Morphological and crystallographic features of Laves phase in advanced 9 % Cr creep-resistant steel

Abstract

The precipitation behavior of the Laves phase in a 9 % Cr martensitic steel during creep at 650 °C was investigated by means of transmission electron microscopy (TEM). The ideal stacking sequence of the Laves phase is interrupted by various defects, including basal, prismatic, and pyramidal planar faults. The high density of coexisting planar defects significantly affects the crystal structure of the C14 Laves phase, leading to the appearance of a five-fold symmetry structure with characteristics of quasicrystalline ordering. The preferred nucleation sites for the Laves phase particles were determined to be the ferrite (α) grain boundaries and the α/M₂₃C₆ (M = Cr, Fe) interphase boundaries. Selected area electron diffraction and high-resolution TEM of extracted carbon replicas were employed to reveal the orientation relationships between the phases. In the case of interfacial precipitation, the lattice mismatch between the Laves phase and the α-matrix is discussed with respect to the adoption of two distinct orientation relationships with M₂₃C₆ carbide: namely, ${\left(01\overline{1}0\right)}_{\mathrm{Laves}}\Vert {\left(110\right)}_{M_{23}{C}_6}\Vert {\left(1\overline{1}1\right)}_{\alpha }$, ${\left(0001\right)}_{\text{Laves}}\Vert {\left(1\overline{1}1\right)}_{M_{23}{C}_6}\Vert {\left(011\right)}_{\alpha }$, and ${\left(0001\right)}_{\text{Laves}}\Vert {\left(110\right)}_{M_{23}{C}_6}\Vert {\left(1\overline{1}1\right)}_{\alpha }$, ${\left(01\overline{1}0\right)}_{\text{Laves}}\Vert {\left(1\overline{1}2\right)}_{M_{23}{C}_6}\Vert {\left(\overline{1}12\right)}_{\alpha }$, taking into account that M₂₃C₆ particles are commonly related to the α-matrix by the Kurdjumov-Sachs orientation relationship. These observations and their interpretations are essential for understanding the heterogeneous precipitation of Laves phase particles in high‑chromium martensitic steels.