Zig-zag martensite formed at low temperatures

Abstract

Introduction Two martensite morphologies, thin plate and lenticular martensites with M s temperatures below room temperature, have been separated according to their substructures (1-3). Thin plate martensite contains completely twinned substructures with a {112} twin plane, while the lenticular martensite contains a high density of dislocations and partial twins along the midribs. In the present work, by means of magnetic susceptibility measurements and optical microscope observations, it has been found that a kind of small size zig-zag martensite formed at rather low temperature when compared with that of lenticular martensite. This type of martensite can easily be separated from normal lenticular martensite via a two-step cooling method. Internal friction results reveal that this type of martensite has similar aging behaviour to that of lenticular martensite. Transmission electron microscopy observations demonstrate that its substructure shows no sign of twins. Experimental Procedures The alloy used in this study was Fe-20Ni-0.73C. The specimens were sealed in high vacuum capsules and austenized at temperature of 1150 °C for three hours followed by a water quench. The M s temperature determined by the magnetic susceptibility measurement was 224 K. The specimens for internal friction measurement were first machined to a size of about 1 x 1 × 36 mm and then polished to a size of about 0.7 x 0.7 x 35 mm using acid HF (Hydrofluoric + Hydrogen Peroxide). The specimens were fixed in a low frequency torsion pendulum before cooling. The average cooling rate was about 14 K per minute. The heating rate during internal friction measurement was about 1.1 K per minute. The strain amplitude during the measurement was less than 10 -5 and the testing frequency was about 0.35 Hz. A Philips-400 Transmission Electron Microscope (TEM) operating at 120 kV was used for this study. TEM specimens were prepared by a two-jet electropolisher. The electrolyte was 5% perchloric acid + 95% ethanol. The specimens for magnetic susceptibility measurements were cut with a slow rate diamond saw and then chemically polished (using acid HF) into bars of about 1 x 1 x 3 mm. The specimen was surrounded by two coils of thin cobalt wire. One of the coils was supplied with an AC current of 100 Hz. The magnetic susceptibility measurement was carried out by monitoring the inductance of the other coil with a lock-in amplifier. A two-step cooling method, shown schematically in Fig. 1, was used in this study. Samples of the complete austenite phase at room temperature (RT) were first cooled to 152 K and then heated to room temperature. After holding at room temperature for a while, the samples were secondly cooled to 82 K and then heated to room temperature. Internal friction measurements were made during these two heating processes. Magnetic susceptibility measurements were performed during both cooling and heating processes. Optical microscopy and TEM have been used to reveal the microstructures of the transformation products.