Single-crystal – oriented-crystal – and epitaxy growth processes; – Processes of growth from liquid or supercritical state – Having moving solid-liquid-solid region
Reexamination Certificate
1999-07-09
2001-04-03
Hiteshew, Felisa (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Processes of growth from liquid or supercritical state
Having moving solid-liquid-solid region
C117S046000, C117S050000, C117S051000, C117S052000
Reexamination Certificate
active
06210478
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to cold-crucible induction melting techniques and apparatuses therefor. More particularly, this invention is directed to the use of a horizontal floating-zone cold crucible for at least levitating and locally melting a material by induction heating to refine and/or analyze the material, and particularly for the purpose of removing insoluble inclusions while leaving desired alloying constituents in tact.
Vertical cold-crucible induction levitation melting techniques are known, an example of which is U.S. Pat. No. 3,702,368 to Hukin. With such techniques, a material is placed in a vertically-oriented metal crucible, melted and then slightly levitated out of contact with the crucible by a radio frequency field generated by an induction coil surrounding the crucible. The walls of the crucible are water-cooled and segmented, the latter of which enables induction heating to occur through the metal crucible walls by interrupting induced current flow in the walls that would otherwise attenuate the induction coil field.
Melting by induction levitation has also been used to purify materials such as silicon (e.g., UK Patent Application 2207061) and high-temperature reactive metals such as titanium, niobium and chromium and their alloys by a process known as floating-zone refining. The chemical reactivity of titanium, niobium and chromium can degrade conventional oxide crucibles as a result of a high negative free energy of the reactive metal oxide formation. While melting and casting operations can be performed in an inert atmosphere or vacuum to avoid reactions with gaseous oxygen and nitrogen, oxygen and nitrogen are generally nonetheless available either in the form of impurities in the processing environment or from the crucible. Significant degradation of the crucible and contamination of a reactive melt becomes more likely as the melting temperature of the alloy and the concentration of reactive elements in the alloy increase. For these reasons, floating-zone refining has been used to purify reactive materials, during which a molten zone is traversed from one end of the material to the other, where elemental impurities become concentrated as a result of solubility differences of the impurities between the liquid and solid phases of the material.
Zone refining can be categorized as either horizontal floating-zone or vertical floating-zone, depending on the orientation of the material and the direction in which the molten zone moves during refining. In both techniques, the material is completely surrounded by the crucible walls in order to contain and achieve a substantially uniform temperature in the molten zone. While horizontal and vertical floating-zone refining techniques have been developed to produce very pure ingots of high-temperature elemental materials that contain soluble elemental impurities, they have not been successfully employed for removing minimal solubility inclusions from alloys. Inclusions of interest include oxides, carbides, nitrides, silicides, carbonitrides, oxynitrides and oxycarbides, which occur in alloys such as nickel-based and cobalt-based superalloys and titanium alloys. Accordingly, it would be desirable if a technique were available for enabling zone refining of high-temperature alloys, including alloys containing reactive elements, for the purpose of removing insoluble inclusions from an alloy but not its desired alloying constituents. It would also be desirable if such a technique employed an improved crucible design that enabled additional processes to be carried out on the alloy being refined.
SUMMARY OF THE INVENTION
According to the present invention, there is provided a method for locally and successively melting regions of an alloy by induction heating using a horizontal floating-zone crucible to refine and/or analyze the alloy. The invention is particularly directed to the removal of insoluble inclusions from high-temperature reactive metals and their alloys, nickel-based and cobalt-based superalloys, and other materials containing reactive elements. As used herein, reactive elements will be understood to mean those elements that readily react with or dissolve species within the processing environment, including crucibles and molds, such as silicon, aluminum, calcium, oxygen, nitrogen, carbon and magnesium.
The method of this invention generally entails the use of a crucible having a chamber with horizontally and oppositely-disposed first and second ends, and having means for generating an electromagnetic field within the chamber. A material is then placed within the chamber, and an electromagnetic field is generated to fully or at least partially levitate the material within the container and to create a localized molten zone within the material, preferably starting at or near one end of the material. The localized molten zone is then translated horizontally through the material in a direction toward an oppositely-disposed second end of the material adjacent the second chamber end, progressively melting successive local regions of the material. Each successive region first melts and then resolidifies as the localized molten zone traverses it.
According to one aspect of the invention, the material is zone refined in a manner that causes physical separation of insoluble inclusions within the molten material as a result of the inclusions having lower densities than the molten material. The inclusions float to the top of the localized molten zone, and are driven toward the one end of the material as the molten zone is translated along the material. This process has been shown to be particularly useful for removing low-density solid inclusions, such as oxides, carbides, nitrides, silicides, carbonitrides, oxynitrides, oxycarbides and mixtures thereof from reactive metals and alloys containing reactive elements. To promote refining, the horizontal chamber of the crucible preferably has a longitudinal opening located within its upper circumferential region, which causes a transverse temperature gradient to exist between the surface of the material facing the crucible wall and the surface of the material facing the opening, with that portion of the material facing the opening being hotter. According to this invention, inclusions have been found to be driven to and become concentrated at the surface of the material facing the opening.
According to another aspect of the invention, the process can be used to determine the solidus and liquidus temperatures of a non-congruent melting alloy by visual and pyrometric inspection of the alloy during melting and resolidification. For this purpose, the longitudinal opening noted above allows for optical (e.g., visual and pyrometric) inspection of the alloy as it is heated through its solidus and liquidus temperatures, which correspond to the extremes of a visible mushy zone of the alloy. By enabling both visual and pyrometric inspection, the locations of the mushy zone extremes can be visually ascertained and the solidus and liquidus temperatures corresponding to these extremes can be measured.
In view of the above, it can be seen that a significant advantage of this invention is that an alloy containing highly reactive but intentionally added alloying constituents can be purified and/or analyzed using a crucible that has a horizontally-oriented melt chamber, as opposed to vertical chambers used by the prior art for these materials. Unlike vertical floating-zone techniques that rely only on surface tension to stabilize the melt within the floating zone, the horizontal floating-zone technique of this invention also makes use of the electromagnetic field to support and stabilize the molten region. Because of the typically elongated sample geometry, there is less of a tendency for distortion or break-up of the melt during processing in accordance with this invention.
Other objects and advantages of this invention will be better appreciated from the following detailed description.
REFERENCES:
patent: 2875108 (1959-02-01), Pfann
patent: 3702368 (1972-11-01), Hu
Bewlay Bernard Patrick
Dalpe Dennis Joseph
Lipkin Don Mark
General Electric Company
Hiteshew Felisa
Johnson Noreen C.
Stoner Douglas E.
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