Investigation of microstructural changes in M250 grade maraging steel using positron annihilation

Abstract

Microstructural evolution of M250 grade maraging steel subjected to various thermal-aging treatments has been investigated using positron annihilation, X-ray diffraction, hardness and electron microscopy studies. Isochronal aging treatment in the range of 600–900 K, in steps of 100 K, was carried out and positron lifetime, austenite volume fraction and hardness values have been measured. The stages corresponding to point defect dynamics and formation of intermetallics have been clearly identified. Based on these results, measurements were performed on specimens isothermally aged at 755 K from 0.1 to 100 h to elucidate the time evolution of microstructural changes, and the results are discussed.

Keywords:

• maraging steel
• electron microscopy
• microstructure
• precipitates
• positron annihilation

Acknowledgements

We thank Dr. P.R. Vasudeva Rao, Director, Chemistry, Metallurgy & Materials Group and S. Mahadevan of NDED, Indira Gandhi Centre for Atomic Research, for cooperation and P. Gopalan for her assistance during positron lifetime measurements.