Titanium Alpha-Beta Alloy Fabricated Material and Process for Preparation

1. Technical Field

This invention relates to titanium alloy fabricated material having improved mechanical properties rendering it more useful, for instance, as rotating components such as impellers and disks for gas turbine engines and the like.

2. Background of the Invention

Turbine engine impellers of Ti-6Al-4V and other titanium alloys are currently being used both by gas turbine engine manufacturing companies in the USA and abroad for use at temperatures of up to 300.degree. C. (570.degree. F.).

3. Disclosure of Invention

This invention is concerned with the provision of titanium alpha-beta alloy fabricated material having improved mechanical properties. Depending on the particular alloy, the fabricated material may be capable of services at temperatures higher than 300.degree. C.

Thus, it has now been discovered that titanium alloys can be prepared, using the process technology of this invention, which are particularly suitable for use as impellers and disks and for other uses involving low cycle fatigue. Significantly improved tensile properties and particularly improved low cycle fatigue properties are obtained, along with modest improvement in fracture toughness and crack growth resistance. Thus, one process variant of the invention gives higher fracture toughness with higher fatigue crack growth resistance and a moderate low cycle fatigue life; while another variant gives improved low cycle fatigue properties and tensile strength with moderate fracture toughness. The alloys are effective at temperatures up to 750.degree. F. (400.degree. C.).

More particularly, it has been discovered that if a Ti-6Al-2Sn-4Zr-6Mo alloy (which can contain minor amounts of oxygen and nitrogen) is formed into a particular microstructure and heat treated at optimum temperatures, improved components can be achieved.

All parts and percentages in this specification and its claims are by weight unless otherwise indicated.

The drawings FIGS. 1-4) are photomicrographs of the alloys resulting from the process conditions listed in Table II. Beta phase (matrix) appears dark and alpha phase (particles) light in the photomicrographs.

FIG. 1 is composed of parts 1A to 1C, showing microstructure, respectively, at center, mid-radius, and rim, all at mid-height, in a 25.4 cm diameter by 6.35 cm thick pancake forging.

FIG. 2 is composed of parts 2A and 2B, both being at the mid-height, mid radius location, one being at twice the magnification of the other, in a 25.4 cm diameter by 6.35 cm thick pancake forging.

FIG. 3 is taken at the mid-height, mid radius location in a 22.9 cm diameter by 13.7 cm thick pancake forging.

FIG. 4 is composed of parts 4A to 4C, showing microstructure, respectively, at center, mid-radius, and rim, all at mid-height, in a 25.4 cm diameter by 6.35 cm thick pancake forging.

In general, alloys for embodiments of the present invention fall under the category, titanium alpha-beta alloys. Examples of alpha-beta alloys are Ti-6Al-4V, Ti-6Al-6V-2Sn (Cu +Fe), Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si, and Ti-6Al-2Sn-4Zr-2Mo, the last being sometimes termed a "near-alpha" alloy.

The invention will be explained below as it applies to the Ti-6Al-2Sn-4Zr-6Mo alpha-beta alloy, with the understanding that those skilled in the art will be able to analogize application of the principles involved to other titanium alpha-beta alloys.

A titanium alloy Ti-6Al-2Sn-4Zr-6Mo which can be used to obtain the improved properties has the following general composition:

5.50 to 6.50% aluminum,

3.50 to 4.50% zirconium,

1.75 to 2.25% tin,

5.50 to 6.50% molybdenum,

0 to 0.15% iron

0 to 0.15% oxygen

0 to 0.04% carbon,

0 to 0.04% (400 ppm) nitrogen,

0 to 0.0125% (125 ppm) hydrogen,

0 to 0.005% (50 ppm) yttrium,

0 to 0.10% residual elements, each

0 to 0.40% residual elements, total, and

remainder titanium.

Products of the invention are achieved via two general routes, namely by

Route 1. .beta.-fabricating plus .alpha.-.beta. solution heat treatment plus aging, and by

Route 2. .alpha.-.beta.-fabricating plus .alpha.-.beta. solution heat treatment plus aging.

Route 1, in general, gives higher fracture toughness with higher fatigue crack growth resistance and a moderate low cycle fatigue life; while route 2 gives improved low cycle fatigue properties and tensile strength with moderate fracture toughness.

To quantify these property characteristics for the Ti-6Al-2Sn-4Zr-6Mo alloy, process route 1 can achieve average values as follows: yield strength greater than (>) 150 ksi (kilopounds per square inch) (1034 MPa), ultimate tensile strength >160 ksi (1102 MPa), elongation >7%, reduction in area >15%, fracture toughness K.sub.Ic >60 ksi.in.sup.1/2 (65.9 MPa m.sup.1/2), low cycle fatigue life >10,000 cycles at a total strain range of 1.0%, and fatigue crack growth rate less than or equal to (.ltoreq.) about 2.times.10.sup.-6 inches per cycle (5.times.10.sup.-8 meters per cycle), and even .ltoreq.1.times.10.sup.-6 inches per cycle (2.5'10.sup.- meters per cycle), at a .DELTA.K=10 ksi.in.sup.1/2 (11 MPa.m.sup.1/2). Extrapolating from our results to this point, we believe that by following process route 1 we should be able to exceed these minimums, respectively maximums, by at least another 10% of the values just stated.

Process route 2 can achieve average values as follows: yield strength greater than (>) 150 ksi (kilopounds per square inch) (1034 MPa), ultimate tensile strength >160 ksi (1102 MPa), elongation >7%, reduction in area >15%, fracture toughness K.sub.Ic >45 ksi in.sup.1/2 (49.4 MPa.m.sup.1/2), low cycle fatigue life >15,000 cycles at a total strain range of 1.0%, and fatigue crack growth rate less than or equal to (.ltoreq.) about 2.times.10.sup.-6 inches per cycle (5>10.sup.-8 meters per cycle), and even .ltoreq.1.times.10.sup.-6 inches per cycle (2.5.times.10.sup.-8 meters per cycle), at .DELTA.K=10 ksi.in.sup.1/2 (11 MPa.m.sup.1/2). Extrapolating from our results to this point, we believe that by following process route 2 we should be able to exceed these minimums, respectively maximums, by at least another 10% of the values just stated.

References here and throughout this specification and its claims to the qualifiers ".beta." or "beta" and ".alpha.-.beta." or "alpha-beta" with respect to fabricating steps mean "carried out within the temperature range of, respectively, the .beta.-phase field and the .alpha.-.beta. phase field where the .alpha. and .beta. phases coexist, both fields being as shown on the phase diagram for the alloy".

For general information on the subject of phase diagrams for titanium alloys such as the Ti-6Al-2Sn-4Zr-6Mo alloy of concern in this invention, refer to the discussion of FIG. 6-53 on page 238 in "Elements of Physical Metallurgy" by Albert G. Guy, Addison-Wesley, Reading, Mass. 1959.

The term "beta-transus" refers to the temperature at the line on the phase diagram separating the .beta.-phase field from the .alpha.-.beta. region of .alpha. and .beta. phase coexistence. "T.sub.62 " is another way of referring to the beta-transus temperature. A term such as "T.sub..beta. -42.degree. C." means "temperature whose value equals (T.sub..beta. minus 42.degree. C.)".

For the Ti-6Al-2Sn-4Zr-6Mo alloy of concern in this invention, T.sub..beta. is around 1750.degree. F. (950.degree. C.). T.sub..beta. may be determined for a given composition by holding a series of specimens for one hour at different temperatures, perhaps spaced by 5 degree intervals, in the vicinity of the suspected value of T.sub..beta., then quenching in water. The microstructures of the specimens are then observed. Those held at temperatures below T.sub..beta. will show the .alpha. and .beta. phases, whereas those hold above T.sub..beta. will show a transformed .beta. structure.

The fabricating mentioned for processing routes 1. and 2. involves plastic deformation of the metal. Forging is one example of a fabricating process. As is well known, forging can involve a progressive approach toward final forged shape, through the use of a plurality of dies, for example preform (or blocker) dies and finish dies. It is of advantage in the present invention to use "hot die" forging, i.e. a die temperature which is e.g. above about 550.degree. C. (1020.degree. F.). An advantage of hot die forging in the present invention is that it avoids formation of a chill zone of different properties than the rest of the metal.

In the case of .beta.-fabrication, i.e. processing route 1., it may be beneficial that the temperature actually fall during fabrication into the range of .alpha.-.beta. coexistence; this is termed "through-transus" .beta.-fabricating, in that the fabrication process starts out at temperatures in the .beta.-region and falls during fabrication such that the .alpha.-.beta.-region is reached.

It will be noted that times and temperatures of elevated temperature operations, for instance forging temperatures and solution and aging treatments, are qualified herein by the term "about", this being a recognition of the fact, for instance, that, once those skilled in the art learn of a new concept in the heat treatment of metals, it is within their skill to use, for example, principles of time-temperature integration, such as set forth in U.S. Pat. No. 3,645,804 of Basil M. Ponchel, issued Feb. 29, 1972, for "Thermal Treating Control", to get the same effects at other combinations of time and temperature.

Fabricated metal is usually returned to ambient temperature by air cooling, although oil quenching may be employed after solution heat treatment steps for improving retention of metastable .beta.-phase.

With reference particularly to the processing of route 1, at least one part of the fabrication is carried out while the alloy is at temperatures in the .beta. phase field. In the case of forging, preferably at least the finish forging is a .beta.-forging. Such finish forging may be preceded by an .alpha.-.beta. preform step. Alternatively, both the preform and the finish forging may be .beta.-forging steps.

For example, the entire forging operation may be carried out at temperatures about in the range of T.sub..beta. +20.degree. C. to T.sub..beta. +75.degree. C. Alternatively, this temperature range may be used only for the finish forging, and the finish forging may be preceded by an .alpha.-.beta. preform at temperatures about in the range of T.sub..beta. -20.degree. C. to T.sub..beta. -120.degree. C.

As indicated above in the section "Processing in General", .beta.-forging steps may be of the "through-transus" type; thus, a forging step may start at a temperature in the above-mentioned range T.sub..beta. +20.degree. C. to T.sub..beta. +75.degree. C. and, by the end of the forging step, be at a temperature below the .beta.-transus, i.e. in the .alpha.-.beta. region. .beta.-forging steps of the through-transus type are advantageous for achieving improved fracture toughness and low-cycle fatigue properties; it is thought that this effect is explainable on the microstructural level as follows: The process reduces precipitation of .alpha.-phase at the grain boundaries, such that .alpha.-phase there is discontinuous; to the extent that .alpha.-phase does form, it is thin-layered as compared to the thick and continuous type of precipitates which occur, for instance, when forging is carried out entirely in the .beta.-phase field, coupled with slow post-forging cooling. In general, the effect is not obtained when the forging start temperature is higher, e.g. T.sub..beta. +50.degree. C., and clearly not at T.sub..beta. +80.degree. C.

.beta.-forging may be followed by an oil quench for the purpose of reducing, or preventing, .alpha.-phase precipitation at grain boundaries.

Fabrication is followed by solution heat treatment and then aging. Solution heat treatment is carried out at temperatures about in the range T.sub..beta. -20.degree. C. to T.sub..beta. -120.degree. C. about for a time in the range 20 to 120 minutes, for the purpose of achieving a coarse transformed beta microstructure and a near-equilibrium mixture of .alpha. and .beta. phases in the upper part of the .alpha.-.beta. field of the phase diagram and a supersaturated state in the subsequent, quenched condition, preparatory to precipitation hardening in the aging step.

Aging is carried out at temperatures about in the range 425 to 650.degree. C. (797.degree. F. to 1202.degree. F.) for a time in the range 2 to 25 hours, for the purpose of precipitating fine .alpha.-phase particles in the retained supersaturated .beta.-phase matrix. This .beta. matrix is then referred to as "aged".

With reference particularly to the processing of route 2, fabrication is carried out while the alloy is at temperatures in the field of .alpha. and .beta. phase coexistence.

In the case of forging, a finish forging may be preceded by one or several preform steps. Both preform and finish forging steps are carried out in the .alpha.-.beta. field.

Preferably, fabrication is carried out in the .alpha.-.beta. field at temperatures about in the range of T.sub..beta. -20.degree. C. to T.sub..beta. -120.degree. C.

Fabrication is followed by solution heat treatment and then aging. Solution heat treatment is carried out at temperatures about in the range T.sub..beta. -5.degree. C. to T.sub..beta. -25.degree. C. about for a time in the range 20 to 80 minutes, for the purpose of achieving a near-equilibrium mixture of .alpha. and .beta. phases in the upper part of the .alpha.-.beta. field of the phase diagram and a supersaturated state in the subsequent, quenched condition, preparatory to formation of transformed beta during quenching and subsequent precipitation hardening in the aging step. During the solution treatment step, a small amount of equiaxed, primary .alpha. is retained as equilibrium alpha-phase, while, during the cooling, or quenching, step, part of the .beta.-phase transforms to acicular to plate-type, or basket-weave, secondary .alpha..

Solution heat treatment may include a stage subsequent to the treatment in the range T.sub..beta. -5.degree. C. to T.sub..beta. -25.degree. C. This subsequent stage is carried at temperatures lower in the .alpha.-.beta. field, for instance at temperatures about in the range T.sub..beta. -40.degree. C. to T.sub..beta. -120.degree. C. about for a time in the range 1 to 3 hours, for the purpose of thickening the transformed .beta. (secondary .alpha.).

As in process route 1, aging is carried out at temperatures about in the range 425 to 650.degree. C. (797.degree. F. to 1202.degree. F.) for a time in the range 2 to 25 hours, for the purpose of precipitating fine .alpha.-phase particles in retained .beta.-phase matrix.

The following examples will serve to illustrate the invention.

Table I provides composition information for the particular Ti-6Al-2Sn-4Zr-6Mo alloys tested. The "max" and "min" values show the compositional ranges to exist among the particular alloys.

Table II reports the thermomechanical processing histories and the microstructures obtained. Resulting mechanical properties are reported in Table III.

All of the examples started with .alpha.-.beta. fabricated and .alpha.-.beta. annealed bar stock. 15.24 cm (6-inch) diameter by 14.2 cm (5.6-inch) to 31 cm (12.2-inch) long bar stock samples were hot die forged (die temperature in the range 1300 to 1600.degree. F., 700 to 875.degree. C.) at a crosshead speed of 51 cm (20 inches) per minute to produce forged dimensions as given in Table II. The 14.2 cm (5.6-inch) length material was used to make pancake forgings measuring 25.4 cm (10.0 inches) diameter by 6.35 cm (2.0 inches) thick, while the 31 cm (12.2-inch) length was fabricated into pancake forgings measuring 22.9 cm (9.0 inches) diameter by 13.7 cm (5.4 inches) thick.

From the data reported in Table III, it can be seen that the alloys of the invention have excellent tensile properties and fracture toughness. Particularly effective are Examples 2 and 4. Table IV reports on fatigue properties, namely low cycle fatigue and fatigue crack growth rate.

While the invention has been illustrated by numerous examples, obvious variations may occur to one of ordinary skill and thus the invention is intended to be limited only by the appended claims.