Abstract: The invention relates to an alloy suitable for making single-crystal castings and to a casting made thereof. The alloy comprises, by weight percent, Chromium 10-16%, Cobalt 4-16%, Molybdenum 2-4%, Tungsten 0-2%, Tantalum 2-8%, the sum of ##EQU1## lying between 5 and 7%, Aluminium 4-6%, Titanium 1-4% the sum of ##EQU2## lying between 5 and 7%. Carbon 0.015-0.05%, Boron 0-0.01%, Zirconium 0-0.01%, the balance being nickel plus incidental impurities.

Abstract: A relatively low-density Nickel based superalloy particularly suitable for use in the form of a single crystal casting comprised by weight percent:Chromium: 7-13%Aluminium: 5-7%Titanium: 2-5%Cobalt: 4-16%Molybdenum and/or Ruthenium: 1-4%Vanadium: 0-2%Carbon: 0-0.05%Balance Nickel plus incidental impurities, the density of the alloy being less than 7.9 kg/dm.sup.3.

Abstract: An alloy composition intended for use in the form of a single crystal casting comprises a modified nickel-based superalloy. The alloy consists essentially of, by weight percent,7-13% Chromium,0-15% Cobalt,1.25-3% Titanium+1/2 Niobium,4.5-6.7% Aluminium,3-12% Tungsten,0-3.5% Molybdenum,0-5% Tantalum,0-2% Hafnium,0-1.5% Vanadium,0.015-0.05% Carbon,0-0.01% Boron,0-0.05% Zirconium,Balance essentially Nickel plus incidental impurities.The invention also contemplates a heat treatment for the alloy comprising initially heating to 1300.degree.-1320.degree. C. followed by cooling to room temperature and ageing the alloy at a temperature of about 870.degree. C. for 16 hours.

Abstract: A selector for use in allowing only one crystal from a directionally-solidified initiating mould cavity to grow into a main casting cavity is described. The selector comprises a ceramic member with a curvilinear groove formed in its surface. The selector is positioned in a two part ceramic shell mould which cooperates with the groove in the selector to form a curvilinear passage which selects the single crystal required.