Title:
Method and apparatus for heat-treating solid alloy material
Kind Code:
A1


Abstract:
A method of heat-treating solid alloy material. Solid alloy material is immersed in liquid lithium at a temperature higher than the solid solution temperature and lower than the melting point of the solid alloy material in an initial heating tank. Then, the solid alloy material is held there until the solid alloy material becomes solid solution. Then, the solid alloy material is immersed in liquid lithium at a temperature lower than the solid solution temperature in a cooling tank. Thus, supersaturated solid solution is obtained. Then, the solid alloy material is re-heated at an aging temperature higher than room temperature.



Inventors:
Ninomiya, Susumu (Kanagawa, JP)
Application Number:
11/143969
Publication Date:
10/12/2006
Filing Date:
06/03/2005
Assignee:
JAPAN THERMOTEC CO., LTD.
Primary Class:
Other Classes:
148/713, 266/120
International Classes:
C22F1/18
View Patent Images:



Primary Examiner:
YANG, JIE
Attorney, Agent or Firm:
FOLEY & LARDNER LLP (WASHINGTON, DC, US)
Claims:
What is claimed is:

1. A method of heat-treating solid alloy material, the method comprising: an initial heating step of heating liquid lithium in an initial heating tank to a temperature higher than the solid solution temperature of the solid alloy material and lower than the melting point of the solid alloy material, at said solid solution temperature the components of the solid alloy material melting and becoming solid solutions, and immersing the solid alloy material directly in the liquid lithium in the initial heating tank and holding the material in the liquid lithium until the solid alloy material becomes a solid solution; a lithium-immersion cooling step of holding the liquid lithium in a cooling tank at a temperature lower than the solid solution temperature and immersing the solid alloy material directly in the liquid lithium in the cooling tank after the initial heating step, thereby cooling the solid alloy material and obtaining a supersaturated solid solution; and a re-heating step of holding the solid alloy material in a re-heating tank maintained at an aging temperature higher than room temperature, after the lithium-immersion cooling step.

2. The method of heat-treating solid alloy material according to claim 1, the method further comprising a second cooling step of cooling the solid alloy material to a temperature lower than the melting point of lithium, after the lithium-immersion cooling step and before the re-heating step.

3. The method of heat-treating solid alloy material according to claim 1, wherein the solid alloy material is α-β titanium alloy or β titanium alloy.

4. The method of heat-treating solid alloy material according to claim 3, wherein: the initial heating step includes holding the liquid lithium in the initial heating tank at a temperature ranging from 700° C. to 1300° C.; and the lithium-immersion cooling step includes holding the liquid lithium in the cooling tank at 500° C. or less.

5. The method of heat-treating solid alloy material according to claim 1, further comprising a step of mechanically processing the solid alloy material after the lithium-immersion cooling step and before the re-heating step.

6. The method of heat-treating solid alloy material according to claim 1, further comprising a step of mechanically processing the solid alloy material before the initial heating step.

7. The method of heat-treating solid alloy material according to claim 1, wherein the re-heating step is performed by immersing the solid alloy material directly in the liquid lithium in the re-heating tank.

8. An apparatus for heat-treating solid alloy material, the apparatus comprising: a heat-treatment chamber filled with inert gas; an initial heating tank arranged in the heat-treatment chamber and containing liquid lithium heated to a temperature higher than solid solution temperature of the solid alloy material and lower than melting point of the solid alloy material, at said solid solution temperature the components of the solid alloy material melting and becoming solid solutions, and in the liquid lithium the solid alloy material being immersed; and a cooling tank arranged in the heat-treatment chamber and containing liquid lithium held at a temperature lower than the solid temperature, in the liquid lithium the solid alloy material being immersed after transported from the initial heating tank.

9. The apparatus for heat-treating solid alloy material according to claim 8, wherein the solid alloy material is α-β titanium alloy or β titanium alloy.

10. The apparatus for heat-treating solid alloy material according to claim 8, the apparatus further comprising a re-heating tank arranged in the heat-treatment chamber and containing liquid lithium held at a temperature lower than the solid solution temperature and higher than the temperature of the liquid lithium in the cooling tank, said solid alloy material coming from the cooling tank being immersed in the liquid lithium contained in the re-heating tank.

11. The method of heat-treating solid alloy material according to claim 1, wherein the solid alloy material is titanium alloy to be used as material of golf-club heads, structural parts of vehicles, structural parts of aircrafts, structural parts of engines or implant parts.

Description:

CROSS-REFERENCE TO A RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-165326 filed on Jun. 3, 2004; the entire content of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for heat-treating alloy material. More particularly, the invention relates to a method and apparatus for performing heat treatment that uses liquid lithium.

2. Description of Related Art

Heat treatments such as quenching and annealing have long been performed on carbon steel and other alloys in order to impart desired mechanical properties to these materials.

Titanium is light and strong, and also resistant to high temperature and corrosion. This is why it has found a widespread use in recent years. The crystal structure of pure titanium is in hexagonal close-packing crystal structure (in a phase) at room temperature. At about 885 ° C., titanium undergoes allotropic transformation, assuming crystal structure of body-centered cube (in β phase). It is known in the art that alloys produced by adding various metals to pure titanium change in crystal structure as they receive heat treatment, and that their mechanical properties change as their crystal structures change (see Toshiyuki Suzuki, Yasuo Moriguchi, The Story of Titanium, revised edition, Japanese Standards Association, 1995, pp. 70-78).

Means for heat-treating metal materials, hitherto employed generally, are the electric furnace, gas furnace and vacuum furnace. Medium for cooling the material, hitherto used generally, are water, oil and gas. In some cases, high-frequency heating, electric-current heating, salt heating/cooling and heating/cooling using fused metals, such as lead, tin and gallium, are carried out. Further, it is proposed that liquid metallic sodium or potassium be used as cooling fluid to heat-treat solid metal materials (see Japanese Patent Application Laid-Open Publications No. 2000-345236 and Japanese Patent Application Laid-Open Publication No. 2002-12917).

The conventional method of heating metal by using a gas heater or an electric heater requires a long time to heat the metal and can hardly heat it uniformly. High-frequency heating requires use of wave-applying devices that conform to the products to be heat-treated. This inevitably raises the manufacturing cost of the products. In salt heating/cooling, the salt must be very carefully handled and managed to prevent environmental pollution. Heating/cooling using fused metals, such as lead, tin and gallium, needs high cost in its control. If the heavy metal sticks to the surfaces of the product being heat-treated, much labor and time will be spent in removing it from the product. Sodium and potassium have low boiling points of 881° C. and 756° C., respectively. If they are used as heating media at temperatures higher than their boiling points, they must be used in a pressurized atmosphere. If they are used in the atmosphere, they may undergo spontaneous ignition.

Generally it is demanded that heavy-metal materials be strengthened through heat treatment. Particularly, titanium products, which are light (having small specific gravity), should be strengthened.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoing. An object of the invention is to provide a novel method and apparatus that can easily provide alloy materials having great strength.

A method of heat-treating solid alloy material according to an embodiment of the present invention comprises: an initial heating step of heating liquid lithium in an initial heating tank to a temperature higher than the solid solution temperature of the solid alloy material and lower than the melting point of the solid alloy material, at said solid solution temperature the components of the solid alloy material melting and becoming solid solutions, and immersing the solid alloy material directly in the liquid lithium in the initial heating tank and holding the material in the liquid lithium until the solid alloy material becomes a solid solution; a lithium-immersion cooling step of holding the liquid lithium in a cooling tank at a temperature lower than the solid solution temperature and immersing the solid alloy material directly in the liquid lithium in the cooling tank after the initial heating step, thereby cooling the solid alloy material and obtaining a supersaturated solid solution; and a re-heating step of holding the solid alloy material in a re-heating tank maintained at an aging temperature higher than room temperature, after the lithium-immersion cooling step.

An apparatus for heat-treating solid alloy material according to an embodiment of the present invention comprises: a heat-treatment chamber filled with inert gas; an initial heating tank arranged in the heat-treatment chamber and containing liquid lithium heated to a temperature higher than solid solution temperature of the solid alloy material and lower than melting point of the solid alloy material, at said solid solution temperature the components of the solid alloy material melting and becoming solid solutions, and in the liquid lithium the solid alloy material being immersed; and a cooling tank arranged in the heat-treatment chamber and containing liquid lithium held at a temperature lower than the solid temperature, in the liquid lithium the solid alloy material being immersed after transported from the initial heating tank.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart illustrating a method of heat-treating solid alloy material according to an embodiment of this invention;

FIG. 2 is a schematic, vertical cross-sectional view showing an apparatus for heat-treating solid alloy material according to an embodiment of the present invention;

FIG. 3 is a graph showing how the temperature of solid metal material, or α-β titanium alloy, changes with time while the material is being heat-treated by the method illustrated in FIG. 1;

FIG. 4 is a graph representing the relation between the face thickness and flexure of a golf-club head;

FIG. 5 is a cross-sectional view of a golf-club head to which the method of heat-treating solid alloy material according to the present invention is applied;

FIG. 6 is a cross-sectional view of a gasoline engine to which the method of heat-treating solid alloy material according to the present invention is applied;

FIG. 7 is a cross-sectional view of a jet engine to which the method of heat-treating solid alloy material according to the present invention is applied;

FIG. 8 is a vertical cross-sectional view of an artificial hip joint to which the method of heat-treating solid alloy material according to the present invention is applied; and

FIG. 9 is a table showing the proof stress and strength actually measured of Ti-6A1-4V material that has been heat-treated by the method shown in FIGS. 1 and 3.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Embodiments of a method and apparatus for heat-treating solid alloy material according to the present invention will be described with reference to FIG. 1 to FIG. 3. FIG. 1 is a flowchart illustrating a method of heat-treating solid alloy material according to an embodiment of this invention. FIG. 2 is a schematic, vertical cross-sectional view showing an apparatus for heat-treating solid alloy material according to an embodiment of the present invention. FIG. 3 is a graph showing how the temperature of solid metal material, or α-β titanium alloy, changes with time while the material is being heat-treated by the method illustrated in FIG. 1.

As shown in FIG. 2, three liquid lithium tanks 12, 14 and 16 are arranged in a heat-treatment chamber 10, in which a lump 50 of solid metal material (e.g., α-β titanium alloy or β titanium alloy) will be heat-treated. The liquid lithium tanks are an initial heating tank 12, a cooling tank 14 and a re-heating tank 16. The initial heating tank 12 contains liquid lithium, the temperature of which is held at a temperature (transformation temperature) that is higher than the solid solution temperature of the solid alloy material to be treated and lower than the melting point of the solid alloy material. The solid solution temperature is the temperature at which the alloy components of the solid alloy material dissolve into solid solutions. If the solid alloy material is α-β titanium alloy, it should better be maintained at a temperature ranging, for example, from 700 to 1000° C. A heater 18 for heating lithium is attached to the initial heating tank 12.

The liquid lithium contained in the cooling tank 14 is held at a cooling temperature that is lower than the solid solution temperature of the solid alloy material and higher than the melting point (181° C.) of lithium. This cooling temperature is preferably 500° C. or less, more preferably a temperature ranging from 181 to 300° C., if the solid alloy material is a-p titanium alloy. To adjust the cooling temperature within a desired range, a heater 20 and a fan 22 are attached to the cooling tank 14 to heat and cool the liquid lithium, respectively.

It is desirable to hold the liquid lithium contained in the re-heating tank 16, at a temperature that is higher than the melting point (181° C.) of lithium. This temperature is, for example, 200 to 650° C., more preferably 300 to 600° C. To adjust the temperature within a desired range, a heater 24 is attached to the re-heating tank 16.

The heat-treatment chamber 10 is filled with inert gas that does not react with the liquid lithium. The chamber 10 has two inlet doors 30 and two outlet doors 32. The inlet doors 30 define an inlet chamber 34 between them. The outlet doors 32 define an outlet chamber 36 between them. With this structure, it is possible to insert the lump 50 of solid metal material into, and to take the same from the heat-treatment chamber 10, while holding the liquid lithium tanks 12, 14 and 16 in the atmosphere of inert gas.

A transport mechanism 38 is arranged in the heat-treatment chamber 10. The mechanism 38 is designed to transport the lump 50 of solid metal material from the initial heating tank 12 to the cooling tank 14 and thence to the re-heating tank 16, the lump 50 being immersed in the liquid lithium contained in each tank.

An embodiment of the method of heat-treating solid alloy material according to this invention will be described, with reference to the flowchart of FIG. 1.

First, a lump of solid metal material, such as titanium alloy, is rolled into a plate having a prescribed thickness in step S1. Alternatively, the lump is cut or shaped in the first step S1. Namely, a mechanical process is performed on the lump of solid metal material, providing a part that has a size and a shape, both prescribed. Processing strain is accumulated in the lump of metal. This helps to form nuclei that develop into fine crystals, ultimately strengthening the metal material. In FIG. 2, the solid metal material to be heat-treated is, for example, a lump 50 of metal.

In the second step S2, contamination and impurities are removed from the surfaces of the solid-metal material lump. This step is important in terms of the purity management of liquid lithium.

The lump 50 of solid metal material is transported into the heat-treatment chamber 10 through the inlet chamber 34. In the third step S3, the lump 50 is then immersed in the liquid lithium contained in the initial heating tank 12. The lump 50 is thereby quickly heated to a temperature higher than the solid solution temperature. At this time, the components of the alloy are instantaneously dissolved entirely, forming a solid solution.

Flash heating and short-time heating suppress the growth of crystal grains. If the solid metal material is, for example, α-β titanium, its phase changes from α-phase, i.e., hexagonal close-packing crystals structure, to β-phase, i.e., body-centered cubic crystal structure. Nonetheless, the growth of β-phase crystals is suppressed.

Next, the lump 50 of solid metal material is taken out of the initial heating tank 12. In the fourth step S4, the lump 50 is immersed in the liquid lithium contained in the cooling tank 14. The alloy components of the lump 50 are quickly cooled to a temperature lower than the solid solution temperatures at which they become solid solutions. The quick cooling suppresses the growth of α-phase crystals of the alloy components. The β-phase crystals in supersaturated state are preserved. That is, the alloy components exist in the form of finer solid-solution crystals.

Subsequently, the lump 50 of solid metal material is pulled out of the liquid lithium contained in the cooling tank 14. In the fifth step S5 (a second cooling step), the lump 50 is cooled to a temperature lower than the melting point of lithium. It is cooled to, for example, room temperature (normal temperature), as it is left to stand in gas. In this step, the lump 50 is held at normal temperature for a predetermined time, rendering the crystals stable. The fifth step S5 is not limited to spontaneous cooling. Instead, it may be forced cooling. Further, the fifth step S5 may not be carried out at all.

After the fourth step S4 or the fifth step S5, the lump 50 of solid metal material can be subjected to mechanical process (not shown) such as rolling, imparting strain in the lump 50 in order to make the crystals smaller. This is desirable for the mechanical process, because the lump 50 is relatively soft, particularly at this time.

Then, in the sixth step S6, the lump 50 of solid metal material is immersed in the liquid lithium contained in the re-heating tank 16 and is heated again. In this step, a chemical compound structure is formed between the metal and the alloy elements, by virtue of aging. As a result, the crystals of the lump 50 become still smaller. This enhances the strength of the lump 50.

Next, the lump 50 of solid metal material is transported from the heat-treatment chamber 10 through the outlet chamber 36. The lump 50 is then cooled to normal temperature in the seventh step S7. This cooling is performed in, for example, a gas atmosphere of normal temperature. Namely, the lump 50 is left to stand in gas.

In the eighth step S8, lithium is removed from the lump 50 of solid metal material. This step can be water washing or ultrasonic-wave washing.

As indicated above, the seventh step S7 (spontaneous cooling) and the eighth step S8 (removal of lithium and surface washing) may be carried out after the lump 50 of solid metal material is transported out of the heat-treatment chamber 10. Alternatively, these steps can be performed in the heat-treatment chamber 10.

The sixth step S6 (re-heating) need not be performed quickly. Hence, the lump 50 can be heated again not in a bath of liquid lithium. Rather, it can be heated again in an electric furnace or a vacuum furnace. In such a case, the lump 50 of solid metal material can be brought into the heat-treatment chamber 10 before it is subjected to the sixth step S6 (re-heating).

As shown in FIG. 2, the solid metal material may be a metal lump 50. Instead, the solid metal material may be a wire, a bar or a plate. In such a case, the portions of the material (not shown) are sequentially transported to the initial heating tank 12, the cooling tank 14 and the re-heating tank 16. Thus, the material can be continuously heat-treated. Although not illustrated in FIG. 2, the heat-treatment chamber 10 may be a glove-box type one. If so, a worker can manually move the lump 50 of solid metal material, not using the transport mechanism 38.

As described above, the material undergoes flash heating, quick cooling, and re-heating, which use liquid lithium, acquiring an extremely great strength.

Some examples of applying the titanium alloy that has been heat-treated as described above will be now described. These examples make use of the properties of the titanium alloy, i.e., small specific gravity and great strength achieved by the heat treatment.

[Application to Material of Golf-Club Head]

Golf clubs should be so designed that its user may hit a ball to land it at a long distance ahead. It is therefore important to lighten the club head as much as possible. Then, the speed of the head increases and the rebound coefficient of rebound increases at the ball-hitting face surface when the golfer swings the club. Conventionally, the face surface was made as hard as possible to increase the coefficient of rebound, utilizing the elasticity of the ball. Recently, it is desired that the club should have a thinner face that has high elasticity. This is because a face surface of high elasticity has been found to have a larger coefficient of rebound.

FIG. 4 shows the relation between the face thickness and flexure of the face of a golf-club head. Obviously, the thinner the face, the greater the flexure is. Thus, the face should be as thin as possible in order to raise the coefficient of rebound. If the face is thin, however, the stress that the face material receives when it touches a ball will increase. It is therefore demanded that the face be made of stronger material.

To meet this demand, titanium alloys are recently used as material of gold-club heads. Titanium alloys are chemically very active and reactive, however. When products made of titanium alloy are heat-treated by the conventional methods, a thick oxide film is formed on the surface as they are heated and cooled. The oxide film is hard to be removed. Further, the quality of the products is much influenced by the change or unevenness of the temperature distribution in the heat-treatment furnace. The temperature control of the conventional heat-treatment furnace is insufficient. This is why the conventional furnace is not generally employed to heat-treat titanium alloys to be used on golf clubs.

Heat-treated titanium alloy is used on some golf clubs. During the conventional heat treatment, however, hydrogen or oxygen is like to diffuse into the alloy while the alloy is heated to high temperature. As a consequence, the alloy thus heat-treated cannot attain a sufficient strength.

If heat-treated by the method according to this invention, titanium alloy material can acquire greater strength than hitherto possible. The method of the invention can therefore helps to provide golf-club heads that are thinner and yet stronger than the existing golf-club heads and enable the users to land the ball at a longer distance.

FIG. 5 depicts a wood-type, golf-club head made by using titanium alloy that has been heat-treated by the method according to the present invention. The golf-club head has a hollow structure, having a cavity 61. The golf-club head comprises a hosel 60, a crown 62, a sole 63 and a face 64. The hosel 60 is connected to a shaft (not shown). The crown 62, which defines the upper part of the cavity 61, is fused with the hosel 60. The sole 63 is connected to the crown 62, defining the lower edge of the cavity 61. The face 64 is connected to the crown 62 and sole 63, defining the front part of the cavity 61. These parts of the golf-club head are welded to one another. The face 64 is a plate-shaped part that bends on touching a ball (not shown) and then pushes the ball by virtue of its elasticity.

This golf-club head is made of titanium alloy material heat-treated by the method according to the invention. The material is stronger than the conventional titanium alloy material. Therefore, the wall thickness of the golf-club head can be made thinner than that of conventional golf-club heads. Hence, the head can be lighter than the conventional ones.

Particularly, the face 64, which has a small wall thickness, not only serves to lighten the golf-club head, but also greatly bends upon touching a ball. In other words, the face 64 can have a large coefficient of rebound. In view of this, only the face 64 may be effectively made of the titanium alloy material heat-treated by the method of this invention.

[Application to Material of Mechanical Parts]

Titanium alloys have a large intensity ratio (strength per unit density). They are therefore used to lighten the bodies of vehicles, the fuselage and wings of aircraft, engines, and the like. The titanium alloy material heat-treated by the method of this invention has a larger intensity ratio than the conventional titanium alloys. It can therefore find use in the car bodies, fuselages, wings, engines and the like.

FIG. 6 is a cross-sectional view of a gasoline engine of a vehicle to which the method of heat-treating solid alloy material according to the invention is applied. A piston 72 in a cylinder 71 is connected to a crankshaft 74 by a connection rod 73. As the piston 72 reciprocates up and down in the cylinder 71, the crankshaft 74 is driven to rotate. An intake pipe 76 and an exhaust pipe 77 are connected to a cylinder head 75 mounted on the top of the cylinder 71. An intake valve 78 is provided to open and close the intake pipe 76. An exhaust valve 79 is provided to open and close the exhaust pipe 77. The intake valve 78 and exhaust valve 79 open and close the pipes 76 and 77, respectively, at times determined by the rotation of a cam 80 and a valve locker 81 driven by the cam 80. The force with which the cam 80 pushes the valve locker 81 is controlled by a valve spring 82 and a retainer 83.

Thus, the lightening of the valves 78 and 79, the valve spring 82, the retainer 83, the connection rod 73 and the piston 72, which reciprocate or swing, thereby minimizing their inertia, greatly contributes to the enhancement of the engine performance. In view of this, it is preferable to apply the method of heat-treating solid metal material according to this invention to at least some of these parts.

FIG. 7 is a cross-sectional view of an airplane jet engine to which the method of heat-treating solid alloy material according to the invention is applied. Fan blades 87 collect air coming to the engine front. Part of the air thus collected is compressed in a compression zone 40. The air, thus compressed, flows into a combustion chamber 41 provided at the back of the compression zone 40. In the chamber 41, the air is mixed with fuel, combusting the fuel, generating high-temperature high-pressure combustion gas. The gas turns the turbine 42 located at the rear of the combustion chamber 41. It is desired that the fan blades 87, the compressor blades 88 and the compressor blade disc 89, i.e., movable parts in the low-temperature zone, and a fan case 90 and vanes 91, i.e., stationary parts in the low-temperature zone, be made of solid alloy material heat-treated by the method according to this invention.

Thus, the airplane can be lightened as a whole when the method of heat-treating solid alloy material according to this invention is applied to parts of the jet engines of the airplane. Moreover, when the rotating parts of each jet engine are thus made light, their centrifugal force decreases. They can then rotate at high speed, which helps enhance the performance of the engine.

[Application to Implant Material]

Most titanium alloys are light and strong. In addition, they exhibit no toxicity to human being. They are therefore used as materials of implants such as parts of artificial bones. The titanium alloy material heat-treated by the method according to this invention has a larger intensity ratio than the conventional titanium alloys. Therefore, it can be used as material of implants.

FIG. 8 is a vertical cross-sectional view of an artificial hip joint to which the method of heat-treating solid alloy material according to the present invention is applied. The artificial hip joint comprises a receptacle 94, a caput 96 and a stem 98. The receptacle 94, caput 96 and stem 98 are made of titanium alloy and ceramic or resin. The titanium alloy excels in affinity to human tissues. The ceramic and resin excel in abrasion resistance. The receptacle 94 is embedded (implanted) in a haunch bone 95 and has a recess 99. The stem 98 is embedded in a thighbone 97. The caput 96 extends from the stem 98 and is fit in the recess 99 of the receptacle 94 and can slide in the recess 99. Since the titanium alloy can have elongation and Young's modulus that are similar to those of man's bones, the artificial hip joint using the titanium alloy can be comfortable artificial bones.

The method of heat-treating solid alloy material according to the present invention can control the physical properties of titanium alloy, ultimately providing artificial bones similar to man's bones in physical properties.

EXAMPLE 1

The heat treatment shown in FIGS. 1 and 3 was performed on Ti-6A1-4V that is an α-β titanium alloy. FIG. 9 shows the results of the heat treatment.

That is, FIG. 9 shows the 0.2%-proof stress and tensile strength, both measured of Ti-6A1-4V alloy before and after the heat treatment. FIG. 9 also shows the 0.2-proof stress and tensile strength of a titanium alloy heat-treated by the conventional method, which are disclosed in Table 4.3 of Toshiyuki Suzuki, Yasuo Moriguchi, The Story of Titanium, revised edition, page 76. As evident from FIG. 9, the titanium alloy heat-treated by the method of this invention is far superior in both proof stress and tensile strength.

[Advantages of the Invention]

In the present invention, rapid heating and cooling can be easily performed on solid alloy material, so quickly as was impossible with the conventional method. The alloy material can thereby attain strength far greater than was possible with the conventional method. Particularly, the use of liquid lithium in heat-treating the solid alloy material is advantageous in the following respects.

(1) Having a boiling point (1342° C.) of lithium higher than the boiling point (881° C.) of sodium, liquid lithium can achieve a heat treatment at higher temperatures.

(2) Being liquid and having high thermal conductivity, metal lithium can heat the material uniformly.

(3) Generating less vapor than liquid sodium, liquid lithium is consumed in a smaller amount.

(4) Liquid lithium can be easily removed from the product by water washing.

(5) Less reactive with water than liquid sodium, liquid lithium is easy to handle.

(6) Having half the specific gravity of liquid sodium, liquid lithium is easy to transport and handle.

(7) Less oxidized in air than liquid sodium, liquid lithium does not undergo spontaneous ignition and is therefore easy to handle.

(8) Liquid lithium reduces oxide, if any, on the surface of a titanium alloy product, thus making the product clean (because lithium is more readily oxidized than titanium).