Microalloying in sintered steels by mechanical alloying: evolution of nanostructure during attrition and thermal stability

Abstract

Microalloying as a technique to improve the mechanical properties has been used for decades in bulk steels. In order to transfer some of the benefits of microalloying to sintered steels, microalloying elements (Nb in this case) are introduced in the iron powder by mechanical alloying. Since mechanical alloying is a solid state alloying technique, the microalloying elements can be added not only in elemental form (Nb) but also in the form of carbides (NbC). Moreover, the use or not of wax as a process control agent during milling is considered. This work studies the evolution of the powder characteristics and nanostructure during milling and the thermal stability of the resultant powder (since for further consolidation the powder will be exposed to temperature). The evolution of the powder characteristics as a function of milling time is monitored by apparent and tap densities, SEM, particle size distribution, DRX, crystallite size and internal strain. The thermal stability of the resultant nanostructured powder is analysed by means of DRX (crystallite size and internal strain) after annealing the milled powder at different temperatures. The purpose of this study is to investigate the temperature influence on crystallite features and the release of stored deformation energy.

Keywords:

• Mechanical alloying
• Microalloying
• Sintered steels
• Nanostructured powder
• Thermal stability

This research has been supported by the Ministerio de Ciencia e Innovación de España through project no. ENE2009-13766-C04-03 and by the Comunidad de Madrid through the programme ESTRUMAT (project no. S2009/MAT-1585) and the programme for researchers PIA. The authors are grateful to Dr M. Zadra, K4Sint Srl, for the production of the sintered specimens.

Notes

This paper is part of a special issue on ‘Euromat 2011: powder synthesis and processing for controlled microstructure’