No abstract

In this paper, details are given for the structural evolution of (Ti amorphous alloys, part of wider program of alloy development by equiatomic substitution. All three alloys initially crystallize by forming a nanoscale icosahedral phase. However, at higher temperatures, their decomposition sequences differ significantly. The nanoscale icosahedral phase in the (Ti 33 Zr 33 Hf 33 ) 70 (Ni 50 Cu 50 ) 20 Al 10 alloy decomposes into a mixture of Zr 2 Cu-type and icosahedral phases. This icosahedral phase still exists after heating up to 970 K, indicating a high thermal stability of this phase. The nanoscale icosahedral phase in the (Ti 33 Zr 33 Hf 33 ) 70 (Ni 33 Cu 33 Ag 33 ) 20 Al 10 alloy also transforms into a mixture of Zr 2 Cu-type and icosahedral phase during the second exothermic reaction but then transforms into a mixture of Zr 2 Cu-type and Ti 2 Ni-type phases. The nanoscale icosahedral phase in the (Ti 25 Zr 25 Hf 25 Nb 25 ) 70 (Ni 50 Cu 50 ) 20 Al 10 alloy decomposes into a mixture of Ti 2 Ni-type and MgZn 2 -type phases during the second exothermic reaction. It is concluded that the formation of the Zr 2 Cu-type phase retards the decomposition of the nanoscale icosahedral phase, which increases the thermal stability. In contrast, formation of Ti 2 Ni-type and MgZn 2 -type phases accelerates the decomposition of the nanoscale icosahedral phase, which decreases its thermal stability.

3-dimensional atom probe (3DAP) analysis enables 3D reconstruction of the chemical variations inside materials with near atomic resolution. This technique has enabled important insights into the phase chemistry and microstructure of a range of complex engineering materials. Although individual planes can be resolved when analysing in a direction close to a low-index pole, the spatial resolution of this technique has not been sufficient for reconstruction of all the original lattice positions. The spatial resolution is thought to be fundamentally limited by small lateral displacements of the atom on the surface of the specimen during the field evaporation process due primarily to interactions with the neighbouring surface atoms. It has therefore been considered that it will be impossible to correctly reconstruct the crystal lattice from 3DAP data. Recently, we have found that a 3DAP reconstruction of a region near a grain boundary in pure tungsten (an element renown for its good imaging and evaporation behaviour) revealed several different sets of crystallographic planes visible at angles up to 45° from the direction of analysis.