Aluminum Alloys and Manufacture Methods
A composition comprises, in weight percent: Al as a largest constituent; 3.0-6.0 Cr; 1.5-4.0 Mn; 0.1-3.5 Co; and 0.3-2.0 Zr.
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This is a continuation of U.S. patent application Ser. No. 14/902,709, filed Jan. 4, 2016, which is a 371 U.S. national stage application of PCT/US2014/045982, filed Jul. 9, 2014, which claims benefit is claimed of U.S. Patent Application No. 61/844,762, filed Jul. 10, 2013, and all entitled “Aluminum Alloys and Manufacture Methods”, the disclosures of which are incorporated by reference herein in their entireties as if set forth at length.
BACKGROUNDThe disclosure relates to aluminum alloys. More particularly, the disclosure relates to aluminum alloys containing an icosahedral phase (I-phase) for use in aerospace applications.
Since the discovery of the existence of an icosahedral phase (I-phase) in 1984, a number of documents discuss the composition and mechanical properties of aluminum alloys containing such a phase. Examples include U.S. Pat. No. 4,772,370 and US Patent Application Publication 2010/0003536A1. More recently, the idea of using I-phase materials for coatings has also surfaced. While many references assert that aluminum alloys with the I-phase have high ductility, these measurements are usually based on bending and such a mode of stress does not, in general, coincide with the ability of a material to deform in pure tension. Tests in pure tension have shown that I-phase materials behave poorly, often exhibiting tensile failures near 1% elongation. This behavior has often been attributed to the high volume fractions (e.g., as high as 80%, see U.S. Pat. No. 6,334,911) of I-phase produced in alloys explored to date. However, a variety of other factors can be involved; that is, hydrogen content, phases that have a low volume fraction, but embrittle aluminum alloys, or the size and distribution of I-phase particles, even at low volume fractions.
It has been documented that transition metal elements such as Co can be added to ternary aluminum I-phase alloys, such as Al—Cr—Co or Al—Mn—Co, and this results in a finer size and distribution of I-phase particles. See, K. Kita, K. Saitoh, A. Inoue, T. Masumoto, “Mechanical Properties of Al Based Alloys Containing Quasi-crystalline Phase as a Main Component”, Materials Science and Engineering, A226-228, 1997, pp. 1004-1007 (hereafter “Kita et al.”). Kita et al. assert that this results in greater strength, although it is not clear that some strength is not derived from the compound Al9CO2.
SUMMARYOne aspect of the disclosure involves a composition comprising, in weight percent: Al as a largest constituent; 3.0-6.0 Cr; 1.5-4.0 Mn; 0.1-3.5 Co; and 0.3-2.0 Zr.
In one or more embodiments of any of the foregoing embodiments, in atomic percent content Co divided by the sum (Cr+Mn) less than or equal to 0.07.
In one or more embodiments of any of the foregoing embodiments, in atomic percent content Co divided by the sum (Cr+Mn) less than or equal to 0.065.
In one or more embodiments of any of the foregoing embodiments, the composition in weight percent comprises: 3.0-6.0 Cr; 1.5-4.0 Mn; 0.1-1.0 Co; and 0.3-1.5 Zr.
In one or more embodiments of any of the foregoing embodiments, the composition in weight percent comprises: 3.7-5.2 Cr; 2.1-3.0 Mn; 0.4-0.6 Co; and 0.7-1.1 Zr.
In one or more embodiments of any of the foregoing embodiments, the composition in atomic percent comprises: 1.9-2.9 Cr; 1.0-1.6 Mn; 0.2-0.3 Co; and 0.2-0.4 Zr.
In one or more embodiments of any of the foregoing embodiments, in weight percent the total of all additional contents is not more than 5.0.
In one or more embodiments of any of the foregoing embodiments, in weight percent no additional individual elemental content exceeds 1.0.
In one or more embodiments of any of the foregoing embodiments, the composition in weight percent, having each of Fe and Si content, if any, does not exceed 0.02.
In one or more embodiments of any of the foregoing embodiments, by weight H content, if any, does not exceed 1 ppm.
In one or more embodiments of any of the foregoing embodiments, the composition has an icosahedral phase (I-phase).
In one or more embodiments of any of the foregoing embodiments, a volume fraction of said I-phase is 15% to 30%.
In one or more embodiments of any of the foregoing embodiments, a characteristic size of said I-phase is less than 200 nm.
In one or more embodiments of any of the foregoing embodiments, Al9Co2 content, if any, is less than 5% by volume.
Another aspect of the disclosure involves a method for manufacturing the composition. The method comprises atomizing a master alloy, pressing the atomized alloy to form a billet, extruding the billet to form an extrusion, and forging the extrusion.
In one or more embodiments of any of the foregoing embodiments, the composition comprises, in atomic percent: Al as a largest constituent; 1.9-2.9 Cr; 1.0-1.6 Mn; 0.2-0.3 Co; and 0.2-0.4 Zr.
In one or more embodiments of any of the foregoing embodiments, the composition has an icosahedral phase (I-phase).
In one or more embodiments of any of the foregoing embodiments, a volume fraction of said I-phase is 15% to 30%.
In one or more embodiments of any of the foregoing embodiments, the composition is a powder metallurgical alloy.
In one or more embodiments of any of the foregoing embodiments, the composition is effective to form a passivating layer when exposed to a salt-fog environment.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims.
Like reference numbers and designations in the various drawings indicate like elements.
DETAILED DESCRIPTIONWe have determined that above a certain Co level, the Co exceeds the solubility limit of the I-phase particles, forms Al9Co2, and embrittles the material. We have also found that the presence of the Al9Co2 phase precludes effective degassing because it forms at temperatures where degassing normally occurs. To preclude the formation of this phase, one is forced to degas at lower temperatures and this results in a high hydrogen content. However, when hydrogen is greater than 1 ppm in nanostructural materials, the ductility suffers. Finally, when Cr and Mn exceeds values that are above those needed to provide a volume fraction of I-phase greater than 25%, ductility also suffers.
First, by reducing Co levels to those needed to provide for a small size and a more even size distribution of the I-phase, without forming Al9Co2, one may obtain alloys that have better ductility than the prior art.
Second, by eliminating or substantially reducing the Co-induced formation of Al9Co2, one may obtain alloys that can be degassed at higher temperatures, thereby resulting in lower hydrogen concentrations, thus leading to improved ductility. We have found that keeping hydrogen below 1 ppm provides for excellent ductility, particularly in nanostructural materials like the I-phase alloys.
Third, by lowering Cr and Mn levels, one may eliminate the formation of primary I-phase particles that come out in powder particles while they are still liquid. Such particles grow rapidly in the liquid (typically to 500 nm and larger) and do not contribute to strength, but only serve to lower ductility. Hence, with lower Cr and Mn levels, only small I-phase particles form (typically 200 nm or less, more particularly, 50 nm or less) and these allow for both good strength and ductility.
Thus, we have determined that above a certain Co level, the Co exceeds the solubility limit of the I-phase particles, forms Al9Co2, and embrittles the material. We have endeavored to more particularly define the relevant Co level relative to Cr and Mn levels. In one approximation, this involves the atomic percentage ratio of Co to the sum of Cr and Mn contents. Above about 0.065 will result in the formation of Al9Co2. When one alters the Cr and Mn levels, the Co level must accordingly change to maintain the ratio to control/limit I-phase.
In general, the Zr serves to thermally stabilize the I-phase. A desirable Zr level is sufficient to prevent thermally-induced I-phase coarsening (such coarsening would lower strength) while not being so high as to form undesirably large Al3Zr particles. Such Al3Zr particles, instead of behaving as fine dispersoids for grain size control, behave more like insoluble particles that lead to reduced ductility and fracture properties.
Table I below shows the measured composition of a tested material (“Test 1”). Weight and atomic percentages of Cr, Mn, Co, and Zr are given. The balance was Al with at most impurity levels of other components. Specifically, the contents of H, Fe, and Si were particularly sensitive with by weight amounts of less than 1 ppm H and less than 0.02% each for Fe and Si. Properties for the Test 1 composition are discussed below and were used to model target nominal values for three further examples. Although the test data shows advantageous performance, the modeling suggests even greater benefit to compositions having at least slightly lower Zr and substantially lower Co.
In an exemplary process of manufacture, the master alloy is formed (See, e.g., US Patent Application Publication 2012/0328470A1).
The master alloy is atomized (See, e.g., US Patent Application Publication 2012/0325051A1).
A vacuum hot-press (VHP) billet is then formed (See, e.g., US Patent Application Publication 2012/0024110A1). Prior to VHP, hot stage X-ray diffraction was used to identify when and if Al9Co2 would form in the powder. Because Al9Co2 began to form at 650 F (343° C.), degassing was at 600 F (316° C.) rather than 700 F (371° C.; 700 F (371° C.) previously being used to keep H content to less than 1 ppm by weight).
After billet production is an extrusion and forging process. An extrusion ratio of between 2:1 and 8:1 was used to limit the adiabatic heating associated with higher ratios. Such heating reduces strength. Such extrusion is discussed in US Patent Application Publication 2012/0325378A1. Exemplary forging is discussed in US Patent Application Publications 2008/0308197A1 and 2012/0328472A1.
In terms of I-phase generation, at 28 volume percent characteristic (e.g., average) I-phase particle size of the Test 1 sample was between 190 and 230 nanometers. At 25 volume percent, the size is calculated to be between 170 and 200 nanometers. At 20 volume percent, the size is calculated to be between 130 and 150 nanometers.
The three example alloys were specifically modeled to provide three different predicted I-phase volume percentages of 20%, 25%, and 28%, respectively, without any substantial Al9Co2. The three example alloys have a lower Zr content than the test alloy selected to preferably eliminate insoluble Al3Zr formation. The three Zr values are identical merely to obtain better data on the effect of Co. Three exemplary compositional ranges are also given to encompass these. A fourth compositional range is selected to also include the Test 1 material. Additional ranges could be formed around the Test 1 alloy or any of the examples by merely providing ±0.30 weight percent variation for each of the four alloying elements Co, Cr, Mn, and Zr. In each range, aluminum would form the majority by weight percent of the composition and, more particularly, substantially the remainder/balance (e.g., enough of the remainder to avoid significant compromise in properties). For example, to the extent any constituents beyond the enumerated Al, Cr, Mn, Co, and Zr are present, they would be expected to aggregate no more than 5 weight percent (more narrowly, no more than 2 weight percent and yet more narrowly, no more than 1 weight percent). Each additional element, individually, would be expected to be no more than 2 weight percent, more narrowly, no more than 1.0 weight percent, more particularly, no more than 0.5 weight percent.
However, as noted above, there are several specific elements for which much lower upper limits may be present. These include H, Fe, and Si. Exemplary maximum H is no more than 10 ppm, more narrowly, 5 ppm, more narrowly, 2 ppm, more narrowly, no more than 10 ppm, more narrowly, 5 ppm, more narrowly, 1 ppm. Exemplary Fe and Si maximum contents are each no more than 0.1 weight percent, more particularly, no more than 0.05 weight percent or 0.03 weight percent or 0.02 weight percent.
As noted above, for any of these ranges the atomic ratio of Co to the sum of Cr and Mn may be at most 0.065, more broadly, at most 0.07 or 0.10, and more narrowly, 0.050-0.065.
Exemplary Al9Co2 content, if any, is less than 5.0% by volume, more particularly, less than 2.0% or less than 1.0%.
Furthermore, exemplary I-phase volume percentage is less than 30%, more particularly, 15% to 30% or 18% to 28%. Exemplary characteristic (e.g., average) I-phase size is less than 1000 nm, more particularly, less than 500 nm or less than 200 nm.
Measured yield strength of the Test 1 alloy show greater yield strength than typical baseline aluminum fan alloys (e.g., 2060-T852 and 7255-T7452) by about 10-20% over a range from about ambient temperature (72 F (22° C.)) to 250 F (121° C.). Yield strength of the Test 1 alloy is slightly less (about 10-20% less) than Ti-6Al-4V over a range from ambient to approximately 600 F. However, specific yield strength exceeds that of both the Ti-6Al-4V and the baseline aluminum alloys over such temperature ranges (e.g., by at least about 10%). This evidences the ability to save weight when replacing either the Ti-6Al-4V or the baseline aluminum alloys.
Similar results are present with elastic modulus. The elastic modulus Test 1 alloy falls between that of the baseline aluminum alloys over the 72-600 F (22-316° C.) range on the one hand and below that of the Ti-6Al-4V on the other hand. However, the specific elastic modulus substantially exceeds these three prior art alloys over such range. The slightly greater advantage at lower temperature than at higher temperature is still at least about a 10% advantage over the Ti-6Al-4V and 7255 at the higher end of that range and at least about 5% over the 2060 at the higher end of that range.
On average, the coefficient of thermal expansion is reduced slightly relative to the two baseline alloys over the 72-600 F (22-316° C.) range. For the Test 1 material, the ductility varied between 5 and 6% elongation with a strength level greater than 100 Ksi (689 MPa). However, this material has high hydrogen (4 ppm, see
Additionally, corrosion resistance of the Test 1 alloy has been observed as improved relative to 7055-T7451, 7255-T7452, 2060-T852, and 6061-T6. This is measured as substantially lower average pit depth in salt-fog testing (e.g., ASTM B117 (neutral PH)). Exemplary average maximum pit depth was less than half of all of these baseline alloys in salt-fog testing from five hundred hours to over one thousand hours. Exemplary pit density (number of pits per unit of surface area) was even more dramatically lower (e.g., less than 10% of the density and potentially down to fractions of a percent).
This improvement in corrosion resistance is associated with a passivating layer forming in the salt-fog chamber because of the composition of the alloy. That is, the bare surface as shown in
In general, freshly exposed metallic surfaces will adsorb and react with oxygen present in the atmosphere almost instantaneously. With aluminum, this forms a thin oxidation layer that is easily breached, simply by handling. The breach leads to further oxidation that is similarly subject to breach.
Hence, processes have been developed such as anodization, which places a thick, hard, oxide layer on the aluminum. This oxide is less easily removed. However, if the anodization layer is breached (e.g., due to a scratch or dent), the area of exposed aluminum will rapidly corrode. The self-passivating ability can form an anodization-like passivating layer with thickness on the order of several micrometers, in distinction to typical oxidation layers which may be two or more orders of magnitude thinner.
An observed self-healing of pits is believed to involve such passivating in that the metal spontaneously forms a thicker oxide (like one would normally apply by anodization) after it has been damaged (e.g., by scratching, impact, erosion, and the like). Where pits do grow, these are precluded from growing further, as shown in
The exemplary tested lower/inner/inboard sublayer has a thickness of about 8 micrometers, more broadly, 5 micrometers to 10 micrometers or at least 5 micrometers. The observed upper/outer/outboard sublayer has a larger thickness of 15 micrometers to 20 micrometers, more broadly, at least 10 micrometers or 10 micrometers to 25 micrometers. The gap has a thickness of about 1 micrometer to about five micrometers, more particularly between 1.5 micrometers and 3 micrometers. Each identified thickness may be a local thickness or a characteristic thickness (e.g., mean, median, or modal, over an exposed area of a part).
The use of “first”, “second”, and the like in the following claims is for differentiation within the claim only and does not necessarily indicate relative or absolute importance or temporal order. Similarly, the identification in a claim of one element as “first” (or the like) does not preclude such “first” element from identifying an element that is referred to as “second” (or the like) in another claim or in the description.
Where a measure is given in English units followed by a parenthetical containing SI or other units, the parenthetical's units are a conversion and should not imply a degree of precision not found in the English units.
One or more embodiments have been described. Nevertheless, it will be understood that various modifications may be made. For example, when applied to an existing baseline, details of such baseline may influence details of particular implementations. Accordingly, other embodiments are within the scope of the following claims.
Claims
1. A composition comprising, in weight percent:
- Al as a largest constituent;
- 3.0-6.0 Cr;
- 1.5-4.0 Mn;
- 0.1-3.5 Co; and
- 0.3-2.0 Zr.
2. The composition of claim 1 wherein, in atomic percent content, Co divided by the sum (Cr+Mn) is less than or equal to 0.07.
3. The composition of claim 1 wherein, in atomic percent content, Co divided by the sum (Cr+Mn) is less than or equal to 0.065.
4. The composition of claim 1 comprising, in weight percent:
- 3.0-6.0 Cr;
- 1.5-4.0 Mn;
- 0.1-1.0 Co; and
- 0.3-1.5 Zr.
5. The composition of claim 1 comprising, in weight percent:
- 3.7-5.2 Cr;
- 2.1-3.0 Mn;
- 0.4-0.6 Co; and
- 0.7-1.1 Zr.
6. The composition of claim 1 comprising, in weight percent:
- 3.7-5.2 Cr;
- 2.1-3.0 Mn;
- 0.4-1.0 Co; and
- 0.7-1.1 Zr.
7. The composition of claim 1 wherein, in weight percent, the total of all additional elemental contents is not more than 5.0.
8. The composition of claim 1 wherein, in weight percent, no additional individual elemental content exceeds 1.0.
9. The composition of claim 1 wherein, in weight percent, each of Fe and Si content, if any, does not exceed 0.02.
10. The composition of claim 1 wherein, by weight, H content, if any, does not exceed 1 ppm.
11. The composition of claim 1 having an icosahedral phase (I-phase).
12. The composition of claim 11 wherein a volume fraction of said I-phase is 15% to 30%.
13. The composition of claim 11 wherein a characteristic size of said I-phase is less than 200 nm.
14. The composition of claim 1 wherein Al9Co2 content, if any, is less than 5% by volume.
15. A method for manufacturing the composition of claim 1, the method comprising:
- atomizing a master alloy;
- pressing the atomized alloy to form a billet;
- extruding the billet to form an extrusion; and
- forging the extrusion.
16. (canceled)
17. The composition of claim 1 having an icosahedral phase (I-phase).
18. The composition of claim 17 wherein a volume fraction of said I-phase is 15% to 30%.
19. The composition of claim 17 being a powder metallurgical alloy.
20. The composition of claim 17 wherein the composition is effective to form a passivating layer when exposed to a salt fog environment.
Type: Application
Filed: Jul 22, 2019
Publication Date: Nov 7, 2019
Applicant: United Technologies Corporation (Farmington, CT)
Inventors: Thomas J. Watson (South Windsor, CT), Iuliana Cernatescu (Glastonbury, CT)
Application Number: 16/518,217