Evolution in the synthesis of Ni-Al by mechanical milling

E. ZELAYA; M.R. ESQUIVEL; D. SCHRYVERS

Rio de Janeiro-Brazil

Conferencia; International Microscopy Congress IMC-17 2010; 2010

Brazilian Society for Microscopy and Microanalysis

The synthesis of elements with very different melting temperatures like Ni and Al could be obtained at low cost by mechanical alloying (MA). Moreover, the processing of metals by MA leads to the formation of equilibrium and non-equilibrium phases that should be carefully studied [1]. Nickel powder (99.999% purity) and aluminium flakes (99.999% purity) were set in a ratio of 65 at.% Ni and 35 at.% Al. The milling was carried out under Ar atmosphere at room temperature using steel balls with a mass relation of the elemental blends of 22.33:1. After certain intervals of time, samples of 200 mg were taken for X-ray and TEM analysis (table 1). Room temperature X-ray diffraction was measured on a Philips PW 1710/01 Instrument with Cu K¦Á radiation. Diffraction patterns were analyzed by the Rietveld method using DBWS software. TEM characterization was performed using a Tecnai F20 G2, field emission microscope and a FEI CM20, operated at 200 keV. After 10 hs of milling only the presence of pure Ni and Al metals is still observed by both X-ray diffractograms and TEM ring diffraction patterns (RDP). After 20 hs of milling, nanocrystalline Ni-Al intermetallics have been synthesized according indexation of TEM RDP (table 1 and fig. 1). However, the X-Ray diffractograms still only show the presence of pure Ni and Al. When a milling time of 50 hs is reached, the presence of Al3Ni2 and Al3Ni5 and the absence of pure Ni and Al are detected in the X-ray diffractograms while the presence of Ni as a pure element was still observed in a HAADF-STEM image. This inhomogeneity in the Ni-Al particles is also confirmed with an EDX line scan (fig. 2). Finally after 100 hs both X-ray and TEM show the complete synthesis of Ni-Al intermetallics. The disagreement between both methods can be related to the difference in probe size for both techniques. X-ray diffraction could not detect crystalline particles smaller than 200 nm. However, X-Ray diffraction is an accurate method to follow the possible intermetallics that are present in the samples once they are well consolidated. Another effect of MA in these samples is the broadening of the intensity distribution for each RDP ring as the milling time increases (fig. 1). Amorphization of the material with the increment of milling time could be a possible explanation for this broadening. However, the present system has a very large negative heat of mixing, so it is not expected to reach amorphization using a low energy mechanical milling. Moreover, a HRTEM image of sample 9 shows many regions with crystalline structures (fig. 3). The broadening of the ring diffraction pattern is thus attributed to strained lattices on the edge of the particles or even the presence of new phases favoured by the large milling time. Both possibilities are in agreement with the Moir¨¦ fringes observed in fig. 3 (a). References [1] S. K. Pabi et al., Materials Science and Engineering, A214 (1996) 146-152.