Synthesis and high-temperature properties of layered solids
Authors:   , Elmeligy Tarek Ali , Barsoum Michel W. 
Published: October 2023

Oxidation resistance and high-temperature creep resistance are two metrics of performance for most mechanical high-temperature applications. Ternary transition metal carbides and borides have shown great potential in said high-temperature applications due to the impressive isothermal oxidation resistance as well as favorable creep performance. These properties can be tuned and improved through careful synthesis of compositional solid solutions. Moreover, modification of processing, and microstructure is another performance enhancing metric. In this thesis, we examine the effect of compositional solid-solution alloying of transition metal layered carbides as well as exploring of ternary borides. The focus is to analyze the effect of MAX phase solid-solution alloying on high-temperature properties as well as explore high-temperature properties of neighboring ternary silico-borides MAB family. Based on computational and material design recommendations, we synthesize various solid-solution phases using a combination of pleasureless-sintering, hot-pressing and spark plasma sintering and test their high-temperatures properties, specifically oxidation and creep behavior. Based on the results obtained from these studies, recommendations are made to fabricate MAX and MAB phases with enhanced high-temperature performance. Alloying 33 at.% of Y on the M layer in (Cr[2/3]Y[1/3])₂AlC changed the oxide species form and concomitantly the oxidation resistance of this chemically in-plane ordered i-MAX compared to ternary Cr₂AlC. Oxidation at 1000-1200 °C produces a mixture of ternary Y₃Al₂(AlO₄)₃ (YAG) and Y₄Al₂O₉ (YAM) and Cr₂O₃. The layers produced are not passivating compared to Al₂O₃ formed on Cr₂AlC. It is concluded that alloying with 33 at.% Y is the driving force for formation of the ternary YAG/YAP mixture that are not oxidation resistant. Similarly, Y alloying in (M[2/3]Y[1/3])₂AlC leads to the formation of Y₂Mo₃O₁₂ at 1300 °C, which is also unprotective. On the other hand, alloying 2-5 at.% of Nb on the M layer in (Ti[x]Nb[1-x])₂AlC does not significantly change the Nb chemical potential and hence the driving force for passivating Al₂O₃ formation is maintained upon oxidation at 1200 °C. Nonetheless, some initial higher weight gain is obtained compared to Ti₂AlC. However, > 25 at.% Nb alloying shifts the driving force towards formation of other competing oxides such as TiO₂ and at much higher Nb loadings, AlNbO₄. The influence of alloying is different in the cases where we alloy the A layer with Ga in Ti₃Al[0.6]Ga[0.4]C solid-solution MAX phase. Ga increases Al activity resulting in formation dense passivating Al₂O₃ at 1000-1200 °C with excellent adherence to the sample corners. The formation of a continuous alumina scale with only 7 at.% Al in the compound is a record and indicates that the Al activity is greatly enhanced by the presence of Ga. The oxidation kinetics are maintained at sub-parabolic near cubic. Microstructure evidence shows a duplex/triplex oxide layer and evidence for the outward Al flux, J[Al], and the inward O flux, J[O], are related such that 2 J[Al] = 3 J[O]. A fraction of these fluxes combine, at the duplex oxide interface, to nucleate small grains. No competing Ga oxides are detected, and no Ga is found in the oxide by electron microscopy or atomic probe tomography. Transitioning from the carbide MAX system to the boride MAB system influenced the oxidation resistance as well. Oxidation of Mn₂AlB₂ at 700 °C yields a pseudo-protective layer of Mn[2-x]Al[x]O₃ that shuts off oxidation kinetics up to at least 150 h of oxidation. Whereas for Fe₂AlB₂, oxidation at 900 °C yields a protective mixture of Al₄B₂O₉/[alpha]-Al₂O₃ layer with excellent corner adherence. The oxidation kinetics are sub-parabolic beyond 60 h of oxidation. It is apparent that the activity of Mn in Mn₂AlB₂ at 700 °C is high enough to incorporate into the oxide, whereas that is not the case for Fe in Fe₂AlB₂ at 900 °C. In our other investigations replacing the A layer elements with Si in a transition metal M rich Mn₅SiB₂ composition influences various other properties compared to MAB phases. At 12-14 GPa, Mn₅SiB₂ and Fe₅SiB₂ possess double the hardness of MAlB phases. The conductivity is metallic in nature at 1.2-1.5 [mu][omega].m at room temperatures, which is nearly double that of MAlB phases. Both compositions show ferromagnetic behavior with an estimated Curie temperature of 441 K for Mn₅SiB₂ and a magnetic configuration transition from spin reorientation at 153 K for Fe₅SiB₂. DFT calculated elastic moduli are lower than those of MAlB phases. The oxidation resistance exhibited by our studied compositions encouraged us to investigate methods to enhance the creep behavior. We fabricated Ti₂AlC and achieved ~97% of preferred grain orientation (i.e. texturing) such that basal planes are parallel to the tensile creep loading. Creep rates were measured in the temperature range of 1000-1150 °C under tensile stresses in the range of 15-50 MPa. The minimum creep rate is given by a power law, with stress exponent of 2.5±0.1 and an apparent activation energy of 320±20 kJ/mol. The textured Ti₂AlC has significantly lower creep rates than their untextured counterparts, and their times to failure are longer. Microstructures show an order of magnitude reduction of dislocation density in deformed samples in comparison to undeformed samples. For conventional alloys, extensive dislocation networks appear after creep, therefore we could not explain the MAX creep deformation in a typical dislocation framework, but we could in a ripplocation one. In a ripplocation framework, deformation progression through formation of ripplocations, ripplocation boundaries, kinking, kink boundaries, delamination, and results in strain in the loading direction.