Impact characteristics and mechanical alloying processes by ball milling: Experimental evaluation and modelling outcomes

An experimental methodology has been developed and tested for evaluating th e impact frequency and the relative velocity of the impacting ball in the c ourse of deformation-mixing processes carried out in vibro- and shaker-mill s undergoing periodic motion. We measure the times at which the reciprocati ng milling vial reaches extreme displacements and the times at which the ba ll collides with either flat end of the vial. These two periodic signals de fine a 'time-lag' parameter from which it is possible to obtain the impact velocity, provided the vial motion is known. The method allows one to verify the operating conditions under which the pe riodicity of the ball motion is achieved and plastic collisions are approac hed. Under these circumstances, the energy released at impact can be safely approximated as the total kinetic energy of the grinding ball. In addition , the collision frequency - another important parameter of the process - is concurrently obtained. Together, their knowledge permits tracking of a mil ling run (impact by impact) and, therefore, reconstruction of the dynamics of the impact events. Experimental results agree well with the outcomes of the kinematic and dynamic models developed for both vibro- and shaker-milli ng devices. In the later part of the paper, these procedures are employed to study the mechanical alloying of nickel-titanium powder mixtures milled under widely differing impact energy regimes. The amorphous fractions are determined by X-ray methods, and the transformation rates depend on the energy intensity defined as the product of the impact energy with the impact frequency. Howe ver, irrespective of the single impact energy, the same amount of the paren t powders was observed to transform as a function of the energy dose (given by the product of the energy intensity and the milling time). A linear kin etic law characterises the dependence of the reaction semi-transformation p eriod - representing the milling time required to amorphize half of the loa d - on the energy intensity parameter. These results extend the validity of relationships previously observed in the copper-titanium system, and sugge st a general transformation path that is common to amorphization processes induced by mechanical treatment.