No abstract

Ti-50Ni and Ti-40Ni-10Cu (at.%) shape memory alloy powders have been fabricated by the ball milling method. Their alloying behaviour and transformation behaviour were investigated by means of optical microscopy, electron microscopy, X-ray diffractometry and differential scanning calorimetry. As-milled Ti-Ni powders fabricated with a milling time of less than 20 hrs were a mixture of pure elemental Ti and Ni, and therefore we were unable to obtain alloy powders because the combustion reaction between Ti and Ni occurred during heat treatment. Since those fabricated with a milling time of more than 20 hrs were a mixture of Ti-rich and Ni-rich Ti-Ni solid solution, it was possible to obtain alloy powders without a combustion reaction during heat treatment. Clear exothermic and endothermic peaks appeared in the cooling and heating curves, respectively, in DSC curves of 20 hr and 30 hr milled Ti-Ni powders. On the other hand, in DSC curves of 1 hr, 10 hrs, 50 hrs and 100 hrs, the thermal peaks were almost discernible. The optimum ball milling time for fabricating Ti-Ni alloy powders was 30 hrs. Ti-40Ni-10Cu alloy powders were fabricated successfully by the optimum ball milling conditions deduced from Ti-50Ni alloy powders.

Changes in the transformation behavior and transformation temperatures of a 40Ti-50Ni-10Zr (at.%) alloy during thermal cycling have been investigated by means of differential scanning calorimetry, X-ray diffraction and thermal cycling tests under a constant load. A 40Ti-50Ni-10Zr alloy showed two stage transformation behavior, i.e., from the B2 to the B19 and then from the B19 to the B19', although they are not clearly separated. With the increasing of the number of thermal cycling, the decreasing rate of the transformation temperature corresponding to the B2-B19 was higher than that corresponding to the B19-B19', and consequently the former overlapped with the latter. Transformation temperatures of 40Ti-50Ni-10Zr greatly decreased (95K) during thermal cycling without applied stress, since only the thermal cycling effect which suppresses martensitic transformation had an influence on transformation temperatures. Decreases in transformation temperatures during thermal cycling with applied stress was smaller than that during thermal cycling without applied stress because both the structural refinement effect which assists martensitic transformation and the thermal cycling effect had an influence on transformation temperatures.

The crystal structure of the (TiHf)Ni phase formed in a 20Ti-50Ni-30Hf (at.%) alloy was investigated by means of transmission electron microscopy and X-ray diffraction, and was found to be rhombohedral whose lattice parameters were a=0.4887 nm and α=83.3 o . The atomic positions of the Ti(Hf) atoms in the rhombohedral structure of the precipitates were assumed to be Ti=0 0 0, x 1/2 0, x 0 1/2, x 1/2 1/2, and those of the Ni atoms were known to be Ni=1/2 1/2 1/2, 0 0 1/2, 0 1/2 0, 1/2 0 0.

Microstructures and rolling contact fatigue properties of STB 2 high carbon chromium bearing steel were investigated by means of electron microscopy, hardness tests and rolling contact fatigue tests. In order to examine the influence of the heat treatment process on microstructures and rolling contact fatigue, two kinds of heat treatment processes, quenching/tempering (QT) and austenitic nitrocarburizing were performed on STB 2 steel. Rolling contact fatigue life of the nitrocarbufized steel was 3.7 times longer than that of the QT treated steel under a clean lubrication condition and was 1.5 times longer under a contaminated lubrication condition. The amount of retained austenite in the nitrocarburized steel was found to be larger than that in the QT treated steel. As the tempering temperature was increased from 150~ to 300~ the decrease in hardness for the nitrocarburized steel was smaller than that of the QT treated steel. This means that the nitrocarburized steel is suitable as bearin~ material for high temperature applications. Very free Fe,(Fe(CN)~)~ carbonitride of less than 100nm in diameter was found in the nitrocarburized steel. An improvement in rolling contact fatigue life in the nitrocarburized steel was attributed to the combination of the formation of very fine Fe4(Fe(CN)6)3 and retained austenite.