Dependence of {112}<111>-type twin density on carbon content in Fe-C martensite

Abstract

In quenched Fe-C (C: 0.0∼2.0 wt.%) binary alloys, the body-centered cubic (BCC) {112}-type twin structure (density, size and morphology) in martensite was investigated by means of transmission electron microscopy (TEM). In the samples quenched to room temperature, the twin density increased as the carbon content increased. In the carbon-free or pure iron sample, no twin structure was observed. In high carbon martensite, a high density of twins could be seen with twin thickness of 1 nm–2 nm, which is of the scale of the smallest α-Fe grain. The twin density variance is discussed based on a detwinning process, which occurs upon cooling. The twin, as an initial product of martensitic transformation, would experience a higher temperature auto-tempering process in low carbon alloys than in high carbon samples. A noticeable detwinning process takes place in low carbon alloys and results in a low density of twins observed at room temperature. Martensite starting (Ms) temperature plays a crucial role in the detwinning or auto-tempering effect on the twins.

Keywords

• Martensite
• Twin
• TEM
• Carbon steels
• Metastable ω-Fe
• Detwinning