HIGH-ENERGY MECHANICAL GRINDING TO PRODUCE Cr₂AlC AND Ti₂AlC MAX PHASES

Abstract

We have developed a method for producing single-phase Cr₂AlC and Ti₂AlC MAX phases using mechanical activation in a planetary mill, followed by heat treatment of the mechanically activated mixture in the temperature range 800–1150 °C. The specific surface area of  the Cr₂AlC and Ti₂AlC obtained phases approximately equaled 3 m²/g. An Al₈₆Cr₁₄ intermetallic phase emerged in grinding of the 2Cr–Al–C powder mixture. The brittle Al₈₆Cr₁₄ phase promoted better refinement of all components and led to the formation of an X-ray amorphous mixture. Gradual heating of this mixture in the temperature range 800–1150 °C produced a single-phase carbide: Cr₂AlC MAX phase. In grinding of the 2Ti–Al–C powder mixture, a mechanically induced self-propagating synthesis (MSS) reaction occurred to form a mixture of the Ti₂AlC MAX phase and titanium carbide TiC. The ratio between the amounts of the MAX phase and titanium carbide depends on the grinding conditions. Reduction in the grinding intensity resulted in a mixture of 70 wt.% Ti₂AlC and 30 wt. % TiC. The MSS reaction does not take place in grinding of the 2Cr + Al + C mixture because the rule DH/C_p (the ratio of the enthalpy of formation to the specific heat) to be more than 2000 K does not hold for this system. The significant plastic deformation of the titanium powder when ground in the presence of carbon and thermodynamically favored formation  process of both  Ti₂AlC and  titanium carbide (DH/C_p more than 2000 K) promote the MSS reaction. A virtually single-phase powder (95 wt.%) of the Ti₂AlC MAX phase with impurities of TiC and intermetallic AlTi was produced by heating of a mechanically activated mixture of titanium, titanium hydride, aluminum, and thermally expanded graphite. Titanium hydride in this mixture prevents the Ti + C = TiC MSS reaction. Titanium hydride reduces the MSS reaction rate, thus inhibiting the formation of titanium carbide and promoting a greater amount of the Ti₂AlC MAX phase.