Space group and atomic structure determination of a nano-sized ordered phase derived from a f.c.c. structure in maraging steel 12Cr–9Ni–4Mo–2Cu using transmission electron microscopy

Abstract

The unique properties of maraging steel Sandvik 1RK91 were attributed to unique precipitation: a nano-sized L phase in addition to the quasi-crystalline R' phase, which differs from any precipitation system in conventional maraging steels. The L phase was observed after ageing at either 748 or 823 K. It has flake morphology with dimensions approximately 100 x 500 x 500 A. In the present study the structure of the L phase was examined using convergent-beam electron diffraction (CBED), energy-dispersive X-ray analysis (EDX) and high-resolution electron microscopy (HREM). The L phase could be described as Ti(19)Fe(9)Mo(9)Al(8)Cr(5)Ni(50) or simply M(50)Ni(50) (M = Ti, Fe, Mo, Al and Cr). The L phase is isostructural to FeNi. Its crystal structure was determined to have the ordered structure of the uAu-I type (L1(0), P4/mmm, a = 3.52, c = 3.63 A and Z = 2) with two Ni atoms at (1/2) 0 (1/2) and 0 (1/2) (1/2), and two M atoms at 0 0 0 and (1/2) (1/2) 0. The crystal structure of the L phase can also be described using a primitive tetragonal cell and lattice parameters: a = 2.49 and c = 3.63 A, Z = 1. The Volume of the primitive tetragonal unit cell is 22.5 A(3) and the density is approximately 6.98 g cm(-3). The present study has demonstrated the possibility of determining the structure of an extremely small crystal by utilizing the information from CBED, EDX analysis and HREM.