The influence of mill energy and temperature on the structure of the TiNi intermetallic after mechanical attrition
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Mechanical attrition of intermetallic compound TiNi powder was carried out in two different ball mills and as a function of milling temperature. The microstructural changes with milling time were followed by x-ray diffraction, TEM, and DSC. The more energetic Spex shaker mill provided a higher degree of lattice strain and rapidly refined the grain size to the nanometer size regime. Amorphization was observed in the Spex mill with a linear increase in the milling time for amorphization with increasing milling temperature. No amorphization was observed in the less energetic vibratory mill, and the grain size saturated to a constant value of 15 nm after 3=60 h of milling. A critical grain size for the amorphization of 4 - 5 nm was estimated from the temperature dependent studies in the Spex mill. The grain boundary energy (706 mJ/m 2 ), estimated from the vibratory mill experiments, and the above critical grain sizes (5 nm) for amorphization were used to calculate the enthalpy supplied by the nanocrystalline grain boundaries. The calculated value of 4.1 kJ/mol was comparable to the measured enthalpy of crystallization of 3.2 kJ/mol. It is concluded that the nanocrystalline grain boundary energy is responsible for driving the crystalline-to-amorphous phase transformation induced by mechanical attrition in TiNi.
I. INTRODUCTION There has been considerable interest in recent years in the synthesis of amorphous structures by high-energy ball milling of either elemental powder mixtures1 or powders of intermetallics.2 In the case of amorphization of intermetallics, defects introduced by the deformation during milling must be responsible for raising the free energy of the crystalline compound to above that of the amorphous phase. Two important sources for this stored energy of deformation are antisite disorder and grain boundary energy of nanoscale grains.3 Dislocations normally don't provide sufficient stored energy to promote amorphization, since typical values of stored energy by conventional deformation processes provide, at most, 1-2 kJ/mol compared with crystallization enthalpies that are typically 5-20 kJ/mol. 4 The one instance in the literature of amorphization induced by dislocations was for TiNi cold-rolled 60% .5 Koike et al.5 observed partial amorphization of the TiNi intermetallic which they attribute to the large dislocation density (10 1 3 -10 1 4 /cm 2 ), as determined by HREM. They estimated the elastic energy associated with a dislocation density of 10 14 /cm 2 to be 2.2 kJ/mol, which is of the
a) Permanent
address: Department of Chemistry, The National Defense Academy, 1-10-20 Hashirimizu, Yosuka, 239 Japan. J. Mater. Res., Vol. 8, No. 6, Jun 1993 http://journals.cambridge.org
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same order as the crystallization enthalpy reported for TiNi6 of 3.03-3.55 kJ/mol. However, Hellstern et al.7 did not observe amorphization in TiNi powder subjected to high-energy ball milling. Instead, after 24 h of milling in a Spex shaker mill, they observed a nanocrystalline grain structure with a 5
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