The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Titanium alloys have been used for aerospace and non-aerospace applications for years due to their high strength, light weight and excellent corrosion resistance. In aerospace applications, the achievement of high specific strength (strength/density) is critically important, and thus weight reduction is a primary consideration in component design and material selection. The application of titanium alloys in jet engine applications ranges from compressor discs and blades, fan discs and blades and casings. Common requirements in these applications include excellent specific strength, superior fatigue properties and elevated temperature capabilities. In addition to properties, producibility in melting and mill processing and consistent properties throughout parts are also important.
Titanium alloys may be classified according to their phase structure as alpha (α) alloys, alpha-beta (α/β) alloys or beta (β) alloys. The alpha phase is a close-packed hexagonal phase and the beta phase is a body-centered cubic phase. In pure titanium, the phase transformation from the alpha phase to the beta phase occurs at 882° C.; however, alloying additions to titanium can alter the transformation temperature and generate a two-phase field in which both alpha and beta phases are present. Alloying elements that raise the transformation temperature and have extensive solubility in the alpha phase are referred to as alpha stabilizers, and alloying elements that depress the transformation temperature, readily dissolve in and strengthen the beta phase and exhibit low alpha phase solubility are known as beta stabilizers.
Alpha alloys contain neutral alloying elements (such as tin) and/or alpha stabilizers (such as aluminum and/or oxygen). Alpha-beta alloys typically include a combination of alpha and beta stabilizers (such as aluminum and vanadium in Ti-6Al-4V) and can be heat-treated to increase their strength to various degrees. Metastable beta alloys contain sufficient beta stabilizers (such as molybdenum and/or vanadium) to completely retain the beta phase upon quenching, and can be solution treated and aged to achieve significant increases in strength in thick sections.
Alpha-beta titanium alloys are often the alloys of choice for aerospace applications due to their excellent combination of strength, ductility and fatigue properties. Ti-6Al-4V, also known as Ti-64, is an alpha-beta titanium alloy and is also the most commonly used titanium alloy for airframe and jet engine applications. Higher strength alloys such as Ti-550 (Ti-4Al-2Sn-4Mo-0.5Si), Ti-6246 (Ti-6Al-2Sn-4Zr-6Mo) and Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr) have also been developed and are used when higher strength than achievable with Ti-64 is required.
Table 1 summarizes the high strength titanium alloys currently used in aerospace applications, including jet engines and airframes, at low to intermediate temperatures, where the densities of the alloys are compared. Ti-64 is used as the baseline material due to its wide usage for aerospace components. As can be seen from the data in Table 1, most of the high strength alloys, including alpha-beta and beta alloys, attain increased strength due to the incorporation of larger concentrations of Mo, Zr and/or Sn, which in turn leads to cost and weight increases in comparison with Ti-64. The high strength commercial alloys Ti-550 (Ti-4Al-2Sn-4Mo-0.5Si), Ti-6246 (Ti-6Al-2Sn-4Zr-6Mo) and Ti-17 (Ti-5Al-2Sn-2Zr-4Mo-4Cr), which are used for jet engine discs, contain heavy alloying elements such as Mo, Sn and Zr, except for Ti-550 that does not contain Zr. A typical density of high strength commercial alloys is 4-5% higher than the baseline Ti-64 alloy. A weight increase tends to have a more negative impact on rotating components than on static components.
TABLE 1Characteristics of various titanium alloysDensityDensityCategoryAlloyCompositiong/cm3lb/in3increase %Remarksα/β AlloyTi-64Ti—6Al—4V4.431.600.0%Comparison-BaselineTi-575Ti—5.3Al—7.5V—0.5Si4.501.631.6%Inventive ExampleTi-6246Ti—6Al—2Sn—4Zr—6Mo4.651.685.0%ComparisonTi-17Ti—5Al—2Sn—2Zr—4Mo—4Cr4.651.685.0%ComparisonTi-550Ti—4Al—2Sn—4Mo—0.5Si4.601.663.8%ComparisonTi-662Ti—6Al—6V—2Sn4.541.642.5%ComparisonTi-62222Ti—6Al—2Sn—2Zr—2Mo—2Cr—0.2Si4.651.685.0%Comparisonβ AlloyBeta CTi—3Al—8V—6Cr—4Mo—4Zr4.821.748.8%ComparisonTi-10-23Ti—10V—2Fe—3Al4.651.685.0%ComparisonTi-18Ti—5V—5Mo—5.5Al—2.3Cr—0.8Fe4.651.685.0%Comparison