Analysis of the different stages during the mechanical alloying of Ni-40%at Al

E.ZELAYA; M.R ESQUIVEL

Kiel, Alemania

Congreso; Microscopy Conference 2011; 2011

Sociedad Alemana de Microscopía

In the nickel aluminium alloy system, Ni Al exhibits a high melting temperature (1580 ºC) [1], a metal-like electrical behaviour and a thermal conductivity relative to other intermetallics. For this reason, it is commercially attractive to develop synthesis processes of Ni Al alloys based on a solid-solid reaction produced by mechanical alloying (MA). Nickel powder (99.999% purity) and aluminium flakes (99.999% purity) were set in a ratio of 60 at.% Ni and 40 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 20 h, 50 h and 100 h , samples of 200 mg were taken for X-ray and TEM analysis (figure 1). Room temperature X-ray diffraction was measured on a Philips PW 1710/01 Instrument with Cu K radiation. Diffraction patterns were analyzed by the Rietveld method using DBWS software [2]. TEM characterization was carried out in a FEI CM200, operated at 200 keV. After 20 h of milling, X-Ray and ring diffraction pattern (figure 1a and 1b) shows the presence of intermetallic NiAl with a disorder B2 structure. Ni and Al could also be detected in the diffractograms but the Rietveld analysis agrees with the presence of a larger proportion of intermetallic respect to the initial components. Even when the indexation of almost all the rings is consistent with the indexation of a disorder B2 structure, a thin beam pointed with a grey arrow in figure 1b could be distinguish. This ring could be indexed as the 200 of a Ni structure. Beside the presence the particles with this characteristic ring diffraction pattern, a few particles like the one show in figure 2 could be observed. The EDS performed over 10 different places of this particle reveals a minimal concentration of 96%at Al. The rest of the components detected were C, O and Cu associated to the grid that supports the particles. Moreover, the microdiffraction pattern performed in the circled area in figure 2 a match with a [212] zone axis indexation of Al. When the milling time of 50 h is reached, pure Al could not be detected neither by X-ray diffraction nor ring diffraction pattern (figure 1c and 1d). Finally after 100 h of milling, the diffractograms shows the presence of the peaks that could be associated only to the B2 structure of NiAl. However, a high resolution micrograph that could be attributed to a [110]Ni zone axis was detected in a Ni rich area of the particle (figure 3b). 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. Figure 1 shows the evolution of this system as the milling time increase, according to both techniques. Previous work of reactive alloying for this composition reported the complete formation of the ordered B2 structure after 30 h of milling [3] or 5 h of milling [4]. The previous work [2,3] and this one obtained the B2 as the final intermetallic, however even after 100 h of milling the presence of Ni could be still detected in this work. The main difference between [3,4] and this study is that this milling reaction was produce with a low energy ball mill. While the equilibrium diagram of Ni-40%atAl shows the NiAl and the Ni3Al5 as the equilibrium phases for this composition, the reactive alloying seems to produce only NiAl phase as a final product. However, calculations of the Gibbs energy at room temperature for this composition revealed that B2 phase becomes most stable [4]. This means that the mechanical alloying product seems to be the single phase with the most negative Gibbs energy at room temperature.   1.        T.B. Massalski, Binary Alloy Phase Diagrams, ASM, 1986. 2.        R.A. Young, A. Sakthivel, T.S. Moss and C. O. Paiva-Santos, Journal of Applied Crystallography, 28 (1995) 366-367. 3.        S. K. Pabi et and B.S. Murty., Material Science and Engeneering, A214 (1996) 146-152. 4.        V.K. Portnoy ,A.M. Blinov, I.A. Tomilin, V.N. Kuznetsov and T. Kulik, Journal of Alloys and Compounds 336 (2002) 196–201.