The mystery of ADI

The considerations of ADI strengthening mechanisms based on results of previous structure investigations are presented. The aim of this elaboration was to discuss which mechanism looks to be most responsible for strengthening of ADI. It was concluded that the high strength of ADI which matrix consist mostly ferrite, austenite and sometimes some amount of martensite is caused by strengthening these three phases. Neglecting high carbon martensite which strength properties are nothing surprising, it is proposed that different mechanism of strengthening are responsible for ferrite and austenite strength increase. In case first of these phases strengthening is caused mainly by strain hardening. It is evidenced with very high dislocations and dislocation loops density which are typical in case of ferrite grains. The mechanism of austenite strengthening is more complicated. First of all hardening of austenite is connected with high supersaturation with carbon. The effect of strengthening is additionally increased with grains refinement caused by twinning and formation of stacking faults. The first of them supply extra coherent grain boundaries and the second lead to distortion of perfect lattice structure. It looks from the analysis, that strengthening rather soft and ductile phases which are ferrite and austenite is high enough to explain high strength properties of ADI.