means of high stress concentration effect.(4) For the X-ray elastic constants of cast iron, the values should be used as they are obtained by submitting an annealed specimen to 3 to 10 stress cycles.The elastic constants obtained by the X-ray method is different from those obtained by the mechanical method. And there is great difference in value between cast iron and iron or steel. Therefore, much caution should be taken in estimating the elastic constants in the X-ray stress measurement on cast iron.