Phase evolution in the Ti-Al-B and Ti-Al-C systems during combustion synthesis: Time resolved study by synchrotron radiation diffraction analysis

Abstract

Self-propagating high-temperature synthesis (SHS), also known as combustion synthesis, is a modern technique for producing refractory compounds and materials [1]. This method is based on the use of internal chemical reaction heat rather than external (furnace) heating for material synthesis and processing. Initial mixture consists of two or more powders, which are able to react exothermically with each other. Being locally initiated, the reaction becomes self-sustaining, heat produced in the reaction front igniting the nearest part of the mixture. Ceramic-intermetallic composites, composed of ceramic grains (TiC, TiB2, etc.) and TiAl intermetallic binder, possess attractive properties for potential applications as cutting tools, aerospace materials, hard and refractory structural materials. As the enthalpies of reaction between Ti and C, B or Al are large, these materials can be produced through combustion synthesis (SHS) by means of heterogeneous reactions in the ternary mixtures of elemental powders. Formation mechanisms of the materials have been studied in the present work. Since the characteristic features of SHS are fast temperature variations and short duration of the product formation (usually from a few seconds to a few minutes), Time-Resolved Synchrotron Radiation Diffraction (TRSRD) is used for monitoring phase transformations. This method, firstly developed for investigation of SHS in the Ni–Al system [2], have been further improved in order to reach extremely short temporal resolution of 5 ms per diffraction pattern [3], or to combine it with thermal vision system [4]. Powders of Ti (∼ 40 μm average size, 99.8% purity), C (carbon black, ∼0.1 μm, 99% purity) and B (amorphous black, ∼0.1 μm, 99% purity) were dry-mixed and pressed into rectangular (20 × 10 × 5 mm) samples with a remainig porosity of roughly 40%. Two initial compositions were studied: 2Ti+Al+2B and 2Ti+Al+C. The samples were put in a specially designed chamber with an X-ray transparent window [4], under He atmosphere at normal pressure. Radiation, produced by one of L.U.R.E. (Orsay, France) synchrotron source, was monochromized and focused on the lateral surface of the sample. Radiation wavelenght was chosen equal to 2.53 A, intensity of the incident beam was ∼1010 photon per second. The sample surface exposed to synchrotron radiation (region of analysis), was a rectangle of 0.1 mm × 1.5 mm, with its long side parallel to the reaction front. Reaction was initiated locally by an electrically heated W wire. A gasless combustion front propagated along the sample with a velocity of 3 cm/s (Ti–Al–B) or 0.4 cm/s (Ti– Al–C), maximum averaged temperatures in the reaction zone were 2200 and 1960 K, respectively. Synchrotron ray diffraction patterns were recorded continuously (before, during and after reaction) by a curved linear detector which covered the range (theta) from 28 to 73 ◦, divided into 512 or 1024 channels. Sequence of 1024 SRD patterns was stored without time interval on a PC during 40.96 s. Hence, every pattern represents information about sample current phase composition during an interval of 40 ms. The stored data files were further converted into bitmap images, where larger intensity of radiation correspond to darker areas. These images give general information about phase evolution during combustion synthesis of the materials. TRSRD pattern of phase transformations in the Ti– Al–B system is shown in the Fig. 1. Lines of Ti and Al can be observed before reaction, amorphous boron does