Characterization of Ti 4 AlN 3
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INTRODUCTION
RECENTLY , we reported on the fabrication of a fully dense, polycrystalline, single-phase layered nitride, Ti4AlN3.[1] Initially, this ternary was believed to have a Ti3Al2N2 chemistry and a hexagonal structure.[2] More recently, it was proposed, based on chemical analysis, that this compound had a Ti3AlN2 chemistry instead and was isostructural with Ti3SiC2.[3] Most recently, we have conclusively shown by high-resolution transmission electron microscopy, chemical analysis, and Rietveld refinement of neutron diffraction that this compound was layered, wherein layers of pure Al atoms are separated from each other by four layers of Ti. The nitrogen atoms occupy the octahedral sites between the Ti layers.[4,5] Based on its structure and the results presented herein, there is little doubt that Ti4AlN3 represents a member (the only one to date) of a third family of the layered carbides and nitrides, with the general formula Mn11 AXn , where n 5 1 to 3, M is an early transition metal, A is an A-group (mostly groups III-A and IV-A) element, and X is either C or N, as reported elsewhere.[6–9] The purpose of this article is to report on the properties of this newly synthesized ternary. Information regarding the oxidation is briefly summarized in this article; a complete account can be found elsewhere.[10] II. EXPERIMENTAL DETAILS The synthesis of Ti4AlN3 is described elsewhere.[1] In brief, TiH2 (TIMET, Henderson, NV; 99.3 pct, 2325 mesh), TiN (Alfa Aesar; 99.8 pct, 2 to 5 mm), and AlN (Alfa Aesar, N 32.0 wt pct minimum, dm ' 3 mm) powders were mixed to the desired stoichiometry. The mixed powders were cold
pressed, annealed under dynamic vacuum at 900 8C for 9 hours (to dehydride the TiH2), sealed under vacuum in borosilicate glass tubes, and hot isostatically pressed (hipped) at 1275 8C for 24 hours under 70 MPa of pressure. To obtain single-phase material, the samples were further annealed at 1325 8C for 168 hours under Ar. All samples were fully dense, with a measured density of 4.58 mg/m3, which compares favorably to the theoretical density of 4.61 Mg/m3. The grain structure of Ti4AlN3 was resolved by etching a polished surface with a 1:1:1 by volume solution of H2O, HNO3, and HF for 15 seconds (Figure 1(a)). The average grain size is on the order of '20 mm. The mechanical tests were completed on a servohydraulic MTS testing machine. All measurements were carried out using a constant crosshead displacement speed of 0.0035 mm/s, which corresponds to an initial strain rate of 1023 s21. The compression tests were carried out on samples of dimensions 2 3 2 3 3 mm3. The flexural samples used in the thermal-shock and damage tolerance measurements were cut to the ASTM-C1161 specification of 2 3 1.5 3 25 mm3. The flexural strength (s) was calculated using the following equation:
s5
3P(l1 2 l2) 2BW 2
[1]
where P is the fracture load; l1 and l2 are the outer and inner spans, respectively; B is the specimen width (2 mm); and W is the specimen thickness (1.5 mm). The aforementioned samples were cut using a diamon
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