Characterization of Ti 4 AIN 3 A.T. PROCOPIO, M.W. BARSOUM, and T. EL-RAGHY Bulk samples of Ti 4 AIN 3 were fabricated by reactive hot isostatic pressing (hipping) of TiH 2 , AlN, and TiN powders at 1275 8C for 24 hours under 70 MPa. Further annealing at 1325 8C for 168 hours under Ar resulted in dense, predominantly single-phase samples, with ,1 vol pct of TiN as a secondary phase. This ternary nitride, with a grain size of ’20 mm on average, is relatively soft (Vickers hardness 2.5 GPa), lightweight (4.6 g/cm 3 ), and machinable. Its Young’s and shear moduli are 310 and 127 GPa, respectively. The compressive and flexural strengths at room temperature are 475 and 350 MPa, respectively. At 1000 8C, the deformation is plastic, with a maximum compressive stress of ’450 MPa. Ti 4 AlN 3 thermal shocks gradually, whereby the largest strength loss (50 pct) is seen at a DT of 1000 8C. Further increases in quench temperature, however, increase the retained strength before it ultimately decreases once again. This material is also damage tolerant; a 100 N–load diamond indentation, which produced an ’0.4 mm defect, reduces the flexural strength by only ’12 pct. The thermal-expansion coefficient in the 25 8C to 1100 8C temperature range is 9.760.2 3 10 26 8C 21 . The room-temperature electrical conductivity is 0.5 3 10 6 (V ? m) 21 . The resistivity increases linearly with increasing temperature. Ti 4 AlN 3 is stable up to 1500 8C in Ar, but decomposes in air to form TiN at ’1400 8C. I. INTRODUCTION pressed, annealed under dynamic vacuum at 900 8C for 9 hours (to dehydride the TiH 2 ), sealed under vacuum in RECENTLY , we reported on the fabrication of a fully borosilicate glass tubes, and hot isostatically pressed dense, polycrystalline, single-phase layered nitride, (hipped) at 1275 8C for 24 hours under 70 MPa of pressure. Ti 4 AlN 3 . [1] Initially, this ternary was believed to have a To obtain single-phase material, the samples were further Ti 3 Al 2 N 2 chemistry and a hexagonal structure. [2] More annealed at 1325 8C for 168 hours under Ar. All samples recently, it was proposed, based on chemical analysis, that were fully dense, with a measured density of 4.58 mg/m 3 , this compound had a Ti 3 AlN 2 chemistry instead and was which compares favorably to the theoretical density of 4.61 isostructural with Ti 3 SiC 2 . [3] Most recently, we have conclu- Mg/m 3 . The grain structure of Ti 4 AlN 3 was resolved by sively shown by high-resolution transmission electron etching a polished surface with a 1:1:1 by volume solution microscopy, chemical analysis, and Rietveld refinement of of H 2 O, HNO 3 , and HF for 15 seconds (Figure 1(a)). The neutron diffraction that this compound was layered, wherein average grain size is on the order of ’20 mm. layers of pure Al atoms are separated from each other by The mechanical tests were completed on a servohydraulic four layers of Ti. The nitrogen atoms occupy the octahedral MTS testing machine. All measurements were carried out sites between the Ti layers. [4,5] using a constant crosshead displacement speed of 0.0035 Based on its structure and the results presented herein, mm/s, which corresponds to an initial strain rate of 10 23 there is little doubt that Ti 4 AlN 3 represents a member (the s 21 . The compression tests were carried out on samples of only one to date) of a third family of the layered carbides dimensions 2 3 2 3 3 mm 3 . The flexural samples used in and nitrides, with the general formula M n11 AX n , where n 5 the thermal-shock and damage tolerance measurements were 1 to 3, M is an early transition metal, A is an A-group (mostly cut to the ASTM-C1161 specification of 2 3 1.5 3 25 groups III-A and IV-A) element, and X is either C or N, as mm 3 . The flexural strength (s) was calculated using the reported elsewhere. [6–9] The purpose of this article is to following equation: report on the properties of this newly synthesized ternary. Information regarding the oxidation is briefly summarized s 5 3P(l 1 2 l 2 ) 2BW 2 [1] in this article; a complete account can be found elsewhere. [10] where P is the fracture load; l 1 and l 2 are the outer and inner II. EXPERIMENTAL DETAILS spans, respectively; B is the specimen width (2 mm); and The synthesis of Ti 4 AlN 3 is described elsewhere. [1] In W is the specimen thickness (1.5 mm). The aforementioned brief, TiH 2 (TIMET, Henderson, NV; 99.3 pct, 2325 mesh), samples were cut using a diamond wheel and used as is TiN (Alfa Aesar; 99.8 pct, 2 to 5 mm), and AlN (Alfa Aesar, (i.e., the corners were not beveled). N 32.0 wt pct minimum, d m ’ 3 mm) powders were mixed The Vickers hardness was measured at a load of 100 N; to the desired stoichiometry. The mixed powders were cold the microhardness (LECO* M-400) tester was measured at *LECO is a trademark of LECO Corporation, St. Joseph, MI. M.W. BARSOUM, Professor and T. EL-RAGHY, Research Assistant loads of 3, 5, and 10 N. To measure the damage tolerance, Professor, are with the Department of Materials Engineering, Drexel Univer- microhardness indentations were placed on polished (1200- sity, Philadelphia, PA. A. PROCOPIO graduated from the Department in grit SiC paper) surfaces of four-point bend specimens, such June of 1999 with an MS thesis. Manuscript submitted May 17, 1999. that one diagonal of the indentation was parallel to the length METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 31A, FEBRUARY 2000—333