Heat Recoverable Alloy

Abstract

An alloy capable of having the property of heat recoverability imparted thereto comprising 49.1 to 50.2 atomic percent of titanium, 2.1 to 4.7 atomic percent of iron and the remainder nickel.

Description

BACKGROUND OF THE INVENTION

In U.S. Patent application Ser. No. 852,722 filed Aug. 25, 1969 now abandoned by John D. Harrison and James E. Jervis, the disclosure of which is incorporated by reference herein, there is disclosed a heat recoverable metallic coupling, especially suitable for use on hydraulic lines in aircraft. The requirements of such a coupling are many. First, it must be heat recoverable, that is, it must recover from a heat unstable configuration to a heat stable configuration upon the application of heat alone. It has been found that certain alloys are capable of having this property of heat recoverability imparted to them if they are formed in the heat stable condition while in the austenitic state, then cooled until they undergo a martensitic transition, and then deformed while maintained in the martensitic state. When heated to a temperature where the alloy is again transformed into the austenitic state, the formed object will revert to its original configuration.

Inasmuch as an alloy is considerable stronger in its austenitic state than in its martensitic, a second requirement of an alloy suitable for use in a coupling is that its transition temperature, that is, the temperature (more precisely, the temperature range) where it changes from its austenitic state to its martensitic state, must be below any expected operating temperature of the coupling. In addition, of course, the material from which the coupling is made must inherently have a yield strength sufficient to endure the operating conditions to which it is to be subjected. Conflicting, however, with the requirement for strength is the requirement for light weight. This is particularly true in couplings to be used in aircraft. What is desired is an alloy that can be fabricated into a coupling having the highest possible strength and the lowest possible weight. The alloy must also be sufficiently workable so that it can be formed into the parts and must not be overly brittle.

Various alloys of titanium and nickel have in the past been disclosed as being capable of having the property of heat recoverability imparted thereto. Examples of such alloys may be found in U.S. Pat. Nos. 3,174,851 and 3,351,463. The alloys disclosed in these patents are binary alloys but ternary alloys have also been suggested. For example, see the article by Goldstein, Buehler and Wiley entitled "Effects of alloying upon certain properties of 55.1 Nitinol" (Published Aug. 1965 by U.S. Naval Ordnance Laboratory, White Oak, Maryland as NOLTR 64-235). None of the alloys disclosed in these patents and publications, however, are satisfactory for use in couplings or other devices for use on aircraft because they do not have the required combination of attributes discussed above, and are particularly deficient with respect to their transition temperatures which in all cases are above the temperature generally considered the maximum for safety -- minus 65.degree.F, and in most cases are substantially above this temperature. In those few cases where the alloys have a transition temperature fairly close to that desired, they had a relatively low yield strength and thus require the use of a substantial volume of metal with the result that the parts are heavier than desired.

SUMMARY OF THE INVENTION

According to the present invention a heat recoverable metal alloy has been provided which has a transition temperature below any temperatures expected during aircraft operations. In addition, the alloy can be worked within the bounds of reasonable manufacturing processes, is not brittle, is not susceptible to embrittlement, and has a high strength to weight ratio. The alloy is thus acceptable for use in aircraft and is particularly useful in constructing hydraulic couplings for such use. It has been found that in order that the alloy have the necessary properties it must contain very close to 50 atomic percent titanium with a small percentage of iron substituted for nickel. Specifically, the alloy must contain from 49.1 to 50.2 atomic percent of titanium and 2.1 to 4.7 atomic percent of iron, the remainder being nickel. The most desirable alloy is obtained when the iron content is between 3.2 and 3.6 atomic percent. The latter alloy will have the maximum strength to weight ratio consistent with the required transition temperature and will still allow practical treatment to permit the impartation of heat recovery to the alloy. From a practical standpoint, the alloy must have a transition temperature which is above that of liquid nitrogen. If it is lower, it becomes commercially impractical if not impossible to maintain the alloy in its martensitic state during deformation and installation of the part fabricated from it.

Alloys may be prepared in accordance with the present invention as follows:

EXAMPLE 1

Equal width and length strips were cut from sheet stock of nickel (International Nickel 270) titanium (Titanium Metals Corporation 35A) and iron (99.9 percent pure). The strips were cleaned to remove any dirt or grease, weighed and assembled in bundles such that the elements were in the ratio of 50 atomic percent titanium, 3 atomic percent iron and 47 atomic percent nickel at each cross section through the longitudinal axis of the bundle. The bundle was then hung in the chamber of a Lepel HCP-F floating zone unit. The chamber was evacuated, then filled with high purity argon to a pressure of 1 atmosphere; this procedure was repeated twice; after the third filling a pressure of + 3 p.s.i. gauge was established and maintained during the melting to minimize air influx.

The lower end of the sample was heated by a single turn induction coil attached to the secondary winding of a 12:1 load matching step down transformer, the primary being powered by a Lepel high frequency induction heating unit (Model T-10-3-DF-E-H) operating in the Kilo Hertz range. Rapid melting resulted from the combination of induction heating and the heat of formation of the intermetallic compound TiNi 0.94 Fe 0.06. The falling droplets of alloy were collected in a cold copper mold, the bundle being fed into the induction coil until it had all been melted and collected in the mold. After cooling, the copper mold and dripcast ingot were removed from the chamber, and the mold was stripped.

The dripped ingot, which was a semi-compact cylinder, was returned to the chamber and an argon atmosphere established as before. A molten zone was passed along the ingot from bottom to top at a rate of about 0.5 cm/minute using the floating zone technique to avoid possible contamination by a crucible. The product was a homogeneous, void-free bar of alloy about 2 cm. diameter, 12 cm. long.

The composition of the alloy of this invention can be described by reference to an area on the titanium nickel and iron phase diagram. The general area of the alloy on the phase diagram is shown by the circled portion of Diagram 1 below. This area of the phase diagram is enlarged and shown in Diagram 2. The compositions at the corners of the quadrilateral are shown in Table 1 below.

ATOMIC PERCENT

Titanium Nickel Iron A 49.1 47.3 3.6 B 49.1 48.8 2.1 C 50.2 46.8 3.0 D 50.2 45.1 4.7

TABLE I ##SPC1##

EXAMPLE 2

In addition to the method described in Example 1, alloys according to the invention may be manufactured from their components by other methods suitable for dealing with high-titanium alloys. The details of these methods, and the precautions necessary to exclude oxygen and nitrogen either by working in an inert atmosphere or in a vacuum, are well known to those skilled in the art and are not repeated here.

It appears however that alloys obtained by the methods and using the materials described will contain small quantities of other elements, including oxygen and nitrogen in total amounts from about 0.05 to 0.2 percent. The effect of these materials is generally to reduce the martensitic transformation temperature of the alloys.

In a Temescal 900 kW electron-beam furnace, bundles of bars, as described in Example 1, containing Titanium (49.6 atomic percent), Nickel (47.2 atomic percent) and Iron (3.2 atomic percent) were melted. On analysis, the resulting ingot was found to contain the same relative proportions.

The properties of the resulting alloy are as follows:

Ms -88.degree.C to -118.degree.C Yield point at room temperature 66,000 psi to 79,000 psi Elongation 20% Hardness Rockwell A 60 to 64

Samples of the composition were hot-workable, could be machined, and displayed no propensity for embrittlement.

The alloy was also capable of at least 5 percent recovery. For example, a hydraulic coupling made of the alloy was provided with a heat unstable diameter of 8% greater than the heat stable diameter.

Claims

What is claimed is:

1. An alloy consisting essentially of 49.1 to 50.2 atomic percent titanium, 3.2 to 3.6 atomic percent of iron and the remainder nickel, the martensitic transformation temperature of said alloy being such as to permit impartation of heat recoverability to an object formed thereof at a temperature above the boiling point of liquid nitrogen.

2. A titanium, nickel and iron alloy within an area defined on a titanium, nickel and iron ternary phase diagram by a quadrilateral with its first corner at 49.1 atomic percent titanium, 47.3 atomic percent nickel and 3.6 atomic percent iron; its second corner at 49.1 atomic percent titanium, 48.8 atomic percent nickel and 2.1 atomic percent iron; its third corner at 50.2 atomic percent titanium, 46.8 atomic percent nickel and 3.0 atomic percent iron; and its fourth corner at 50.2 atomic percent titanium, 45.1 atomic percent nickel and 4.7 atomic percent iron, the martensitic transformation temperature of said alloy being such as to permit impartation of heat recoverability to an object formed thereof at a temperature above the boiling point of liquid nitrogen.