The Metallurgical Synthesis of a New Generation of Deep Hardenable Titanium Alloys. The Metastable Beta Ti-Mo-V-Cr-Al System — design and Properties

Abstract

Titanium alloys are attractive for use in aircraft applications because of the weight savings arising from their high strength-density ratio. To optimize these weight savings, a number of heat treatable alpha-beta alloys have been developed to provide still higher strength-density ratios than annealed materials. However, generally low fracture toughness and deep hardening limitations have restricted the use of heat treated parts of this alloy type. In contrast, the heat treatable beta titanium alloys are inherently deep hardening because of their high alloy content which reduces the decomposition kinetics and makes the hardening reaction controllable. Furthermore, they exhibit excellent fracture toughness, far better than the alpha-beta grades. Several new heat treatable beta titanium alloys have been developed in recent years, Ti-11.5 Mo-6Zr-4.5Sn (Beta III), Ti-8Mo-8V-2Fe-3A1 and Ti-3A1-8Mo-6Cr-4Mo4Zr,1 all of which can develop high strengths in large sections. The heat treatable beta alloys have two deficiencies which limit their potential for use in heavy sections. These are: (i) low aged tensile ductility, particularly in the transverse direction, and (ii) a low elastic modulus:density ratio. The first limitation is in large degree one of tradition since fracture mechanics tests indicate that the beta alloys are tougher than the alpha-beta alloys despite their lower tensile ductility. The second limitation is a result of the large amount of beta stabilizers in these grades which both increases the density and the volume fraction of the β -phase which has a low elastic modulus. Thus the heat treatable beta grades are not as effective in stiffness-critical parts.