GB2153848A - High strength hot corrosion resistant single crystals - Google Patents

Abstract

The alloy comprises 13-15.6% Cr 5-15% Co 2.5-5% Mo 3-6% W 4-6% Ti 2-4% Al balance Ni with no intentional additions of C, B or Zr. Single crystal articles of the above composition display a resistance to hot corrosion and a rupture life at least 3 x and 5 x respectively that of a conventional polycrystalline material of similar composition but with C, B and Zr additions.

Description

SPECIFICATION High strength hot corrosion resistant single crystals Technical Field This invention relates to the field of nickel base superalloy articles which are resistant to hot corrosion and have good mechanical properties at elevated temperatures. This invention also relates to the field of single crystal superalloy articles.

Background Art Increasing demands for efficiency in gas turbine engines have resulted in demands for materials capable of withstanding more severe operating conditions. In particular increased temperature capabilities are required for certain applications along with resistance to hot corrosion. U.S. Patent No. 3,494,709 describes the fabrication of gas turbine components in single crystal form for improved performance. U.S. Patent No. 4,116,723 describes how the essential elimination of carbon from single crystal articles improved certain mechanical properties. U.S. Patent No. 3,615,376 describes a moderate strength superalloy material commercially known as Ren6 80 which has moderate mechanical properties in combination with good corrosion resistance.

Disclosure of Invention Alloys having a composition within the range of 13-15.6% chromium, 5-15% cobalt, 2.5-5% molybdenum, 3-6% tungsten, 4-6% titanium, 2-4% aluminum, balance essentially nickel without intentional additions of carbon, boron or zirconium are fabricated into single crystals. The carbon content must be less than 500 ppm and is preferably less than 300 ppm.

The boron and zirconium contents should each be less than 100 ppm. The single crystal articles have exceptional resistance to hot corrosion and improved mechanical properties relative to those of similar compositions cast in polycrystalline form. In terms of the hot corrosion at 1650"F the invention material has less than one-third of the corrosion rate of conventionally cast carbon containing material of similar composition and has a rupture life at 1500"F/55 ksi which is at least 5 times that of conventionally cast carbon containing materials.

Other features and advantages will be apparent from the specification and claims.

Best Mode for Carrying Out the Invention The present invention comprises directionally solidified single crystal nickel base superalloy articles having an exceptional combination of hot corrosion resistance and elevated temperature mechanical properties. The composition of the article is essentially that composition known in the art as Ren6 80. This composition is described in U.S. Patent No. 3,615,376 and comprises 13-15.6% chromium, 5-15% cobalt, 2.5-5% molybdenum, 3-6% tungsten, 4-6% titanium, 2-4% aluminum, balance essentially nickel. In the commercial Ren6 80 material and in the composition described in U.S. Patent No. 3,615,376, intentional additions are made of carbon, boron and zirconium. These so-called minor elements are added for the purpose of strengthening grain boundaries.In the present invention, however, no intentional additions are made of these elements, and the maximum tolerable levels of the elements, as impurities, appear to be 500 ppm and preferably 300 ppm for carbon and 100 ppm for each of boron and zirconium.

Other preferred relationships between the alloy constituents are also set forth in the patent.

These include a requirement that one-half of the tungsten plus all the molybdenum should be in the range of 5-7% to avoid the formation of detrimental sigma phases. The ratio of titanium to aluminum should be greater than one but less than three to avoid the formation of detrimentai eta phases. Further, the sum of aluminum and titanium should lie in the range of 7.5-9%.

An essential aspect of the present invention is the fabrication of material in this composition (but without carbon) into single crystal form. Casting may be performed according to the teachings of various prior art patents including U.S. Patent Nos. 3,700,023, 3,763,926 and 4,1 90,094 which are incorporated herein by reference. Typically, the casting orientation will have the < 100 > crystal axis parallel to the principal stress access of the component. After being cast in single crystal form, the article will be invariably heat treated in order to improve the mechanical properties by controlling the gamma prime particle morphology in accordance with the teachings of U.S. Patent No. 4,116,723 which is incorporated herein by reference.

The primary feature of the present invention is the exceptional degree of corrosion resistance which is obtained through elimination of minor elements and fabrication in single crystal form.

Table I shows comparative corrosion test results between a material according to the invention and the prior art alloy known as Ren6 80 described in U.S. Patent No. 3,615,376. Corrosion testing was performed at 1650"F in a corrosive gaseous environment produced by the combustion of jet A fuel (30:1 air to fuel ratio) with additions of 20 ppm of ASTM sea salt and sufficient sulfur dioxide to produce a sulfur content equivalent to a 1.3% sulfur content in the fuel. The corrosion data given in Table I is the hours of exposure to a corrosive media required to produce a 1 mil depth of corrosion attack. It can be seen that the corrosion resistance of the invention material is substantially greater than that of the very similar conventionally cast polycrystalline prior art material which contained minor elements.The reason for this substantial improvement is not understood.

According to the data in Table I, the average improvement obtained in comparing the present invention material with the prior art Ren6 80 material was an improvement by a factor of about 6.8. This represents the average of the three tests reported in Table I.

TABLE I Depth of Corrosion Time in Penetration/ Resistance Test/Hrs. Mils Hrs/Mil Ratio Invention 287 4 72 6.6 Rend 80 287 27 11 Invention 512 7 73 10.4 Ren6 80 392 53 7 Invention 498 8 63 3.5 Rend 80 449 25 18 6.8 Table II is a comparative table which is based largely on Table VII of U.S. Patent No.

3,615,376. In the table in the prior patent there is listed a series of alloys which were compared to the invention claimed in that patent (carbon containing conventionally cast) Ren6 80 in terms of hot corrosion resistance. The testing procedure described in the prior patent is similar to that used to evaluate the invention material. Table II in the present application consists of the material from Table VII (only the maximum penetration values are listed since that was the measurement made in the testing of the material in the present invention). The previously mentioned figure of 6.8X was used to predict the behavior of the invention material under the testing conditions which resulted in the data in the prior art patent. While this is not conclusive or definitive, it serves to place the performance of the invention material in perspective both with the prior art Rene 80 material and with other commonly used superalloys.

In the field of hot corrosion as it applies to superalloys it is recognized that there is a relationship between the chromium content in the alloy and the resistance to hot corrosion. The greater the chromium content the more resistant the alloy is to hot corrosion. However, additions of chromium limits the strength attainable so that the higher chromium superalloys also generally have lower strengths. In the experience of the inventors the present invention material has the highest resistance to hot corrosion ever observed for an alloy containing 12-14% chromium.

TABLE II Mils of Attack in 1000 Hrs.

16000F 17500F 2 Rene 80 12 9 B Ren/e- 77 47 44 A Rene 100/IN100 (1) (1) C NASA TRW VI A (1) 43 D SEL 1 47 20 E Alloy 713 (2) 39 Invention(2) 1.8 1.3 (1) No data, catastrophic attack (2) Predicted Very significantly, this improvement in hot corrosion resistance is also accompanied by an increase in creep rupture properties. Table Ill sets out the creep rupture properties of the invention materials as compared with the prior art polycrystalline Ren6 80 material. It can be seen that the creep rupture properties have been improved by factors ranging from 8 to 20 times by the invention composition/process.From Table Ill it can also be observed that the optimum heat treatment temperature for the invention material to produce good properties of 1400"F appears to be about 1850do while the heat treatment temperature from material intended for use at 1600to appears to be about 1975"F. The invention material tested received a solution treatment at a temperature of about 2250"F prior to the heat treatment shown in the table. The conventionally cast Rene' 80 material cannot be solutionized for reasons discussed in U.S. Patent No. 4,116,723. The conventional Rene 80 was given a 4 hour treatment at 1925"F, typical of that which is normally employed.

Table IV in the present application represents a comparison of data obtained from material of the present invention with data presented in the prior art patent U.S. 3,615,376 relating to conventional Rene 80 material. The data in Table IV for the numbered alloys comes from Table Ill of the prior patent and shows rupture life under two different testing conditions for a series of alloys falling within the range of the prior patent (data from the prior patent for alloys outside of the range claimed were deleted). Added to this data is an entry for invention material of the composition 14% chromium, 3.8% tungsten, 3% aluminum, 5% titanium, 9.6% cobalt, 4% molybdenum, balance essentially nickel with no additions of carbon, boron, zirconium or hafnium.The data for the invention material was interpolated in the case of the 1500"F data and extrapolated in the case of 1800"F data using the Larson-Miller parameter technique inasmuch as testing for the invention material was performed at slightly different temperature and stresses. Data has also been added for alloy IN 1 00. The results clearly point out the mechanical property superiority of the invention material. At 1500"F at 55 ksi applied load the invention material would show a life of about 6500 hours which should be contrasted with the maximum of about 900 hours for the best composition reported in the prior patent.At 1800"F/27.5 ksi the invention material would display a life of approximately 1 35 hours which should be contrasted with the best reported life for material in the prior patent of about 50 hours. This again clearly points out the superiority in mechanical properties of the present invention when contrasted with the prior art material.

TABLE III Alloy Creep-Rupture Properties Coating Time to Alloy Diffusion Cycle Test Condition 1% Strain Rupture Life Invention 2050 F/4 hrs 1600 F/55 ksi 36.4 hrs 205.6 hrs 2050 F/4 hrs 1400 F/100 ksi 25.1 200.1 1975 F/4 hrs 1600 F/55 ksi 88.0 451.3 1975 F/4 hrs 1400 F/100 ksi 18.4 302.6 1850 F/4 hrs 1600 F/55 ksi 59.3 446.7 1850 F/4 hrs 1400 F/100 ksi 23.8 590 René 80 1925 F/4 hrs 1600 F/55 ksi 5.5 55.0 1925 F/4 hrs 1400 F/100 ksi 3.0 22.0 TABLE IV Stress Rupture Life, Hrs 15000F/55 KSI 18000F/27.5 KSI 6 744 49 5 896 50 3 836 33 8 670 43 Invention 6500 135 IN 100 550 52 Thus it can be seen from the tables that the present invention achieves an exceptional combination of hot corrosion resistance and high temperature mechanical properties. Notice, that with respect to alloy IN 100 that the invention material has superior corrosion resistance combined with substantially greater rupture life.

It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit and scope of this novel concept as defined by the following claims.

Claims (3)

1. A high strength hot corrosion resistant nickel base superalloy single crystal article consisting essentially of 13-15.6% Cr 5-15% Co

2.5-5% Mo 3-6% W 4-6% Ti 2-4% Al balance nickel with no intentional additions of C, B or Zr said article having a resistance to hot corrosion (in a gaseous environment at 1650 F) which is at least 3X that a conventional (carbon containing) polycrystalline Rene' 80 material and a rupture life at 1500 F/50 ksi which is at least 5X that of conventional (carbon containing) Ren6 80 material.

2. A high strength hot corrosion resistant nickel base superalloy single crystal article consisting essentially of 13-15.6% Cr 5-15% Co

2.5-5% Mo 3-6% W 4-6% Ti 2-4% Al less than 500 ppm C less than 100 ppm B less than 100 ppm Zr balance nickel.

3. An article as in claim 2 having a carbon content which is less than about 300 ppm