THE BEHAVIOR OF OXIDATION AND PRECIPITATI ON HARDENING OF VERY GOLD RICH GOLD-TITAN ALLOYS

The oxidation behavior of gold rich Au-Ti alloys with contents of tita nium between 0.5 and 1.3 weight-% was investigated in synthetic and na tural air in dependence on the oxidation temperature and time. A close d homogeneous layer of titanium oxid was not formed. The nucleation of TiO2 (Rutil) starts at prefered sites (grain boundaries, structure de fects) below the surface of the alloys. The precipitations grow withou t formation a gapless cover of the grain boundaries. Between the preci pitations the concentrations of titanium and oxygen are zero. The tita nium is transported out of the interior of the specimen by volume diff usion. Based on an experimentally observed linear relationship between the oxygen uptake and the oxidation time the speed constants could be determined for 750 degrees C, 825 degrees C and 900 degrees C. Betwee n the natural logarithm of the speed constant and the reciprocal tempe rature a linear connection exists. For this reason it is possible to c alculate the kinetics by a well-defined mathematical expression for th e speed constant of the titanium oxidation in dependence on the titani um content and the oxidation temperature. The connection between the s peed constant and the optical outfit of the surface is illustrated. Fu rthermore the precipitation hardening of the alloys at temperatures be tween 100 degrees C and 700 degrees C is described. A noticable increa se of the hardness is already produced at relatively low temperatures. Whith increasing temperatures the hardness maximum is shifted to shor ter times. With increasing titanium content the hardness maximum incre ases and it is obtained after shorter times. Cold working before the a geing, raises the hardness maximum and accelerates the precipitation. The precipitations are formed to little plates with high coherency to the matrix preferentialy at the grain boundaries. X-ray analysis sensi tively responds to the kinetics of precipitation and gives reliable in formation about the concentration variations of the matrix. The result s concerning the oxidation behaviour and the precipitation hardening m ake it possible to find the conditions of an optimized heat treatment with high hardness but without negative changes of the surface.