Synthesis of MAX Phases in the Zr-Ti-Al-C System

Abstract

This study reports on the synthesis and characterization of MAX phases in the (Zr,Ti)AlC system. The MAX phases were synthesized by reactive hot pressing and pressureless sintering in the 1350-1700 °C temperature range. The produced ceramics contained large fractions of 211 and 312 (n = 1, 2) MAX phases, while strong evidence of a 413 (n = 3) stacking was found. Moreover, (Zr,Ti)C, ZrAl, ZrAl, and ZrAl were present as secondary phases. In general, the lattice parameters of the hexagonal 211 and 312 phases followed Vegard's law over the complete Zr-Ti solid solution range, but the 312 phase showed a non-negligible deviation from Vegard's law around the (Zr,Ti)AlC stoichiometry. High-resolution scanning transmission electron microscopy combined with X-ray diffraction demonstrated ordering of the Zr and Ti atoms in the 312 phase, whereby Zr atoms occupied preferentially the central position in the close-packed MX octahedral layers. The same ordering was also observed in 413 stackings present within the 312 phase. The decomposition of the secondary (Zr,Ti)C phase was attributed to the miscibility gap in the ZrC-TiC system.

The double solid solution (Zr, Nb)2(Al, Sn)C MAX phase: a steric stability approach

“…An alternative approach to obtain MAX phase pure material is to stabilize this stoichiometry by making (Zr, M) 2 AlC and Zr 2 (Al, A)C solid solutions 11 . Stable solid solutions on the M-site have been reported for M = Nb and Ti 12–14 , while recently the in-plane ordered (V 2/3 , Zr 1/3 ) 2 AlC i-MAX phase was synthesized 15 . Horlait et al .…”

Predictive theoretical screening of phase stability for chemical order and disorder in quaternary 312 and 413 MAX phases

“…Fig. 4b 42,45 Our theoretical results indicate that disorder is preferred, though a more in-depth discussion of these discrepancies will be presented below.…”

“…17,48 We have previously shown that o-MAX phases can be stable both upon limited intermixing of M′ and M″, and at off-stoichiometries. 15,24 Ti 2 ZrAlC 2 or (Ti 0.67 Zr 0.33 ) 3 AlC 2 is another example where both order 45,49 and disorder 42,51 have been claimed. Our results indicate that Ti and Zr prefer disorder or possibly semi-order, based on the moderate T disorder = 357 K combined with a similar electronegativity of Ti and Al.…”

Processing of MAX phases: From synthesis to applications

“…Another alternative is to use metal hydrides, such as TiH 2 or ZrH 2 , 142 since they seem to be beneficial for compositions that are difficult to synthesize such as Zr 2 AlC or Zr 3 AlC 2 . 143,144 However, the use of metallic hydrides might be dangerous because of the liberation of H 2 during the thermal process, so safety has to be strongly considered for large quantities. As "A" source, elemental powders such as Al or Si are extensively used due to the low prices, and an excess up to 20 at.…”