Method for fabrication of superplastic composite material having metallic aluminum reinforced with silicon nitride

A method for the fabrication of a superplastic composite material having metallic aluminum reinforced with silicon nitride includes thoroughly mixing silicon nitride with metallic aluminum, pressure-sintering the resultant mixture, further heating and pressing the sintered mixture, hot extrusion-molding the resultant sintered article, subjecting the molded article, when necessary, to a heat treatment such as the T6 treatment thereby forming a superplastic composite material, and deforming the composite material in a temperature region in which the material exhibits superplasticity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the fabrication of a superplastic 
composite material having metallic aluminum reinforced with silicon 
nitride whiskers or minute silicon nitride particles by utilizing its own 
superplasticity. 
2. Description of the Prior Art 
The conventional ceramic short fiber-reinforced aluminum-based composite 
material exhibits high specific strength and high specific elastic modulus 
and excels in abrasion resistance and heat resistance and, therefore, 
promises extensive utility as a structural material for the aerospace 
industry. This composite material, however, is deficient in capacity for 
fabrication. 
In view of the deficiency, there have been made various studies on the 
production of composite materials exhibiting superplasticity. 
Typical superplastic composite materials so far developed as the result 
are: (a) a composite material of 2124 Al alloy reinforced with silicon 
carbide (SiC) whiskers [T. G. Nieh, C. A. Henshall and J. Wadsworth, 
"Superplasticity at high strain rates in a SiC whisker reinforced Al 
alloy" Scripta Metallurgica Vol. 18 (1984 pp 1405-1408], (b) a composite 
material of 7475 Al alloy reinforced with SiC whiskers [M. W. Mahoney and 
A. K. Ghosh, "Superplasticity in SiC reinforced Al alloys" Six 
International Conference on Composite Materials ICCM & ECCM Vol. 2 
(1986)], and (c) a composite material of 7064 Al alloy reinforced with 
minute SiC particles [M. W. Mahoney and A. K. Ghosh, "Superplasticity in a 
high strength powder aluminum alloy with and without SiC reinforcement" 
Metallurgical Transactions A Vol. 18A (1987) p 653]. 
These composite materials are invariably manufactured by a complicated 
process called a fabrication heat-treatment method (solid-solution 
treatment aging treatment--hot rolling work--recrystallization treatment). 
Further, it has been reported that the transformation superplastic 
fabrication method (the technique of inducing superplastic deformation by 
repeating a heating treatment and a cooling treatment alternately) brings 
about superplastic deformation in (d) a composite material of 6061 Al 
alloy reinforced with SiC whiskers. 
The superplastic material which is produced by this method, however, has a 
slow deforming speed. The composite material (a) indicated above, when 
subjected to superplastic deformation, tends to form a liquid phase and 
give rise to cavities in the metallic Al matrix and, after the 
superplastic deformation, suffers from deficiency in mechanical 
properties. 
The composite material (b) indicated above is manufactured by a procedure 
which comprises applying the whiskers to a foil of the 7475 Al alloy and 
causing the applied whiskers to disperse on and adhere fast to the foil. 
Thus, it is considered to have dubious stability in mechanical properties. 
The present inventors formerly invented a method for the production of a 
superplastic composite material having metallic aluminum reinforced with 
silicon nitride and excelling the known superplastic composite materials 
in practicability, applied this invention for a U.S. patent under Ser. No. 
07/497,884 and won the issue of a notice of allowance dated June 20, 1990. 
SUMMARY OF THE INVENTION 
The inventors have found that the superplastic composite material 
manufactured in accordance with the formerly invented method exhibits in a 
uniaxial tensile test high ductility exceeding 200% in terms of the amount 
of deformation when the temperature and the deforming speed are limited to 
respective specific values. 
Moreover, in this case, since the superplastic temperature region falls 
below the solidus curve, the matrix is in a solid state and the region of 
superplastic strain speed is as high as to fall on the order of 10.sup.-1 
(1/second). 
This invention has been perfected on the basis of this knowledge. 
To be specific, this invention is directed to a method for the fabrication 
of a superplastic composite material having metallic aluminum reinforced 
with silicon nitride, which comprises wet-mixing silicon nitride of at 
least one member selected from the group consisting of powder of 50 .mu.m 
under and whiskers with metallic aluminum powder of 50 .mu.m under by the 
use of a solvent, then depriving the resultant mixture of the solvent, 
pressuresintering the mixture freed of the solvent under a vacuum, further 
heating and pressing the resultant sintered mixture, hot extrusion-molding 
the sintered article, subjecting the resultant molded article, when 
necessary, to a heat treatment such as the T6 treatment thereby forming a 
superplastic composite material having the metallic aluminum reinforced 
with silicon nitride, and deforming the superplastic composite material in 
a temperature range in which the material exhibits superplasticity. 
The above and other objects and features of the invention will become 
apparent from the following detailed description with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The superplastic composite material obtained during the course of the 
method of this invention exhibits a high superplastic strain speed in its 
superplastic temperature region. This high superplastic strain speed is 
considered to be ascribable to the grain boundary slippage occurring 
between adjacent minute crystal particles in the material. 
Now, the method of this invention will be described in detail below. 
The composite material produced during the course of the method of this 
invention has metallic aluminum as its matrix. Though the aluminum purity 
of this metallic aluminum is not specifically defined, it is practically 
desired to be not less than 80%, preferably not less than 85%. If the 
aluminum purity is unduly low, the produced composite material is 
deficient in the characteristics of the metallic Al as the matrix. 
The silicon nitride is incorporated in the composite material for the 
purpose of enhancing the strength of the produced composite material. It 
is not allowed, however, to bring about any adverse effect upon the 
superplasticity of the produced composite material. 
The largest allowable content of silicon nitride in the composite material 
is approximately 35% by volume. Preferably, the silicon nitride content is 
in the range of 15 to 20% by volume. 
For the present invention, it is necessary that the metallic aluminum 
powder and silicon nitride should be thoroughly mixed. To ensure 
thoroughness of mixture, they must be subjected to wet mixing. 
Specifically, the thorough mixture can be attained, for example, by 
placing the metallic aluminum powder and silicon nitride in such an 
organic solvent as alcohol or acetone and subjecting them as contained in 
the solvent to a treatment using ultrasonic waves. Then, the resultant 
mixture having the metallic aluminum powder and silicon nitride contained 
in the organic solvent is deprived of the organic solvent, to obtain a 
homogeneous mixture of the two components. This mixture is 
pressure-sintered and the resultant sintered mixture is further heated and 
pressed. Practically, the conditions for the heating and pressing are at 
least 200.degree. C. and at least 50 MPa, preferably 400.degree. to 
650.degree. C. and 300 to 500 MPa. The sintered article consequently 
obtained is treated again practically under the conditions of at least 
200.degree. C. and at least 50 MPa, preferably 400.degree. to 650.degree. 
C. and 300 to 500 MPa, and is then hot extrusion-molded. The practical 
conditions for the extrusion molding are at least 5 of extrusion ratio and 
at least 300.degree. C. of temperature, preferably 30 to 100 of extrusion 
ratio and 400.degree. to 600.degree. C. of temperature. The extrusion 
molded mixture, when necessary, is subjected to a heat treatment such as 
the T6 treatment. 
In consequence of the series of treatments mentioned above, there is 
obtained a superplastic composite material having metallic aluminum 
reinforced with silicon nitride. The superplastic temperature region of 
the composite material is such that the matrix phase of the material falls 
below the solidus curve in the phase diagram. In this specific superplastic 
temperature region, the plastic strain speed is high. 
Specifically, when the metallic aluminum of the 6000 and 7000 series of the 
AA Standard is used, the superplastic temperature region is 500.degree. to 
560.degree. C. and the 
strain speed in that temperature region is at least 10.sup.-1 (1/second). 
When the composite material containing the 6061 aluminum alloy is subjected 
to tensile deformation at the superplastic temperature of 545.degree. C., 
it exhibits a strain speed of 1.5.times. 10.sup.-1 (1/second). In the case 
of the composite material containing the 7064 aluminum alloy subjected to 
tensile deformation at the superplastic temperature of 525.degree. C., the 
strain speed thereof is found to be 1.7.times. 10.sup.-1 (1/second). 
Now, the present invention will be described more specifically below with 
reference to working examples. 
EXAMPLE 1 
Silicon nitride whiskers and powder of the 6061 aluminum alloy of the AA 
Standard having a particle size of not more than 44 .mu.m were measured 
out in volumes such that the whiskers accounted for a volume content of 
20%, and they were homogeneously mixed in ethanol as exposed to vibration 
of ultrasonic waves. The resultant mixture was deprived of ethanol and 
then dried. The resultant dried powdery mixture was pressure-sintered in a 
hot press under a vacuum at 600.degree. C. and 200 MPa for 20 minutes. 
Subsequently, in the open air, the sintered mixture was compressed again 
at 600.degree. and 400 MPa for 20 minutes. The compressed sintered mixture 
was placed in an aluminum tube, hot extrusion-molded statically at 
500.degree. C. at an extrusion ratio of 44 and then subjected to the T6 
heat treatment (8 hours' standing at 500.degree. C. followed by water 
cooling and 16 hours' standing at 500.degree. C. followed by air cooling), 
to obtain a superplastic composite material. 
When the composite material was deformed by tension at 545.degree. C., it 
showed a strain speed of 1.5.times. 10.sup.-1 (1/second) and a total 
elongation of 250%. Since the solidus curve temperature of the 6061 
aluminum alloy is 582.degree. C., the results obtained herein indicate 
that the matrix phase underwent superplastic deformation in the solid 
state. Thus, the possible degradation of mechanical properties after the 
superplastic deformation could be precluded. 
FIG. 1 is an 800-magnification photomicrograph illustrating a metallic 
texture of a superplastic composite material produced in Example 1. In the 
photomicrograph, the black parts represent silicon nitride whiskers and the 
white parts the matrix of the 6061 Al alloy. 
FIG. 2 is a graph showing the relation between the total elongation (%) and 
the strain speed obtained in the tensile deformation at 545.degree. C. of 
the composite material produced in Example 1. 
EXAMPLE 2 
Silicon nitride whiskers and powder of the 7064 aluminum alloy of the AA 
Standard having a particle size of not more than 44 .mu.m were measured 
out in volumes such that the whiskers accounted for a volume content of 
20%, and they were subjected sequentially to mixing--pressure 
sintering--second compression--hot extrusion molding--T6 heat treatment 
under the same conditions as in Example 1, to obtain a composite material 
of the 7064 Al alloy reinforced with silicon nitride whiskers. When the 
composite material was deformed in the open air at 525.degree. C., it 
showed a strain speed of 1.7.times. 10.sup.-1 (1/second) and a total 
elongation exceeding 200%, a sign characteristic of superplastic 
deformation. Again in this case, the superplastic temperature region was 
below the solidus curve. The composite material deformed exhibited 
substantially the same strain speed as in Example 1. 
FIG. 3 is a graph showing the relation between the total elongation (%) and 
the strain speed obtained in the tensile deformation at 525.degree. C. of 
the composite material obtained in Example 2.