Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
1999-10-18
2002-05-14
Jenkins, Daniel J. (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C419S012000, C419S013000, C419S014000, C419S029000, C148S514000
Reexamination Certificate
active
06387196
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a particle-reinforced titanium alloy which is reinforced by ceramic particles having a thermodynamically stable property in titanium alloy.
2. Description of the Related Art
There has been known particle-reinforced titanium alloy which is reinforced by particles. As a technique for producing this type titanium alloy, Japanese Unexamined Patent Publication 10-1,760 has been provided. This Patent Publication technique includes: (1) using titanium alloy which is reinforced by dispersing ceramic particles having a thermodynamically stable property, such as titanium boride, in a matrix, and (2) heat-treating this titanium alloy to dissolve a colony grain structure and to generate a minute-acicular &agr; phase structure. According to the process disclosed in this publication, the above-mentioned particle-reinforced titanium alloy is produced by way of the steps including: (1) heating the titanium alloy in a temperature range not less than &bgr;-transus temperature; (2) quenching the titanium alloy with water from the temperatures range not less than &bgr;-transus temperature to room temperature or to under room temperature; and (3) heating the titanium alloy in a two phase region of (&agr;+&bgr;) formed between &bgr;-transus temperature and 800° C. The quenching step indicates a considerably rapid cooling rate.
Also, Japanese Unexamined Patent Publication 3-73,623 discloses another process for heat-treating a &agr;+&bgr; type titanium alloy. This process includes: (1) heating the titanium alloy having a &agr;+&bgr; type in a temperature range which is 10-60° C. lower than &bgr;-transus temperature; and (2) cooling the titanium alloy at a cooling rate of 0.1-5° C./second to less than 500° C. so as to improve toughness thereof. When heating temperature is not less than &bgr;-transus temperature, a phase of &bgr; easily becomes a large-size. In this publication technique, it is guessed that heating temperature is set at temperatures which is 10-60° C. lower than &bgr;-transus temperature for avoiding a large-sized phase of &bgr;.
SUMMARY OF THE INVENTION
The technique disclosed in Japanese Unexamined Patent Publication 10-1,760 intends to improve fatigue strength of titanium alloy; however, it does not intend to improve creep resistance. When the heat treatment disclosed in this publication is carried out, acicular &agr; phases are parted and then change into broken-up structures; therefore, creep property is deteriorated in spite of high fatigue strength. Generally, it is thought that a finer microstructure leads to improved fatigue strength and that a larger microstructure leads to suppressed creep deflection and improved creep resistance.
Also, the technique disclosed in Japanese Unexamined Patent Publication 3-73,623 intends to improve toughness; however, it does not intend to improve creep resistance. Further, the titanium alloy disclosed in this publication does not contain particles such as titanium boride particles, and heating temperature does not exceed &bgr;-transus temperature.
The present invention has been accomplished in view of the aforementioned circumstances. It is therefore an object of the present invention to provide a process for producing particle-reinforced titanium alloy which is excellent in creep resistance while ensuring fatigue strength.
The present inventors have eagerly researched titanium alloy and have developed the present invention by experimentally confirming the following phenomenon. When the present inventors have carried out: using titanium alloy in which ceramic particles are dispersed having a thermodynamically stable property; heating the titanium alloy in a temperature range of not less than &bgr;-transus temperature; and cooling the titanium alloy at a cooling rate of 0.1-30° C./second: titanium alloy is improved in creep resistance while ensuring fatigue strength.
The reason for obtaining the above-mentioned characteristics is not surely clear. However, this reason is guessed as follows:
It is thought that a larger microstructure contributes to suppress creep deflection and to improve creep resistance, and that a finer microstructure contributes to improve fatigue strength. The present invention uses the titanium alloy in which ceramic particles having a thermodynamically stable property are dispersed. Therefore, the present invention prevents abnormal growth of the old of &bgr; grain, in spite of the complete acicular formation of microstructures, even when the titanium alloy is heated in a temperature range of not less than &bgr;-transus temperature. Also, since the titanium alloy is cooled from the range of not less than &bgr;-transus temperature, and since the titanium alloy passes through &bgr;-transus temperature at an appropriate cooling rate of 0.1-30° C./second, the microstructure size of titanium alloy is appropriate in such a manner that both creep resistance and fatigue strength are ensured.
The present invention provides a process for producing a particle-reinforced titanium alloy, which comprises the steps of: heating a titanium alloy in which ceramic particles having a thermodynamically stable property are dispersed in a temperature range of not less than &bgr;-transus temperature; and cooling the heated titanium alloy to pass through the &bgr;-transus temperature at a cooling rate of 0.1-30° C./second.
The present invention can provide a particle-reinforced titanium alloy in which creep resistance is excellent while fatigue strength is ensured.
PREFERABLE MODE OF THE PRESENT INVENTION
The present invention employs titanium alloy in which ceramic particles having thermodynamically stable property are dispersed.
The titanium alloy may be a sintered compact formed by sintering a green compact, a forged product formed by forging the sintered compact, a cast product, or a forged product formed by forging the cast product. As for forging, hot forging can be used.
The titanium alloy can include an &agr; phase-stabilizing element, for example aluminum (Al), and a &bgr; phase-stabilizing element. The titanium alloy can contain, at least, 3-6% of aluminum (Al), and 2-6% of tin (Sn) by weight, when a matrix of titanium alloy is 100% by weight. However, the present invention process is not limited within these contents.
The microstructure of matrix of the titanium alloy in an ordinary-temperature region may be a microstructure wholly formed of &agr; phases, a microstructure mainly formed of &agr; phases, or a microstructure formed of &agr; phases being mixed with &bgr; phases. The &agr; phase may be an acicular &agr; phase, or an acicular &agr; phase mixed with an equi-axed &agr; phase.
The ceramic particles having a thermodynamically stable property may be titanium boride (TiB and TiB
2
), titanium carbide (TiC and TiC
2
), titanium silicide, and titanium nitride (TiN). In particular, titanium boride is preferable in such ceramic particles. Titanium boride can work as a hard particle or a reinforcing particle in a matrix of titanium alloy. Titanium boride has good congeniality for the matrix of titanium alloy; so, it is suppressed that a weak reactive phase for causing fatigue crack is formed in an interface between the titanium boride and the matrix of the titanium alloy.
Proportion of the ceramic particles having a thermodynamically stable property, such as titanium boride, can be chosen depending on applications, etc. An upper limit of the proportion may be 10% or 7% by volume, and a lower limit may be 0.1% or 0.4% by volume, in the case where the whole titanium alloy with ceramic particles dispersed therein is 100% by volume. However, the proportion of the ceramic particle is not limited within these ranges.
An average particle size of ceramic particles having a thermodynamically stable property, such as titanium boride, can be chosen depending on applications, etc. For example, an upper limit of the average particle size of the ceramic particle may be 50 &mgr;m. A lower limit of the average particle size of
Furuta Tadahiko
Saito Takashi
Sakurai Kouji
Yamaguchi Toshiya
Jenkins Daniel J.
Oliff & Berridg,e PLC
Toyota Jidosha & Kabushiki Kaisha
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