Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C14/00—Alloys based on titanium

Definitions

• the invention relates to ah alpha-beta titanium-base alloy having an outstanding combination of tensile strength, including shear strength and ductility.
• tensile strength implies "useable" tensile strength, i.e., at an acceptable ductility level. Since strength and ductility vary inversely with each other, as is the case for virtually all hardenable metal systems, one usually has to make trade-offs between strength and ductility in order to obtain an alloy that is useful for engineering applications.
• Standard (uniaxial) tensile properties are usually described by four properties determined in a routine tensile test: yield strength (YS), ultimate tensile strength (UTS, commonly referred to simply as “tensile strength”), % Elongation (%EI) and % Reduction in Area (%RA).
• yield strength YS
• UTS ultimate tensile strength
• %EI % Elongation
• %RA % Reduction in Area
• Double shear strength Another tensile property often cited, particularly in reference to fastener applications, is "double shear" strength, also reported in ksi. For this property, ductility is not determined, nor is a yield strength. In general, double shear strength of titanium alloys are approximately 60% of the uniaxial tensile strengths, as long as uniaxial ductility is sufficient. [005] When attempting to make comparisons of tensile properties from different alloys heat treated to a range of tensile strength/ductility combinations, it is convenient to first analyze the data by regression analysis.
• the strength / ductility relationship can usually be described by a straight-line x-y plot wherein the ductility (expressed as either %EI or %RA) is the dependent variable and the strength (usually UTS) is the independent variable.
• %EI or %RA the dependent variable
• UTS the strength
• the accepted practice is to produce smaller lab-sized heats of both the experimental alloy formulations and an existing commercial alloy formulation and compare results on a one-to-one basis.
• the key is to choose a commercial alloy with exceptional properties.
• the commercial alloy designated as "Ti-17" (Ti-5A1 - 2Sn - 2Zr - 4Cr - 4Mo) was chosen as the baseline commercial alloy against which the experimental alloys would be compared. This alloy was chosen because of the exceptional strength/ductility properties demonstrated by this alloy in bar form.
• Table 1 Tensile and Shear Strength Data from a commercial high strength titanium alloy (Ti-17) processed to bar*
• Table 1 provides tensile and double shear property data for Ti-17 0.375 inch diameter bar product produced from a nominal 10,000 lb. full-sized commercial heat. The combinations of tensile strength, shear strength and ductility exhibited in this Table are clearly exceptional for any titanium alloy. Note also that the double shear strength values average very close to the 60% of UTS value cited earlier. Summary of the Invention:
• the ultimate goal of this alloy development effort was to develop a heat treatable, alpha-beta, titanium alloy with improved ductility at high strength levels compared to heat treatable titanium alloys that are commercially available today, such as Ti-17.
• the goal could be further defined as such: to develop an alloy that exhibits at least a' 20% improvement In ductility at a given elevated strength level compared to Ti-17.
• the titanium alloy in order for titanium to offer a nominal 40% weight savings by replacing steel with titanium in a high strength aerospace fastener, the titanium alloy must exhibit a minimum double shear strength of 110 ksi. In order to do so, considering the typical scatter associated with such tests, the typical values should be at least approximately 117 ksi. With the aforementioned correlation that titanium alloys exhibit a double shear strength that is typically about 60% of the tensile strength, in order to produce a double shear strength range of at least 117 ksi (to support a 110 ksi min.), one would expect this to require a tensile strength of at least 195 ksi.
• an alpha-beta, titanium-base alloy having a combination of high strength and ductility and exhibiting at least a 20% improvement in ductility at a given strength level compared to alloy Ti-17, as defined herein.
• the alloy may exhibit a double shear strength of at least 110 ksi, as defined herein.
• the alloy may further exhibit a tensile strength of at least 195 ksi. More specifically, the tensile strength may be within the range of 195 to 215 ksi.
• the alpha-beta, titanium-base alloy in accordance with the invention comprises, in weight percent, 3.2 to 4.2 Al, 1.7 to 2.3 Sn, 2 to 2.6 Zr, 2.9 to 3.5 Cr, 2.3 to 2.9 Mo, 2 to 2:6 V; 0.25 to 0.75 Fe, 0.01 to 0.8 Si, 0.21 max.
• the alpha-beta, titanium-base alloy may comprise, in weight percent, about 3.7 Al, about 2 Sn, about 2.3 Zr, about 3.2 Cr, : about 2.6 Mo, about 2.3 V, about 0.5 Fe, about 0.06 Si, about 0.18 max. Oxygen and balance TPand incidental impurities.
• This alloy may exhibit a tensile strength of over 200 ksi and ductility in excess of 20% RA and double shear strength in excess of 110 ksi.
• Table 2 provides a summary of the formulations that were produced in the first iteration of laboratory size heats.
• the baseline Ti-17 formulation is Heat V8226. Note that the Ti-17 baseline alloy has no vanadium addition; a low (less that 0.25%) iron addition; no intentional silicon addition (0.014 represents a typical "residual" level for titanium alloys for which no silicon is added); and an oxygen level in the range of 0.08-0.13, which conforms to common industry specifications concerning Ti-17.
• V8226 %RA 101.0 - 0.39.66 UTS 0.62 15.7 23.7
• V8227 %RA 49.1 - 0.1513 UTS 0.20 16.5 19.6
• V8228 %RA 138.0 - 0.5315 UTS 0.66 23.7 34.6
• V8229 %RA 181.7 - 0.77089 UTS 0.85 13.5 29.8
• V8230 %RA 125.1 - 0.4915 UTS 0.48 19.4 28.6
• V8231 %RA 134.5 - 0.5325 UTS 0.71 20.0 30.7
• Table 3 summarizes the uniaxial tensile results obtained from the first iteration of experimental alloy formulations noted in Table 2 that were processed to bar and heat treated.
• Table 4 provides a regression analysis of the Table 3 data.
• the first item to note is a comparison of the tensile properties of the Ti- 17 material cited in Table 3 (laboratory size Ti-17 heat) vs. those cited in Table 1 (production-sized Ti-17 heat). Note that the calculated %EI values of the lab-sized heat are 78% and 83% of those from the full sized heats at 195 ksi and 215 ksi respectively and the calculated %RA values are 67% and 62% at the same respective strengths. This data clearly confirms the significant drop-off of laboratory size heats vs. full-sized heats and reinforces the need to compare results from comparable sized heats. .
• V8251 and V8252 examine lower aluminum levels (about 0.5% less than V8547), in one base at the same silicon level (V8251 ) and another (V8252) at the higher silicon level.
• V8247 %RA 130.2 - 0.5047 UTS 0.87 21.1 31.3
• V8248 %RA m.2 - 0.4084 UTS • 0.62 23.4 31.5
• V8250 %RA 53.5 - 0.0993 UTS ' 0.21 32.1 34.1
• V8251 %RA 13639 - 0..5726 UTS 0.84 13.8 25.2

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