ISSN 1061-3862, International Journal of Self-Propagating High-Temperature Synthesis, 2009, Vol. 18, No. 3, pp. 194–199. © Allerton Press, Inc., 2009.

Hot Forging of MAX Compounds SHS-Produced in the Ti–Al–C System1 A. M. Stolina, D. Vrelb, S. N. Galysheva, A. Hendaouib, P. M. Bazhina, and A. E. Sytscheva aInstitute

of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia bLIMHP, Université Paris 13, CNRS, Institut Galilée, 99 Av. Jean-Baptiste Clément, Villetaneuse, 93430 France Received June 22, 2009

Abstract—Investigated was the forgeability (extent of compression) of still hot SHS products (MAX phases) formed in the Ti–Al–C system at relatively low applied pressures (1.5–15.0 MPa in a press punch) in conditions of free SHS compression. The extent of compression was measured as a function of time delay (between the end of reaction and compression) and applied pressure. Characterized were the microstructure and phase composition of thus obtained MAX compounds. Keywords: SHS, hot forging, Ti-Al-C system, MAX phases PACS numbers: 81.05.Mh, 81.20.Ka DOI: 10.3103/S1061386209030108 1

1. INTRODUCTION

Ti2AlC and Ti3AlC2 are ternary compounds–the socalled MAX phases, from their chemical composition Mn + 1AXn, where M is a transition metal, A is a IIIA or IVA group element, X is either C or N, and n ranges from 0 to 2–have attracted considerable interest primarily because of their unique properties combining many merits of both metals and ceramics [1–6]. Like metals, they show high thermal and electrical conductivity, and are easy to machine, resistant to thermal shock, and tolerant to damage. Like ceramics, they possess low densities, a high Young modulus, a low thermal expansion coefficient, high strength at high temperatures, and excellent oxidation resistance. All of these properties make Ti2AlC and Ti3AlC2 promising materials for hightemperature structural and functional applications. Among different methods to produce these ceramics, SHS has been used as a very cost-effective method since the early 90s [7, 8], and recently we succeeded in producing these phases with no detectable amounts of TiC as a residual phase, which opened up new possibilities for the use of SHS [5, 6]. As for many materials synthesized by SHS, the final porosity observed in the product may be a serious drawback of this synthesis method, although other production methods may require much longer processing time and/or high operating cost devices (e.g. Hot Isostatic Pressing or Spark Plasma Sintering). In this work, we investigated the formability of burned Ti–Al–C green pellets at relatively low applied pressures (1.5–15.0 MPa in a press punch) in condi1 The

article is published in the original.

tions of free SHS compression, using a hydraulic press, and without any supplemental heating device, the reaction being the only source of heat release, with the aim of producing dense samples rich in MAX phases. 2. EXPERIMENTAL Pelleted green samples (12 mm in diameter, 15 mm high, relative density 60%) were prepared from 20.0 wt % Al + 71.1 wt % Ti + 8.9 wt % C powder blends (intended to obtain Ti2AlC as an end product). We have shown in previous study [5] that, upon a slight increase in the relative amount of aluminum in the green mixture, a higher purity product (i.e. a greater extent of conversion to Ti2AlC with less residual TiC) can be obtained in normal SHS conditions. We also studied another composition, namely Ti2Al1.5C with respective mass percentages of 64.6 wt % Ti, 27.3 wt % Al, and 8.1 wt % C, to determine if an increase in product purity could be attained when forging was applied after SHS reaction. The combustion temperature was measured with a W–Re thermocouple at Tc = 2110 K (Ti2AlC), indicating the formation of a liquid phase during combustion (TmAl = 933 K, TmTi = 1950 K), which is favorable for the product ductility during processing time. After ignition from the top, the samples were subjected to pressing at P = 1.5–15.0 MPa as shown in Figs. 1, 2. Pressure is applied to the oil pushing the punch of the press, 75 mm in diameter. An equivalent pressure on the sample is rather difficult to estimate, as the diameter of the sample increases during the process, from the initial 12 mm to a maximum diameter of

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