Microstructure and residual elastic strain at graphite nodules in ductile cast iron analyzed by synchrotron X-ray microdiffraction

Abstract

The microstructure and residual elastic strain at graphite nodules (GNs) in ductile cast iron produced using either a fast or slow cooling rate have been characterized using synchrotron 3D X-ray Laue microdiffraction. The results show that thermal stress is introduced during cooling and that part of this stress is relaxed by plastic deformation of the polycrystalline ferrite matrix. It is found that the plastic deformation is accommodated by the formation of dislocations and dislocation boundaries, which are organized in a cell structure. The dislocation density quantified based on the microstructure is most pronounced at the GN/matrix interface around small GNs in the fast cooled sample. Residual elastic strain is also present, which is mainly compressive with a maximum of 6.0-9.9 × 10-4 near the GNs. Gradients of plastic deformation and elastic strain field around the GNs are observed. The results document for the first time that both the elastic strain field and the plastic strain field averaged over the grains around the GNs is approximately scaling with GN size and not affected by the cooling rate. The experimental data are compared with simulations by a finite element method, and agreement and disagreement are discussed in detail.

Relation between microstructural heterogeneities and damage mechanisms of a ferritic spheroidal graphite cast iron during tensile loading

“…The subsurface microstructural heterogeneities could affect the location of crack initiation and the fracture path observed, 8,16,22 for example, a graphite nodule located below the observation's surface may induce higher deformation at the adjacent surface and the subsequent development of a crack in the metallic matrix. Under the experimental methodology used, this effect would be unnoticed.…”

“…The results show that the thermal contraction mismatch between the graphite nodule and the metallic matrix during solid‐state cooling leads to a complex residual stress state along the microstructure, particularly near the graphite particles. However, this complex residual stress state is relevant at small strains only 19,22 …”

Analysis of the correlation between micro-mechanical fields and fatigue crack propagation path in nodular cast iron

“…Concerning the matrix, it is worth remarking that the mean size of the grains surrounding a graphite nodule is expected to roughly equal to half of the nodule equivalent diameter [22].…”

“…This was done in order to include the residual stress state associated with the mismatch in thermal contraction between the nodules and the matrix [22] into the subsequent simulations.…”

“…On the other hand, experimental characterization of the full 3D deformation field at the level of the microstructure is very challenging. The reason is that the limited number of matrix grains between neighboring nodules precludes the application of energy dispersive X-ray diffraction [21], whereas measurements using X-ray Laue micro-diffraction [22,23] require long time to probe a volume sufficiently large to be statistically relevant and this may lead to stress relaxation during the measurement. In addition, absorption tomography combined with digital volume correlation, which have proved to be very effective for the determination of the local stress intensity factor in SGI [24][25][26], lacks spatial resolution if the nodules alone are used as markers.…”