XRD-Analysis of the Relation of Stacking Fault Formation and the TRIP-Effect in ADI

Abstract

The mechanical properties of ausferritic ductile iron are strongly influenced by the ability of the austenite to undergo martensite formation. Strain-induced martensite formation occurring under the right circumstances results in transformation induced plasticity (TRIP) that improves ductility and strength. TRIP has been shown to depend on the austenite’s stacking fault energy (SFE), which describes the crystals micromechanical behaviour. In austenitic FeMnAlSi-TRIP steels, TRIP as a reaction to mechanical load only occurs for SFE <20 mJ/m2. For carbon-stabilised austenite as in ADI, the relationship between stacking faults, mechanical properties and martensitic transformation has not yet been established. To investigate the TRIP-effect in ADI, unalloyed ADI with 3.43 wt% C, 2.52 wt% Si and 0.21 wt% Mn was ausferritised and subjected to tensile tests at temperatures between −180 °C and 200 °C. The amount of martensite produced by thermal and mechanical activation, crystalline microstrain and stacking fault density were measured on deformed and undeformed regions of the specimen by XRD and the resulting SFE calculated. Between −70 and 20 °C, the elongation at fracture exceeded 10 % compared to below 2 % in the temperature range of −130 to −180 °C. At temperatures above 80 °C, elongation at fracture gradually decreased to 4.5 % at 200 °C. High sample ductility was associated with lower SFEs as low as 35 mJ/m2, indicating a correlation of stacking fault formation and strain-induced martensite formation in ADI.