MICROSTRUCTURAL CHARACTERIZATION OF RHENIUM POWDER OBTAINED AT VARIOUS TEMPERATURES Rodolfo L. Mannheim and Jorge L. Garin Universidad de Santiago de Chile Department of Metallurgical Engineering Casilla 10233, Santiago, Chile Keywords: Rhenium powder, crystallite size, microstructure Abstract: This work deals with the measurement of particle-size distribution of rhenium powder obtained by reduction of ammonium perrhenate under hydrogen flux. Depending upon the reduction temperature, variable amounts of hydrogen, nitrogen and oxygen remain interstitially located in the material, causing large X-ray diffraction line broadening, which accounts for particle-size domains in the microstructure of the metallic particles. The present study was carried out in the range of 400 to 1000 ºC, where the oxidized species can be reduced to rhenium powder. All calculations regarding the line-broadening effects made use of the powerfull Warren-Averbach method, while the experimental diffraction profiles were modeled by non-linear least-squares fitting, using pseudo- Voigt functions. The average particle size increased from 8.7 to 120 nm with increasing temperature from 400 to 750 °C, with no further change above this last temperature. On the other side, strain distribution within the same temperature range varied from 15 to 735x10-5. Introduction Rhenium is a very beneficial alloying addition to form solid solutions with the refractory metals because it greatly enhances the ductility and tensile strength [1-5]. The use of rhenium in these alloys improves their performance at high temperatures and helps to increase the operating temperature and thereby improves the efficiency of the materials in aerospace and energy system applications [6-12]. Since pure rhenium does not exhibit a ductile-to-brittle transition it can be mechanically formed and shaped to some extent at room temperature, giving rise to several methods of manufacture to produce various structural shapes [13]; however, the cold deformation of Re and Mo-Re alloys results in a very rapid work-hardening rate [14]. Because of its high melting point of 3,180 °C and its high chemical activity, rhenium is currently obtained by powder metallurgy techniques [15, 16]; hence the metallic powder is synthesized through a two-stage process: during the first stage a partial reduction of NH4ReO4 at 400 °C occurs stripping away the ammonia from the salt, according to the following reactions 2NH4ReO4 Re2O7 + 2NH3(g) + H2O(g) xRe2O7 + (7x – 2y)H2(g) 2RexOy + (7x – 2y)H2O(g) with 3 < y/x < 3.5. If y/x = 3, then RexOy oxide becomes ReO3, while y/x = 3.5 provides the ReO2 oxide. During the second step which usually takes place at 1,000 °C, the “intermediate” rhenium oxide is completely reduced to metallic powder according to RexOy + yH2(g) xRe + yH2O(g) Although thermodynamic data are not available for the last equation, it has been found that the reaction takes place at any temperature in the range from 400 to 1,000 °C; however, depending upon the temperature at which the reduction process (second stage) is carried out, increasing relative amounts of impurities such as hydrogen, nitrogen and oxygen remain interstitially dissolved in the hexagonal structure of the metal, as the temperature departs further down from 1,000 °C. This effect produces dislocation arrays which subdivide the original grains into small coherent domains (crystallites), causing large X-ray diffraction line broadening effects which allows for determination of the domains size distribution. Since this phenomenon is somehow related to the properties of the metallic powder, the scope of this paper was to investigate the crystallite size distribution of Re powder obtained at several temperatures in the range from 400 to 1,000 °C. Experimental procedure The starting material used in this research was high-purity ammonium perrrhenate (99.99 wt %). Measured amounts of the salt were placed into alumina boats, which were fed to a tube furnace on a predetermined heating schedule. The conditions at the first stage included a hydrogen presure of 1 at, temperature in the range of 300-400 °C, flow rate of 2 l/min and 1 hr process time. The second step was carried out at increasing temperatures from 400 to 1,000 °C, flow rate of 4 l/min and 1 hr per run. After each run the furnace was cooled to room temperature under a slow hydrogen current. The resulting samples were chemically analyzed for H, N and O by means of LECO TC-436 and RH-402 instruments. All samples were then subjected to X-ray powder diffraction experiments to determine the peak broadening effects in terms of the process temperature. Powder data were collected at room temperature on a SIEMENS D5000 diffractometer equipped with control and data reduction software DIFFRAC Plus. Results and discussion A group of seven samples was considered as representative of a larger number of experiments. All of the obtained samples showed cluster-like particles with size distribution centered around 1 to 2 mm in diameter. Bulk impurity data analysis of hydrogen, nitrogen and oxygen in the rhenium powder, as a function of the temperature during the second stage, are summarized in Table 1. The influence of using a lower temperature on the impurity content of rhenium is clearly evident. The interstitial impurities obtained at 1,000 °C, which is the current temperature used in the industrial process, are within acceptable levels as demanded by international standards. Table 1. Impurity contents in Re powder in terms of reduction temperature Temperature (°C) Hydrogen (ppm) Nitrogen (ppm) Oxygen (ppm) 400 529 1,606 18,760 500 220 1,503 5,243 600 197 268 5,388 700 161 86 4,123 800 189 59 1,500 900 60 23 902 1,000 30 13 420 Figure 1 shows the three strongest peaks of the Re diffraction patterns corresponding to the samples obtained at 400 and 1,000 °C. The effect of the process temperature on the full width at half maximum intensity (FWHM) can be observed; hence the lower reduction temperature during the second stage causes systematic peak broadening and therefore formation of smaller domain sizes. The characterization of the domain size distribution and microstrains in the different specimens can be evaluated from line profile analysis of the X-ray diffraction data using the Fourier formulism, currently known as the Warren-Averbach method [17]. The diffracted power can be expressed as a Fourier series where the cosine coefficients, An , can be split into two parts, one that contains information about the average column lengths in the sample and a second one that contains information about microstrains: (101) (002) (100) Relative intensity 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Two-theta (°) (101) Relative intensity (100) (002) 35 36 37 38 39 40 41 42 43 44 45 46 47 48 Two-theta (°) Fig. 1. Three strongest X-ray diffraction peaks of rhenium reduced at 400 °C (top) and 1,000 °C (bottom). size strain An = An xAn 1 size with An = Nn/N3 2 strain An = <cos2pmZn> 3 where N3 is the average number of cells per column in the a3 direction, Nn is the average number of th cells in the a3 direction which have an n neighbor in the same column, Zn is a relative distortion between cells that are n cells apart and m is the order of the observed reflections. By taking the logarithm of Eq. 1 this can be written as: size strain ln An = ln An + ln An 4 strain for small m and small Zn ln An can be expanded as strain 2 2 2 2 2 2 ln An = ln (1 - 2p m <Zn >) » -2p m <Zn > 5 then according to Eq. 4 size 2 2 2 ln An = ln An - 2p <Zn >m 6 Separation of crystallite size and microstrain based upon the above equations can be achieved if 2 two peaks of the same (hkl)-set are available; a plot of ln An versus m enables to find the values for the microstrain parameter Z for each n from the slope of the curves. Strains are expressed in terms of eL = DL/L = a3Zn(a3)/a3n(a3) 7 where eL is a relative strain at the average column length L in the crystallites. Since it is assumed that the strains would broaden the peak profile symmetrically, there must be positive and negative values of eL. Therefore the parameter evaluated with the Warren-Averbach method is the mean- 2 square-root microstrain, Ö<eL > . Regarding Eq. 6 the information about average column length can 2 be obtained from the vertical interceptions of the curves An = f(m ); according to the used method size the first derivative of An with respect to n is size ¥ dAn 8 = -N3 ò pi di dn n where pidi is the fraction of columns with a length between i and i +di cells. Since the integral over the distribution function pi is equal to unity for n = 0, N3 can be determined by the initial slope of size the curve An = f(n). In practice n is again replaced by a length. The instrumental broadening corrections demanded by the various cited calculations were performed by using the sample obtained at 1,000 °C as the standard for all measured reflections. The X-ray diffraction profiles were obtained by fitting a pseudo-Voigt function [18] to each measured reflection. The crystallite size and microstrain data for Re powder as calculated by means of the WIN-CRYSIZE software [19] are presented in Table 2 in terms of the process temperature, while the corresponding statistical distribution of Re particles obtained at 400 °C is disclosed in the typical four-window plot shown in Fig.