Study of the Influence of Sintering Atmosphere and Mechanical Activation on the Synthesis of Bulk Ti2AlN MAX Phase Obtained by Spark Plasma Sintering

Abstract

The influence of the mechanical activation process and sintering atmosphere on the microstructure and mechanical properties of bulk Ti₂AlN has been investigated. The mixture of Ti and AlN powders was prepared in a 1:2 molar ratio, and a part of this powder mixture was subjected to a mechanical activation process under an argon atmosphere for 10 h using agate jars and balls as milling media. Then, the sintering and production of the Ti₂AlN MAX phase were carried out by Spark Plasma Sintering under 30 MPa with vacuum or nitrogen atmospheres and at 1200 °C for 10 min. The crystal structure and microstructure of consolidated samples were characterized by X-ray Diffraction, Scanning Electron Microscopy, and Energy Dispersive X-ray Spectroscopy. The X-ray diffraction patterns were fitted using the Rietveld refinement for phase quantification and determined their most critical microstructural parameters. It was determined that by using nitrogen as a sintering atmosphere, Ti₄AlN₃ MAX phase and TiN were increased at the expense of the Ti₂AlN. In the samples prepared from the activated powders, secondary phases like Ti₅Si₃ and Al₂O₃ were formed. However, the higher densification level presented in the sample produced by using both nitrogen atmosphere and MAP powder mixture is remarkable. Moreover, the high-purity Ti₂AlN zone of the MAX-1200 presented a hardness of 4.3 GPa, and the rest of the samples exhibited slightly smaller hardness values (4.1, 4.0, and 4.2 GPa, respectively) which are matched with the higher porosity observed on the SEM images.

Keywords: MAX phase; Ti2AlN; mechanical activation; milling; spark plasma sintering.

Figure 1

SEM images of powders: (a) as-received Ti, (b) as-received AlN, (c) Non-Mechanically Activated Powder mixtures (NMAP) and, (d) Mechanically Activated Powder mixtures (MAP); (e) XRD of NMAP and MAP powder mixtures.

Figure 1

SEM images of powders: (a) as-received Ti, (b) as-received AlN, (c) Non-Mechanically Activated Powder mixtures (NMAP) and, (d) Mechanically Activated Powder mixtures (MAP); (e) XRD of NMAP and MAP powder mixtures.

Figure 2

XRD patterns of the bulk Ti₂AlN MAX phase specimens: (a) MAX-1200, (b) MAX-1200M, (c) MAX-1200N, and (d) MAX-1200NM.

Figure 3

Macroscopic view of the samples and their micro-XRD results for: (a,e) MAX-1200, (b,f) MAX-1200M, (c,g) MAX-1200N, and (d,h) MAX-1200NM.

Figure 4

SEM images for: (a,b) MAX-1200, (c,d) MAX-1200M, (e,f) MAX-1200N and (g,h) MAX-1200NM.