Titanium nitride dispersion strengthened alloys
BACKGROUND OF THE INVENTION 
     This invention relates to titanium nitride dispersion strengthened alloys and their production. 
     It is known that the creep strength of certain titanium-containing alloys can be increased by the presence of titanium nitride dispersions and that such dispersions can be introduced by treatment of the alloys in powder form. It is also known that when the alloys also contain chromium and nitriding is attempted by heating in ammonia relatively small penetration of the titanium nitride front normally occurs. 
     FEATURES AND ASPECTS OF THE INVENTION 
     According to the present invention particles of titanium-containing alloy powder which also contains chromium are heated in the presence of ammonia to form a layer of chromium nitride(s) on the particles and then in an inert atmosphere at a higher temperature to dissociate the chromium nitride(s) and convert substantially all titanium present to titanium nitride. 
     It is considered that the chromium nitride(s) (CrN/CrN 2 ) forming the layer on the particles after treatment with ammonia can provide a high activity source of nitrogen in an envelope around each particle for reaction with the titanium present and that this is an improvement on a previously proposed route which depended upon transport of nitrogen from nitrided (CrN/CrN 2 ) particles to un-nitrided particles in a blended mixture of the two. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention is considered particularly applicable to titanium containing stainless steel and nickel based alloy powders which are subsequently to be formed into fuel element containers or other nuclear reactor components and have a particle size between 30 and 120 microns. An example of a stainless steel is a 20Cr/25Ni alloy containing up to 2  wt  /o Ti. An example of a nickel based alloy is that known as Nimonic PE16. Such components may be formed by conventional powder metallurgy techniques, for example, powder extrusion. In particular they may be formed into tubing. A suitable temperature for the treatment with ammonia is about 700° C. and for the subsequent homogenisation between 1000° C. and 1150° C. By homogenisation is meant the high temperature transport of nitrogen from the chromium nitride layer. It includes dissociation, diffusion and chemical reaction processes and can be achieved by heating the powder rapidly to the dissociation temperature, for example, by pouring the powder into a hot furnace or by increasing the temperature of the nitriding furnace. The atmosphere during the homogenising stage may be a hydrogen/nitrogen mixture to maintain a suitable nitrogen activity. Preferably the duration of treatment is extended beyond completion of the formation of the titanium nitride by changing the atmosphere to hydrogen to remove excess nitrogen which could form embrittling phases in service if allowed to remain in solid solution.