Cyclic phase transformations of mechanically alloyed Co75Ti25 powders
Introduction
Since 1983, the mechanical alloying (MA) [1] method has been successfully employed for the preparation of several amorphous alloy powders [2], [3], [4], [5], [6], [7], [8], [9], [10], carbides [11], [12], nitrides [13], [14] and nanocomposite materials [15], [16], using the ball milling and/or rod-milling techniques. It has been reported that further milling of amorphous Ti75Al25 [17], Al80Fe20 [18], Fe78Al13Si9 [19] and Ti50Al25Nb25 [20] alloy powders leads to amorphous–crystalline phase transformation (crystallization) and the formation of crystalline phases.
The present study has been undertaken as part of an investigation into the structural changes that take place upon high-energy ball milling of a mixture of elemental Co75Ti25 powders at room temperature. For the purpose of the present work, X-ray diffraction, scanning and transmission electron microscopes, and differential thermal analysis have been used to detect the structural changes and the thermal stability of the milled products. In addition, the change of magnetization for the ball-milled powders has also been measured to follow the structural changes at the several stages of milling.
Section snippets
Experimental procedure
Pure elemental powders (99.9%) of Co (70 μm) and Ti (50 μm) were mixed to give the nominal composition of Co75Ti25 (at. %) in a glove box under purified argon atmosphere and sealed in a stainless steel vial (SUS 316, 250 ml in volume) together with 50 stainless steel balls (SUS 316, 10 mm in diameter). The ball-to-powder weight ratio was maintained as 17:1. The MA process was performed in a high-energy planetary ball mill (Fritsch P5) at a rotation speed of 4.2 s−1. However, some samples were
XRD analyses
The XRD patterns of as-milled Co75Ti25 powders are shown in Fig. 1 after selected mechanical alloying times. In contrast to the initial mixture of polycrystalline hcp-Co and hcp-Ti [Fig. 1(a)], a broad diffuse and smooth halo appears after 11 ks of MA time [Fig. 1(b)], suggesting the formation of an amorphous phase. When this sample was annealed under argon gas in a DTA system at 1200 K, it revealed an fcc structure [Fig. 2(a)] that corresponded to the equilibrium phase of Co3Ti [21].
Discussion
The present results confirm that the cyclic amorphization reaction takes place upon high-energy ball milling elemental powders of Co75Ti25 under an argon gas atmosphere at room temperature. Fig. 12 summarizes the results obtained by milling the powders at a rotation speed of 4.2 s−1. Obviously, a homogeneous amorphous phase is formed after only a few kiloseconds of milling [Fig. 1(b)]. The amorphous phase formed at this stage is not stable against the impact and shear forces, which are
Conclusion
In conclusion, we have demonstrated cyclic amorphous-to-crystalline phase transformations during ball milling of elemental Co75Ti25 powder. After a few kiloseconds of milling, a solid-state reaction takes place at the fresh interfaces of Co/Ti layers and an amorphous phase of Co75Ti25 is formed after 11 ks. This amorphous phase transforms into an ordered fcc-Co3Ti upon heating to 880 K (crystallization). Further milling (86 ks) also leads to an amorphous–crystalline transformation
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