THERMOCHEMICAL PROCESSING WITH HYDROGEN OF SUPER-ALPHA-2 ALLOY

Thermochemical processing (TCP) with hydrogen, including hydrogenation , intermediate heat treatment and dehydrogenation, of super-alpha2, a type of Ti3Al-based titanium aluminide alloy, was carried out in order to examine the microstructural variations in the alloy. It is shown t hat a number of microstructural changes, such as the amount, morpholog y and distribution of different phases in the alloy, can be brought ab out by TCP. Acicular or Widmanstatten alpha2 was found to form in the retained B2 matrix under TCP with an intermediate heat treatment of fu rnace cooling from high temperature, instead of mainly equiaxed alpha2 under the normal treatment. The precipitation in the microstructures for alloys undergoing TCP with the intermediate beta-solution heat tre atment followed by ageing while dehydrogenating was found to be more r egular in morphology and homogeneous in distribution. Both the amount of primary alpha2 and the size of the primary beta grains in the micro structure could be controlled by suitable TCP. Some of the reasons lea ding to the microstructural variations caused by TCP are discussed.