Process for crushing hafnium crystal bar

A process for crushing a hafnium (Hf) crystal bar comprises the steps of maintaining the Hf crystal bar at an extremely low temperature by holding the crystal bar in contact with a cryogenic refrigerant and crushing the crystal bar at the extremely low temperature by clamping and compressing the crystal bar between nickel (Ni)-base superalloy members. An apparatus for crushing the Hf crystal bar comprises a Ni-base superalloy-made container for containing the cryogenic refrigerant, the container having a bottom portion capable of being selectively opened and closed, a heat insulator for covering the container filled with the cryogenic refrigerant so as to maintain the interior of the container at the extremely low temperature, Ni-base superalloy-made pressing terminals for clamping the Hf crystal bar therebetween in the container, and a pressing device for exerting pressure on the pressing terminals so as to compress and crush the Hf crystal bar.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to a process for crushing a hafnium crystal bar, and 
more particularly to a process for crushing a hafnium crystal bar in order 
to produce a starting material for the production of a high-purity fine 
powder of hafnium having superior toughness and heat resistance. 
2. Background Art 
Recently, hafnium (Hf) has drawn attention in various fields because of its 
superior toughness and heat resistance. For instance, in the field of 
precision casting, unidirectionally solidified materials of super 
heat-resistant nickel-base alloys with Hf contained therein are being 
commercialized. In the field of powder metallurgy, also, not only 
Hf-containing heavy alloys and dispersion-strengthened alloys but HfC- or 
HfN-containing composite carbides are being commercialized. 
In the former case, hafnium has been added in the form of crystal bars in 
the production of a master ingot as a starting material or a raw material. 
The Hf crystal bars in their uncrushed state have led to low yields or 
have caused segregation. 
In the latter case, on the other hand, it has been the common practice to 
reduce a hafnium salt by hydrogen to form Hf or then form a carbide 
therefrom. In the process of production of the alloys or carbides, 
however, the decomposition or escape of unrequired elements or groups 
contained in the Hf salt has often resulted in formation of vacancies and 
a disordered crystal structure in the final product. 
The above-mentioned problems are solved if there is a crushed product of Hf 
crystal bars of maximum purity as the starting or raw material. Because of 
the high hardness, high toughness and the close-packed hexagonal crystal 
structure of the Hf crystal bars, however, there has not been a 
conventional technique to crush the Hf crystal bars, and commercialization 
has therefore been carried out simply by crushing Hf sponge. 
When the Hf sponge is crushed for use as a raw material for a variety of 
uses, the physical properties and workability of the final product are 
lowered, because of the high nitrogen and oxygen contents of the raw 
material and the susceptibility of hafnium to the effects of interstitial 
impurities such as nitrogen and oxygen. 
In addition, in the process of producing the Hf sponge, chlorine and 
magnesium are left in the Hf sponge. Therefore, the Hf sponge has a high 
content of chlorine and magnesium, which leads to a deterioration of the 
physical properties of the final product. 
SUMMARY OF THE INVENTION 
It is accordingly an object of this invention to provide a process for 
crushing a hafnium crystal bar by which it is possible to obtain a crushed 
product of Hf crystal bars of maximum purity as a raw material. 
Because of the high hardness, high toughness and the close-packed hexagonal 
crystal structure of the hafnium crystal bars, it has not hitherto been 
contemplated to crush the hafnium crystal bars by utilizing 
low-temperature brittleness. One aspect of the present invention therefore 
resides in recognition that the embrittling effect of low temperature on 
hafnium can be positively used, which effect has heretofore been 
considered to be slight. 
One mode of the process for crushing a hafnium crystal bar according to 
this invention comprises the steps of maintaining the Hf crystal bar at an 
extremely low temperature by holding the crystal bar in contact with a 
cryogenic refrigerant, and crushing the Hf crystal bar at the extremely 
low temperature by clamping and compressing the crystal bar between nickel 
(Ni)-base superalloy members. In this process, with the Hf crystal bar 
maintained at the extremely low temperature by holding the crystal bar in 
contact with the cryogenic refrigerant, the low-temperature embrittlement 
effect is enhanced, and the heat generation upon application of pressure 
to the crystal bar is restrained. In this condition, the Hf crystal bar is 
clamped and compressed between the Ni-base superalloy members, whereby the 
Hf crystal bar is crushed through the generation of permanent strain, 
because the Ni-base superalloy is superior to hafnium in hardness and 
toughness and is insusceptible to low-temperature embrittlement. 
The process of the present invention may be carried out using apparatus for 
crushing a hafnium crystal bar which comprises a container made of a 
Ni-base superalloy for containing a cryogenic refrigerant, the container 
having a bottom portion capable of being opened and closed as desired, a 
heat insulator for covering the container filled with the cryogenic 
refrigerant so as to maintain the interior of the container at an 
extremely low temperature, pressing terminals made of a Ni-base superalloy 
for clamping the Hf crystal bar therebetween in the container, and 
pressing means for exerting a pressure on the pressing terminals to 
compress and crush the Hf crystal bar. In this apparatus, the container is 
formed of the Ni-base alloy, whereby the cryogenic refrigerant is safely 
contained. With the container covered by the heat insulator, the interior 
of the container filled with the cryogenic refrigerant is maintained at 
the extremely low temperature. The Hf crystal bar is clamped between the 
Ni-base superalloy-made pressing terminals in the interior of the 
container maintained at the extremely low temperature, and a pressure is 
exerted on the pressing terminals by the pressing means to compress the Hf 
crystal bar, whereby the Hf crystal bar is crushed through the generation 
of permanent strain therein. Since the bottom portion of the container is 
capable of being opened and closed as desired, it is easy to remove the 
crushed Hf crystals from the container. 
As has been described above, according to this invention, it is possible to 
obtain a crushed product of Hf crystal bars of maximum purity as a raw 
material.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
One preferred embodiment of this invention will now be described below, 
based on the accompanying drawings. 
Referring first to FIGS. 1 and 2, the present description deals with one 
embodiment of the apparatus for crushing a hafnium crystal bar used in 
carrying out the process according to this invention. As shown in the 
figures, disposed on a base 1 is a crushing container 3 for containing a 
cryogenic refrigerant 2 therein. The cryogenic refrigerant 2 may be, for 
example, liquid argon. The container 3 is formed of a Ni-base superalloy, 
and comprises a side wall consisting of a tubular cylinder 4a and a 
circular disk-like bottom portion 4b. The cylinder 4a is, for example, 100 
mm in diameter and 180 mm in height. The cylinder 4a is detachably fitted 
to the bottom portion 4b. The outer periphery of the side portion of the 
container 3 is covered with a heat insulator 5 so as to maintain the 
interior of the container 3 at an extremely low temperature. A hafnium 
crystal bar 7 to be crushed is disposed in the container 3. A pair of 
circular disk-like pressing terminals 8 for clamping the Hf crystal bar 7 
therebetween are provided in the container 3. The pressing terminals 8 are 
formed of a Ni-base superalloy. As shown, the pressing terminals 8 are 
located respectively on the upper and lower sides of the Hf crystal bar 7. 
The pressing terminal 8 on the lower side is disposed on the bottom 
portion 4b of the container 3, whereas the pressing terminal 8 on the 
upper side is contacted by pressing means 9 which exerts a pressure on the 
upper pressing terminal 8 to compress and crush the Hf crystal bar 7 
clamped between the upper and lower pressing terminals 8. Pressing means 9 
is employed that includes a press head 10 of a 300-ton press (300-T press) 
which is 98 mm in diameter. Numeral 11 in the figure denotes a pressing 
guide as an aid to vertical compression and stroke in the container 3. 
The process for crushing a hafnium crystal bar according to this invention, 
as carried out with the use of the apparatus constructed as described 
above, will now be explained in detail below referring to FIG. 3. First, 
the Hf crystal bar 7 with a 35 mm diameter is cut (20) to a size of 
40.+-.5 mm by a high-speed cutter. Next, the thusly cut Hf crystal bar 7 
is mixed with dry ice within a heat-insulated, hermetically sealed 
container (not shown) separately prepared, followed by sealing off the 
heat-insulated, hermetically sealed container to perform primary cooling 
(21) to a temperature of -50 degrees C. (.degree.C.). The Hf crystal bar 7 
subjected to primary cooling (21) then undergoes secondary cooling (22) to 
a temperature of about -150.degree. C. or below by placing the crystal bar 
7 in another heat-insulated, hermetically sealed container filled with 
liquid argon and sealing off the liquid argon-filled container. After the 
secondary cooling (22), the lower pressing terminal 8 is disposed on the 
bottom portion 4b in the crushing container 3. The Hf crystal bar 7 which 
had been subjected to the second cooling (22) is then placed on the lower 
pressing terminal 8, and the upper pressing terminal 8 is located on the 
Hf crystal bar 7 to clamp the Hf crystal bar 7 between the pressing 
terminals 8. Simultaneously, liquid argon is poured into the container 3 
to bring the Hf crystal bar 7 into contact with the cryogenic refrigerant 
2, thereby maintaining the Hf crystal bar 7 at an extremely low 
temperature of not higher than -150.degree. C. The container 3 is made of 
the Ni-base superalloy, whereby the cryogenic refrigerant 2 is safely 
contained. Further, with the container 3 covered with the heat insulator 
5, the interior of the container 3 filled with the cryogenic refrigerant 2 
is maintained at the extremely low temperature of -150.degree. C. or 
below. Thereafter, a pressure of about 9 kg/mm.sup.2 is exerted on the 
upper pressing terminal 8 by the press head 10 of the 300-T press used as 
the pressing means 9, thereby compressing the Hf crystal bar 7 in a single 
direction by the upper and lower pressing terminals 8, with the result of 
crushing (23) of the Hf crystal bar 7. When the Hf crystal bar 7 is 
maintained at the extremely low temperature through contact with the 
cryogenic refrigerant 2 such as liquid argon, the low-temperature 
embrittlement effect is enhanced, and the heat generation upon application 
of the pressure to the crystal bar 7 is restrained. When the Hf crystal 
bar 7 in this condition is clamped and compressed between the upper and 
lower pressing terminals 8 made of the Ni-base superalloy, the Hf crystal 
bar 7 is crushed through the generation of permanent strain, because the 
Ni-base superalloy is superior to Hf in hardness and toughness and is 
insusceptible to low-temperature embrittlement. The cylinder 4a of the 
container 3 not only contains the cryogenic refrigerant 2 but serves to 
aid the vertical compression and prevent the scattering of the crushed Hf 
crystals. The steps of primary cooling (21), secondary cooling (22) and 
low-temperature crushing (23) are repeated in series three or four times. 
It is possible to perform a continuous crushing of three or four pieces of 
the cut Hf crystal bars 7. Subsequently, the cylinder 4a of the container 
3 is detached from the bottom portion 4b, and the crushed Hf crystals are 
swifly taken out and are stored (24) in a circulating type desiccator (not 
shown). 
The characteristic values in this invention are optimal values obtained 
from various experimental results. The basic feature of the values lies in 
that the Hf crystal bar 7 is cooled to and maintained at a temperature of 
not higher than -150.degree. C. to embrittle the crystal bar 7 and to cool 
the large quantity of heat generated upon release of the bonding energy of 
the Hf crystal, thereby enhancing the crushing efficiency so as to enable 
crushing of the Hf crystal bar under a compressive pressure of about 9 
kg/mm.sup.2. A temperature higher than -150.degree. C. hinders the 
enhancement of the embrittling effect and makes it impossible to crush the 
Hf crystal bar with a compressive pressure less than about 9 kg/mm.sup.2. 
The crushed Hf crystal product thusly obtained has the following merits. 
When the crushed product is used as is as an alloying additive in the 
production of a master ingot for obtaining precision castings, such as 
directionary solidified castings or single crystal castings, or in the 
production of an electrode alloy for obtaining a forging alloy, a high 
yield can be expected in comparison with the prior approach of adding Hf 
crystal bars. Namely, whereas the yield with the addition of the Hf 
crystal bars is 70 to 80%, the yield with the addition of the crushed Hf 
crystal product produced according to this invention is 99 to 100%. For 
this purpose, Hf sponge with a high N, O, Cl or Mg content is not usable. 
In addition, the crushed Hf crystal product produced according to this 
invention may be used as a raw material in a "Process For Producing 
High-Purity Fine Powder of Reactive Metal and Apparatus Therefor," 
disclosed in Japanese Patent Application Nos. 210620/1988 and 218486/1988, 
respectively filed on Aug. 26, 1988 and Sept. 2, 1988, both owned by the 
present assignee, the entire disclosures of which are incorporated by 
reference herein. When the crushed Hf crystal product is used after being 
pulverized by the process for producing a high-purity fine powder of a 
reactive metal, the fine powder obtained is usable as a raw material for a 
variety of uses. It is impossible to compare such a use with a 
corresponding use according to the prior art because there is not any 
conventional use of the Hf crystal material in pulverized form as a raw 
material. The use of the crushed Hf crystal product obtained according to 
this invention after pulverization as a raw material, however, definitely 
leads to markedly suppressed penetration of impurity elements into the 
atomic arrangement of the final product, as compared to the case where 
hafnium carbide (HfC) is used as a raw material, namely, the case where a 
Hf compound is reduced by hydrogen to where Hf and HfC is produced 
therefrom. Moreover, when the crushed Hf crystal product obtained 
according to this invention is used after pulverization as a raw material, 
the final product obtained is free of disorder in the arrangement of atoms 
arising from the escape of impurity elements or formation of vacancies and 
has stable qualities and properties with good reproducibility.