SOLID-STATE NITRIZATION REACTION OF AMORPHOUS TANTALUM ALUMINUM NITRIDE ALLOY POWDERS - THE ROLE OF AMORPHIZATION BY REACTIVE BALL-MILLING

Amorphous tantalum aluminium nitride alloy powders have been synthesized by high energy ball milling under a purified nitrogen gas (N2) flow at room temperature. The modes of amorphization and crystallization of the TaAlN alloy powders have been investigated by means of X-ray diffraction, optical metallography, scanning electron microscopy, transmission electron microscopy and differential thermal analysis. The mechanical alloying process via the reactive ball-milling technique is classified into three stages of milling. During the first stage the elemental powders of Ta and Al particles grow in size to form layered composite particles of larger diameter as a result of cold welding. In the second stage the elemental Al powders completely diffuse into the Ta matrix to form a b.c.c. TaAl solid solution. This solid solution expands with increasing the milling time to give a saturation value of the lattice parameter a, of 0.3306 nm after 55 ks of milling. At this stage the powders are disintegrated into several particles which have new or fresh surfaces that are able to absorb nitrogen gas, so that both the b.c.c. TaAl solid solution and the unprocessed b.c.c. Ta powder particles react with nitrogen to form a b.c.c. TaAl solid solution nitride and h.c.p. TaN respectively. Further milling creates mechanical deformations such as point defects and dislocations which lead to an increase in the free energy of the nitride phase of the b.c.c. TaAl solid solution to the less stable phase of amorphous TaAlN. In addition, solid state amorphization reactions between h.c.p. TaN and the b.c.c. TaAl solid solution occur. During the final stage of milling (72 ks) a homogeneous phase of amorphous TaAlN is formed containing 18 at.% N2. The crystallization characteristics represented by the crystallization temperature T(x) and the enthalpy change of crystallization, DELTAH(x), are 1115 K and -95 kJ mol-1 respectively.