Contribution of shear deformation to grain refinement and densification of iron powder consolidated by high pressure torsion

Y J Zhao,T Grosdidier,L S Toth,R Massion

doi:10.1088/1757-899x/63/1/012032

Abstract

High Pressure Torsion (HPT) experiments were performed for consolidation of water-atomized pure iron powder (99%) with initial particle sizes of 20-100 gm. The experiments were carried out successfully at room temperature, achieving both low level of residual porosity and significant grain refinement, thanks to the intense shear strain and hydrostatic pressure applied in HPT. X-ray diffraction analysis was carried out on the consolidated samples which revealed no significant proportion of oxides. Considering the inherent heterogeneity of the imposed shear strain in HPT, different positions across the diameters of sample disks were selected for mechanical property and microstructure investigations. The effect of shear deformation on the microstructure and texture was investigated by metallography, scanning electron microscopy, electron backscattered diffraction (EBSD). The micro-hardness and porosity of the samples as a function of shear strain at constant hydrostatic pressure were also measured. The grain size distributions showed homogeneous microstructures with significant grain refinement due to shear deformation. The texture measurements revealed that a shear texture typical to the shear of bcc iron was obtained during HPT compaction of iron powder.

Highlights

Through the assembly of individual particles or atoms, powder metallurgy (PM) is capable of producing bulk components with very small grain sizes and with special compositions that are not achievable with conventional metallurgical processes
While reports have produced the knowledge with regard to microstructure and properties of different metallic powders consolidated through high pressure torsion (HPT), the mechanisms of grain refinement and the texture evolution, which are crucial for the property improvement [10], are rarely reported
We focus on the role of shear strain and the hydrostatic pressure on the compaction process, which could lead to a further understanding to the mechanism of powder densification and microstructure refinement during HPT

Summary

Introduction

Through the assembly of individual particles or atoms, powder metallurgy (PM) is capable of producing bulk components with very small grain sizes and with special compositions that are not achievable with conventional metallurgical processes. It has already been shown that SPD techniques which imposes severe shear deformation and high hydrostatic pressure to the samples is a very efficient process for consolidating powder [3,4,5]. Compared to the conventional PM approach, the SPD technique is viable to be carried out and achieve fully dense products at room temperature [3], and to avoid grain growth. While reports have produced the knowledge with regard to microstructure and properties (density distribution [8], hardness [9], etc.) of different metallic powders consolidated through HPT, the mechanisms of grain refinement and the texture evolution, which are crucial for the property improvement [10], are rarely reported. The evolution of the crystallographic texture of the consolidated samples is discussed in the present work

Experimental materials and procedures

Porosity and Vickers hardness

Microstructure

Crystallographic texture

Conclusions