Characterization of the governing stages during the low energy mechanical alloying of a mixture of intermetallics

M. RODRIGUEZ; M.R. ESQUIVEL; G. MEYER

Ciudad de Guarujá - Brasil

Congreso; VII Encontro SBPMat - 7th Brazilian MRS Meeting; 2008

Sociedade Brasileira de Pesquisa de Materiais

The governing stages during the mechanical alloying of a mixture of (LaCeNdPr)Ni5 and La(Ni,Al)5 were identified. The effects of fracture and cold welding on microstructure parameters were discussed. Four stages were identified by combined scanning electron microscopy, differential scanning calorimetry and x-ray diffraction. Initial stage is governed by the fracture of the larger particles of La(NiAl)5. Progressive increment of the presence of cold welding is observed at intermediate stage. Final stage is reached at integrated milling times longer than 50 h under shared control of both processes. Completion stage occurs after 100 h of integrated milling time. Depending on the starting constituents, the synthesis of AB5´s can be achieved by two alternative paths: from mixtures of either metals/alloys (A + 5B) or intermetallics (AB5 + AB5´). Independently of the path selected, the formation process occurs through four successive stages [1,2]. The identification of these steps is more complicated in the second path because of the chemical and structural similarities of the constituents [3]. In order to identify the governing mechanisms of the synthesis, mechanical alloying was done in a low energy mill. In these devices, the process is slower allowing the discrimination of the stages. Phase identification and microstructure parameters were analyzed by both X-ray diffraction (XRD) and scanning electron microscopy (EDS). The morphology and particles size distribution were analyzed by scanning electron microscopy (SEM). Thermal behavior of samples was analyzed by differential scanning calorimetry (DSC). In this work, the mechanical alloying process of a (LaCeNdPr)Ni5 - La(Ni,Al)5 mixture was studied. The initial stage occurs from 0 to 10 h of integrated milling time (ti). The step was governed by fracture which affected selectively the larger particles La(Ni,Al)5. The fracture and cold welding of both intermetallics governs the intermediate stage. At this stage, La(Ni,Al)5 serves as  a matrix for smaller (LaCeNdPr)Ni5 particles. This alloying step occurs between 10 h and 50 h.  At this ti value, the final stage is reached. The combined fracture and cold welding led to a spheroid like shape for particles and to a final homogeneous chemical composition.  Completion stage was obtained at ti values of 100 h. Refinement of the particles is achieved as observed.After identification of governing stages, an alternative method of synthesis was proposed. The intermetallics mixture is mechanically alloyed for 50 h until final stage is reached followed by low temperature annealing at 600 °C for 24 h.