The microstructure and crystallography of lath martensite with Greninger-Troiano orientation relationship in a Fe-12.8Ni-1.5Si-0.22% C steel

Duanjun Sun a,b, Zhenghong Guo a,b,*, Jianfeng Gu b,c,*

a School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China
b Institute of Materials Modification and Modelling, Shanghai Jiao Tong University, Shanghai 200240, China
c Shanghai Key Laboratory of Materials Laser Processing and Modification, Shanghai Jiao Tong University, Shanghai 200240, China

ABSTRACT

The excellent matching of strength and toughness with respect to lath martensite is closely related to its hierarchical configuration and crystallographic features. Recently, a few literatures reported that the Greninger- Troiano (G-T) relationship, instead of the generally accepted Kurdjumov-Sachs (K–S) relation, was determined in some lath martensite steels. However, the underlying reason is still waiting to be explored. In the present study, systematic characterizations were performed in water-quenched Fe-12Ni-1.5Si-0.2C steel firstly, then a modified version of crystallographic model developed by Morris et al. was proposed to clarify the microstructure and the crystallography with G-T relationship. As the main crystallographic features, the orientation relationship between prior austenite and martensitic variants is close to G-T rather than K–S relation. Further characterization shows that the block interface is parallel to the annealing twin boundary of prior austenite, indicating the {111}γ habit plane of blocks. Naturally, three crystallographically distinct blocks with {111}γ interfaces can be configured into a packet with self-accommodation manner to minimize macro strain. In fact, such a hierarchical configuration with G-T relationship can be understood clearly by the modified version of crystallographic model. Firstly, regarding the bivariant block as the basic element which undertakes the invariant plane shear, the total shear strain within each packet region can thus be effectively cancelled out on the common habit planes of {111}γ due to the self-accommodation effect, leading to an essentially shear strain-free pattern; Secondly, a higher hierarchy is accompanied with a significant decrease of strain and only a homogenous dilation of 0.02 remains in a prior austenite grain, which is closely related to the unique hierarchy; Thirdly, the G-T relationship is natural preferred for the present steel, which satisfies two conditions in this crystallographic model: (i) a minimum of angular deviation between the {111}γ and the habit plane of bivariant blocks; (ii) three
slip systems of dislocations with 〈111〉α’ Burgers vectors involved during the martensitic transformation, which is also universally observed in the TEM examinations. Besides that, compared with the case of K–S relationship, a potential influence of G-T relationship on mechanical properties may embody in inducing a lower ductile-brittle transition temperature and better toughness of lath martensite steels.