Elucidation of the Strengthening Mechanism of Cast Iron using a High-Intensity Neutron Beam
—Atomic Level Observation of the Microstructure Behavior of Cast Iron through an In Situ Neutron Diffraction Experiment—
Cast iron (spheroidal graphite cast iron) is widely used in various parts of automobiles, hydraulic parts of heavy equipments, etc.; therefore, a long service life under severe conditions such under large external forces is required. Spheroidal graphite cast iron is well known to increase its strength by cyclic tensile-compressive deformation simulating a severe environment, but the mechanism has long been a mystery.
To reveal this mechanism, the Japan Atomic Energy Agency (JAEA), J-PARC, Hitachi Construction Machinery, and a research group at Kyoto University carried out an in situ neutron diffraction experiment of a spheroidal graphite cast iron under a cyclic tensile-compressive deformation, and the crystallographic response to the external force of each constituent phase of the spheroidal graphite cast iron was observed. The results showed that, with increasing number of tension-compression cycles, the accumulation of crystal defects (dislocations) in ferrite, the constituent phase with major fraction in the spheroidal graphite cast iron, was observed, resulting an increase in the ferrite phase stress which contributed greatly to the increase in the whole strength of the spheroidal graphite cast iron.
This discovery contributes a big progress in understanding the characteristics of cast iron under extreme external forces. This may also provide feedbacks for the material design of cast iron for improving safety performance and service life, and for the development of cast iron materials suitable for particular purposes.