Process for densifying powder metallurgical product

A process for densifying a powder metallurgical product comprising steps of preparing a powdery starting material, pre-sintering the powdery starting material at a relatively low temperature, executing a pore-eliminating process for eliminating pores resulting from the preceding step on the powdery starting material, and sintering the powdery starting material at a relatively high temperature. It is beneficial to produce a product having a large dimension, a desired shape, and excellent mechanical properties, and being appropriate for or capable of suffering any post-treatment.

FIELD OF THE INVENTION 
The present invention relates to a process for manufacturing a product, and 
more particularly to a process for densifying a powder metallurgical 
product. 
BACKGROUND OF THE INVENTION 
Ni--Al intermetallic compounds, such as Ni.sub.3 Al or the like, have been 
attracting people's attentions for advanced applications recently due to 
their many extraordinary properties, such as a high melting point without 
the transformation of the solid solution commonly observed in nickel-base 
superalloys, and increasing yield strength with temperature due to 
thermally activated cross-slip pinning process. In addition, the intrinsic 
brittleness of polycrystalline Ni.sub.3 Al compound at ambient 
temperatures has been eliminated by microalloying with boron (B, 0.1 
percent by weight). These make it extremely attractive for aviation and 
structural applications at elevated temperatures. However, the superalloy 
used in turbine industries has not been replaced by Ni.sub.3 Al. The major 
problem in manufacturing with melting and casting technique is the strong 
tendency of Al to oxidize at elevated temperatures, which causes 
metal-crucible and metal-ceramic interactions during vacuum melting and 
vacuum investment casting, respectively, as during the regular processing 
of turbine blades. The increasing yield strength with temperature 
characteristics also causes the problem of selecting the suitable die 
material for the post-ingot wrought deformation as a wrought material 
after ingots have been formed. 
Powder metallurgical methods have been alternatively studied by melting raw 
metals into an alloy and atomizing the alloy into intermetallic powders. 
It is therefore called pre-alloying powdering method. The thus obtained 
intermetallic powders adopted as a starting material for powder 
metallurgical process have the following shortages of suffering from: 
1. a required high energy consumption due to an additional process for 
melting; 
2. difficulties on molding to powder compacts due to the hardness of the 
obtained pre-alloying powders; 
3. a high wearing rate caused by them to the mold; 
4. a tendency of getting oxidized; and 
5. a required high sintering temperature. 
There is another method called "mechanical alloying" to finely grind and 
uniformly mix pure elemental powders by using a high power ball mill. 
However, the hardening of the ground powder particles makes them not easy 
to be molded and shaped. In addition, the contamination resulting from the 
oxidization of the powders during grinding and the degradation of the 
surface of the balls in the ball mill or of the inner wall of the ball 
mill are unavoidable. Instead, some of those skilled in the art also set 
forth the related study by taking pure elemental powders, such as pure Ni 
and pure Al powders, as a starting material. Three of the representative 
prior arts, Powder Metal. Int., Vol. 20, No. 3, 25, 1988, J. of Metals, 
14, Sep., 1988, and U.S. Pat. No. 4,762,558, all were disclosed by R. M. 
German et al., report a process, called reactive sintering, executed under 
a low sintering temperature by taking advantage of the evolved heat and a 
temporarily formed transient liquid phase during the reaction of the 
powders. This method is still far from serving as a practical usage, and 
lacks reports about the mechanical properties of the sintered products to 
be foundedly supported. Furthermore, a large amount of pores, about 20% of 
pore density, are formed when the sintering temperature is directly raised 
to about or above 800.degree. C. Besides, a compound NiAl is possibly 
formed accordingly to provide a product, being hard and having a low 
ductility, difficult for further processing. Even at an elevated 
temperature, the product is still hard. Owing to the low ductility and the 
high pore density of the sintered product, the product is too hard to 
process and too brittle to free from cracking so that the cracks of the 
product have already been resulted before the pore having been able to be 
healed during processing. It is a common problem people have to face upon 
manufacturing an intermetallic compound product. 
In summary, the shortages of the prior processes include: 
1. The product produced thereby has defects in structure; 
2. The product produced thereby has poor mechanical properties; 
3. One cannot control the temperature distribution in the product during 
the sintering process so that it is unable to inhibit the formation of the 
unwanted compounds; 
4. The product produced thereby is unsuitable for further hot or cold 
processes; 
5. It is unable to effectively eliminate the pores in the formed product; 
6. The product produced thereby is difficult to be molded or shaped; and 
This invention is affordable to improve the product density to prevent the 
product from cracking and capable of solving the aforementioned problems. 
SUMMARY OF THE INVENTION 
An object of the present invention is to offer a process for densifying the 
obtained product. 
Another object of the present invention is to offer a process to obtain a 
product having excellent mechanical properties. 
Another object of the present invention is to offer a process to obtain a 
desired product by effectively controlling the temperature distribution 
therein. 
Another object of the present invention is to offer a process to obtain a 
product suitable for further hot or cold processes. 
Another object of the present invention is to offer a process to 
effectively eliminate the pores in the obtained product. 
Another object of the present invention is to offer a process capable of 
easily molding or shaping the product. 
In accordance with the present invention, a process for densifying a powder 
metallurgical product comprising: preparing a powdery starting material, 
compressing and mixing the powdery starting material, introducing a heat 
absorbent to be in contact with the powdery starting material, 
pre-sintering the powdery starting material at a relatively low 
temperature, executing a pore-eliminating process for eliminating the 
preceding formed pores on the pre-sintered powdery starting material, 
sintering the pre-sintered powdery starting material at a relatively high 
temperature, and proceeding another pore-eliminating process for the 
sintered powdery starting material and an annealing process for further 
annealing the sintered powdery starting material. 
In accordance with another aspect of the present invention, the powdery 
starting material comprises Ni and Al, Fe and Al, Ti and Al, or Ni and Ti 
elemental powders. 
In accordance with another aspect of the present invention, the 
pre-sintered powdery starting material has a relative small amount of a 
high-Ni content compound and a relative large amount of a low-Ni content 
compound wherein the high-Ni content compound is Ni.sub.3 Al and the 
low-Ni content compound is Ni.sub.2 Al.sub.3 or NiAl.sub.3 wherein 
Ni.sub.2 Al.sub.3 is preferably. 
In accordance with another aspect of the present invention, the sintering 
process results in a reaction of the low-Ni content compound with the Ni 
powders. 
In accordance with another aspect of the present invention, the 
pore-eliminating process is capable of condensing the pre-sintered powdery 
starting material to have a reduced cross-section and is rolling, 
calendering, drawing, extruding, forging, or pressing, and the 
pore-eliminating process is either a cord or a hot deformation process. 
In accordance with another aspect of the present invention, a temperature 
of the pre-sintered powdery starting material is controlled under 
800.degree. C. and preferably under 700.degree. C. 
In accordance with another aspect of the present invention, the heat 
absorbent is a material being inert to the powdery starting material and 
is a ferrous alloy, a stainless steel, Cu, Cu-based alloys, Ni, Ni-based 
alloys, or a mixture thereof. 
In accordance with another aspect of the present invention, the heat 
absorbent is in contact with the powdery starting material in a way of 
encompassing an outer surface of the powdery starting material or being 
embedded to an interior of the powdery starting material, wherein the 
interior of the powdery starting material is a tubular hollow. 
In accordance with another aspect of the present invention, the heat 
absorbent is formed as a tube, a sealing bag, a washer, a liner, a mold, 
or a combination thereof. 
In accordance with another aspect of the present invention, the heat 
absorbent further contains a cooling system therewith comprising a pipe 
and a coolant flowing therethrough. 
In accordance with another aspect of the present invention, the relatively 
low temperature is ranged from 500.degree. C. to 800.degree. C. and 
preferably ranged within 650.degree..+-.50.degree. C. 
In accordance with another aspect of the present invention, the relatively 
high temperature is ranged from 1000.degree. C. to 1465.degree. C. and 
preferably ranged within 1133.degree. C. to 1250.degree. C. 
In accordance with another aspect of the present invention, the powdery 
starting material is further introduced therein with additional elementary 
powders such as pure B powders or Ni--B alloy powders. 
In accordance with another aspect of the present invention, the additional 
elementary powders are, via a process of mixing or electroless plating 
processes, introduced into the powdery starting material. 
In accordance with another aspect of the present invention, the 
pre-sintering process gives an intermediate product of Ni.sub.2 Al.sub.3 
+B. 
In accordance with another aspect of the present invention, a residual 
unreacted Ni phase is formed after said pre-sintering process. 
In accordance with another aspect of the present invention, the sintering 
process gives a final product having a dual phase. 
In accordance with another aspect of the present invention, the sintering 
process gives a final product selected from a group consisting of Ni.sub.3 
Al+0.1% B and a two-phase mixture of Ni.sub.3 Al+0.1% B and NiAl+0.1% B. 
The present invention may be best understood through the following 
description with reference to the accompanying drawings, in which:

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
This invention discloses a method comprising multi-stage sintering 
processes. In a primary experiment by the Inventors, a temperature of 
about 650.degree. C. (slightly below the melting point of pure Al) was 
controlled for the preliminary sintering process. The formed pores within 
the preliminary phases of the compact powders were collapsed with plastic 
deformation after the preliminary sintering process. Then a high 
temperature normal sintering process, a single or multiple cycles of a 
pore-eliminating process and an annealing process were applied to the 
compact powders. We found that the obtained first semi-product (sintered 
first compact powders without any heat absorbent), if being subjected to a 
cold rolling, cracked when suffering only a minor deformation of about 
7.5%. After being identified via an X-ray diffraction analysis of the 
first semi-product, the cause of cracking was found to be attributed to 
the brittleness thereof resulting from the formation of a NiAl phase, as 
shown in FIG. 4 and in FIGS. 1a-1c. Therefore, there would be problematic 
to proceed the original experiment. After further studies, the Inventors 
found that when the compact powders without any heat absorbent were 
sintered at 650.degree. C., the evolved heat gave rise to an elevation of 
a local temperature of the sintered compact powders to a temperature above 
800.degree. C., as shown in FIG. 3. The phenomenon of locally heating the 
sintered compact powders would be effectively overcome by introducing a 
heat absorbent inert to the compact powders. The heat absorbent, such as a 
stainless steel shell, a tube, or a mold, when encompassing the compact 
powder, absorbed or dispersed the heat evolved from the reaction to lower 
the temperature of the compact powders to about 700.degree. C., as shown 
in FIG. 6. Through an X-ray diffraction analysis, we found in the 
semi-product with a heat absorbent that there existed a Ni.sub.2 Al.sub.3 
phase of a large amount and a Ni.sub.3 Al phase of a small amount, as 
shown in FIG. 7. When the semi-product was subject to a cold work, it did 
not crack or break even under a deformation of above 30%. Therefore, the 
porosity of the compact was effectively collapsed and reduced. The 
subsequent normal sintering process was executed at an elevated 
temperature of about 1200.degree. C. to transform phases of the 
semi-product into a Ni.sub.3 Al phase and to form a liquid phases from 
Ni.sub.2 Al.sub.3 which fills into the cracks and crazes formed in the 
compact to eliminate them. During the normal sintering process, a less 
amount and size of pores were formed and were easily eliminated through a 
post-treatment such as a cold work, annealing, or homogenization process. 
Accordingly, the purpose for densifying a Ni--Al intermetallic compound 
having a high strength through a powder metallurgical process was 
satisfied. 
In general, this invention is achieved via a way of controlling the 
temperature of the compact powders during the preliminary sintering 
process to prevent the compact from dramatically elevation of its 
temperature and to inhibit the formation of a high-Ni content compound 
such as NiAl which is notoriously responsible for causing the brittleness 
to the compact powders so that they are unsuitable for further processing 
to mechanically eliminate or heal the pores therein. A metal or the other 
material inert to the starting material is utilized as an heat absorbent 
which is capable of absorbing or dispersing instead of liberating heat 
during reaction to decrease the reaction rate of the starting materials. 
The semi-product obtained thereby has a large amount of a low-Ni content 
compound which is anticipated to assure the toughness, softness, and 
ductility for the semi-product. Therefore, it is capable of suffering 
deformation caused by any of cold or hot works such as a rolling, forging, 
calcining, extruding, drawing, or pressing process to reduce or eliminate 
a large number of pores remaining in the semi-product. The semi-product is 
further subjected to a second stage process of a normal sintering. The 
low-Ni content compounds react with the unreacted pure Ni powders in the 
semi-product to give a product having a phase of Ni.sub.3 Al during the 
normal sintering process. The product is subject to a pore-eliminating 
process and an annealing process to be further densified. The 
aforementioned processes to achieve for the product the above-mentioned 
characteristics therewith construct and promote the multi-stage sintering 
powder metallurgy (MSPM) process of this invention. 
It is shown in FIG. 1 that the strength of a Ni.sub.3 Al intermetallic 
compound increases with the elevation of the environmental temperature, 
which differs remarkably from any of the prior results. It is due to the 
dislocation cross-slip pinning process in the micro-structure occurring at 
an elevated temperature. In FIG. 1, a strength curve versus temperature of 
Type 316 stainless steel is compared to that of one product according to 
the prior disclosures. In the applications of the aerospace industry, a 
high-temperature durable material such as a Ni-based alloy has a high 
strength due to the fact that it has a secondary phase of Ni.sub.3 Al 
around 40% or less in volume. The Ni.sub.3 Al compound can also be 
produced if adopting a process of mixing Ni and Al powders and then 
directly raising the temperature to above 1000.degree. C. (or about 
1200.degree. C. in general) to reactively sinter the mixed powders, or 
adopting a process of sintering the mixed powder at a lower temperature of 
about 650.degree. C. and homogenizing it at an elevated temperature of 
about 1000.degree. C. However, the resultant pores in the product are 
still significant and cause the product to have a porosity of about 20% or 
more, i.e., the product density of Ni.sub.3 Al is less than 80% referring 
to the theoretic density, so that the size or scale of the product is 
unable to be precisely controlled. To seek after for the reasons, it can 
be explained from the Table 1 which shows the published or calculated 
volume changes of various Ni and Al compounds during reaction. 
TABLE 1 
______________________________________ 
Volume change of various compounds of Ni and Al 
Number Reaction Volume change (%) 
______________________________________ 
1 3Al + 2Ni .fwdarw. Al.sub.3 Ni.sub.2 
-4.2 
2 Al.sub.3 Ni.sub.2 + Ni .fwdarw. 3NiAl 
-9.72 
3 NiAl + 2Ni .fwdarw. Ni.sub.3 Al 
-2.73 
______________________________________ 
Practically, it has been observed in experiments that there were still 
5%-15% of porosity more than those listed above. The melting point of Al 
is about 660.degree. C. When Al reacts and incorporates with Ni, the heat 
evolving from reaction causes an elevation of the specimen temperature. If 
the reaction is proceeded at a temperature about 600.degree. C., the 
evolved heat will raise the temperature to cause the Al particle to form a 
transient liquid phase, as shown in FIG. 2-(a) and 2-(b). The Al particle 
of the transient liquid phase will diffuse and penetrate into the space 
among adjacent Ni powder grains through a capillary force, as shown in 
FIG. 2-(c). A diffused Al grain reacts with its surrounding Ni atoms to 
form an Al.sub.3 Ni.sub.2 phase and leaves a pore of almost the same size 
as that of the Al grain at its original site, as shown in FIG. 2-(d). The 
pore has to be healed during the powder metallurgical process so that the 
obtained product has required density and mechanical strength. The 
Inventors tried to mix and compact Al powders together with Ni and Ni-B 
alloy powders to obtain a specimen, designated as A-specimen hereinafter. 
In the experiment, A-specimen was heated up and pre-sintered to a 
temperature about 650.degree. C. and the temperature of A-specimen was 
recorded as a function of time, as shown in FIG. 3. It is shown from FIG. 
3 that a reaction occurred when the temperature was raised only to about 
600.degree. C., and the temperature was dramatically raised by an 
increment of about 200.degree. C. with the assistance of the exothennic 
heat from the reaction. An X-ray diffraction analysis of pre-sintered 
A-specimen, as shown in FIG. 4, shows that a large amount of NiAl and a 
small amount of Ni.sub.3 Al phases, both of which are high-Ni content 
compound, were formed and an unreacted pure Ni phase was remained. 
Preliminarily sintered A-specimen was further subjected to a light cold 
rolling and had a reduction of about 7.5% in area, as shown in FIGS. 
1a-1c. Although a large amount of unreacted pure Ni existed, due to the 
presence of a brittle NiAl phase in A-specimen, it could be found that 
many cracks and crazes were formed on A-specimen. From photographs taken 
at a rolling surface and a longitudinal cross-section, crazes developing 
along the direction perpendicular to the rolling direction could be seen 
and the crazes are concentrated at the NiAl phase and its surroundings. 
That is, cracks and crazes easily occur at the NiAl phase. Growth of the 
cracks in the specimen makes the specimen be formed into broken pieces. 
Basically, the pure two-stage sintering process cannot manufacture thereby 
an ideal Ni--Al intermetallic compound through the powder metallurgical 
processes depicted and discussed above. 
We may look at the phase diagram of the Ni--Al binary system as shown in 
FIG. 5. A Ni.sub.2 Al.sub.3 phase exists at the left side of the NiAl 
phase. The Ni.sub.2 Al.sub.3 phase did not cause people's attention 
because of its low melting point of about only 1133.degree. C. The 
Inventors tried another experiment to let Al, Ni, and Ni.sub.3 B powders 
be mixed and compacted, and then the compact powders were canned into a 
304 stainless steel washer. It is designated as B-specimen. The weight 
ratio of the compact powders and the stainless steel is about 1:1.+-.10%. 
B-specimen was subjected to a preliminary sintering at a temperature of 
about 650.degree. C. The variation of the temperature of the compact 
powders was recorded, as shown in FIG. 6. The compact powders did not 
react until the temperature reached about 620.degree. C. Owing to the 
majority of the exothermic heat resulting from the reaction was absorbed 
and diluted by the 304 stainless steel shell, the specimen temperature 
raised only about 70.degree. C. The X-ray diffraction analysis of 
B-specimen after the preliminary sintering process was shown in FIG. 7. It 
shows that a large amount of a low-Ni phase such as a Ni.sub.2 Al.sub.3 
phase and a residual unreacted pure Ni phase were formed. While 
investigating the SEM photograph of B-specimen after the preliminary 
sintering process, shown in FIGS. 2c-2f, the B-specimen containing a large 
amount of Ni.sub.2 Al.sub.3 did not develop any severe crack thereon after 
suffering a 30% deformation from processing and its integrity was 
maintained. Pores formed thereon in the preliminary sintering process were 
collapsed and the powder particles adjacent to each pore were jointed to 
each other during the work. The collapsed pores were healed during a 
subsequent normal sintering process at an elevated temperature. Although 
some minor crazes were formed on the B-specimen, the propagation of the 
developing crazes was confined by the surrounding Ni phase. 
The main feature and the technique of this invention are to adopt a 
multi-stage sintering process to manufacture a Ni--Al intermetallic 
compound having a high density and a high strength. In the multi-stage 
sintering process, a powdery starting material comprising Ni and Al 
powders is mixed and compacted into a compact powders and placed in 
contact with a heat absorbent which is inert to the compact powders. The 
heat absorbent is sufficient to absorb and dilute the exothermic heat 
resulting from the reaction of Ni with Al in the compact powders to 
prevent the temperature thereof from being excessively increased so that 
the desired pre-sintered semi-product having a low-Ni content compound 
such as a Ni.sub.2 Al.sub.3 phase and a pure Ni phase is obtained during 
the preliminary sintering process. The pre-sintered semi-product has a 
reduced area and forms big pores therein due to the particles of the 
transient Al liquid phase penetrating and diffusing into the space among 
its adjacent particles. The desired pre-sintered semi-product has a tough 
low-Ni content compound phase to offer its required toughness to be free 
from cracking and a soft pure Ni phase to offer its ductility to be able 
to suffer any hot or cold work such as a rolling, forging, extruding, 
drawing, calendering, or pressing. Therefore, the pre-sintered 
semi-product will not crack or break during the pore-eliminating process 
and be capable of being shaped to obtain a desired or a similar shape of 
the final product. Further subjecting the semi-product to a normal 
sintering process will heal the collapsed pores and transform the low-Ni 
content compound reacting with the pure Ni powders into a sintered product 
having a desired phase such as a Ni.sub.3 Al phase. If a temperature above 
1133.degree. C. is used for normal sintering, say 1200.degree. C., the 
liquid phased from Ni.sub.2 Al.sub.3 will be developed and be much 
beneficial to heal or to eliminate any pores, cracks, or crazes. If adding 
B or Ni.sub.3 B powders into the starting material, the sintered product 
will have a Ni.sub.3 Al+B phase. The sintered product can be subjected to 
one or more works or annealing treatments to thoroughly eliminate the 
minute pores caused by the volume constriction of the product to increase 
its density. Through the aforementioned processes, a product of a Ni--Al 
intermetallic compound having a large dimension and any desired geometric 
shape, such as a sheet, a plate, a rod, a fiber, a wire, or a tube, is 
obtained accordingly. FIG. 8 illustrates a process, according to the 
present invention, starting from mixing the weighted Al, Ni, and B or 
Al.sub.3 B powders into a mixture, and FIG. 9 presents a process, 
according to the present invention, setting forth from Ni--B electroless 
plating one or both of the Al and Ni powders, drying them, vacuum 
degassing, e.g., dehydrogenating, from them by heating, and then mixing 
them into a mixture. The mixture obtained in either way depicted above, is 
shaped into a powder compact by a cold rolling process or by other cold 
work. The shaped compact attaching thereto a heat absorbent is subjected 
to a preliminary sintering process, and then the heat absorbent is removed 
from the compact before the subsequent pore-eliminating process being a 
hot or cold work, a normal sintering process, and one or more cycles of 
pore-eliminating and annealing processes. 
The present invention will now be described more specifically with 
reference to the following examples. It is to be noted that the following 
descriptions of examples including preferred embodiments of this invention 
are presented herein for purpose of illustration and description only; it 
is not intended to be exhaustive or to be limited to the precise form 
disclosed. 
EXAMPLE 1 
(A) Preparation of an intermetallic mixture: 
Take and mix 76.84 gm of Ni powders having a purity of above 99.9% and an 
average diameter of about 5 .mu.m, 11.40 gm of Al powders having a purity 
of above 99.5% and an average diameter of about 22 .mu.m, and 1.87 gm of 
Ni.sub.3 B powders having a purity of above 99.5% and an average diameter 
of about 60 .mu.m as a starting material. The starting material is placed 
into a cylindrical polyethylene mixer to proceed a mixing process at a 
speed of 90 rpm for about 2 hours. The thoroughly mixed powder specimen 
has a composition of Al being 24.0 at %, B being 0.12 wt %, and Ni being 
the remaining. 
(B) Shaping: 
The mixed powder specimen is thermally treated to desorb the gases from 
them in a vacuum environment and at about 400.degree. C., then is canned 
and mechanically sealed into a 304 stainless steel tube in air. The tube 
and the specimen contained therein are cold rolled with a deformation of 
about 60% into a piece of a steel jacket having the specimen therein. The 
weight ratio of the specimen to the stainless steel is about 1:1.04. 
(C) Preliminarily reactive sintering: 
The obtained steel jacket with the specimen therein is put into a vacuum 
thermal furnace for a preliminary sintering process at about 650.degree. 
C. for about 30 minutes. The elevation rate of the furnace temperature is 
about 10.degree. C./min. During the process, the temperature variation of 
the specimen is measured by a inserted K-type thermocouple and a 
PC-controlled multi-meter (Model: HP-3457A of Hewlett Packard). 
(D) Rolling and homogenization: 
The pre-sintered specimen is stripped off the steel jacket and subjected to 
a first cold rolling with a deformation of about 30%, then is put into a 
vacuum thermal furnace for a normal sintering at a high temperature of 
about 1200.degree. C. for about 2 hours for the purposes of healing pores 
and transforming the specimen to obtain a uniformly distributed Ni.sub.3 
Al product. 
(E) Testing: 
The mechanical properties of the produced ASTM standard specimen at each 
stage, as shown in FIG. 10, are obtained by an MTS test machine. The 
pre-sintered specimen, named as Specimen S1 hereinafter, has an ultimate 
tensile strength of about 567.+-.8 MPa, an elongation of about 
2.9.+-.0.5%, a relative density of about 93.86.+-.0.03% referring to the 
theoretic density. The specimen, after the normal sintering process, 
further suffering a second cold rolling with a reduction of about 15% in 
area and a first annealing process at about 1200.degree. C. for 2 hours 
and being named as Specimen T1 hereinafter, has an ultimate tensile 
strength of about 654.+-.14 MPa, an elongation of about 8.4.+-.0.5%, and a 
relative density of about 96.29% referring to the theoretic density. The 
specimen being subjected to a third cold rolling with a reduction of about 
10% in area and a second annealing at about 1200.degree. C. for about 2 
hours after experiencing the normal sintering, the second cold rolling, 
and the first annealing, named as Specimen U1 hereinafter, has an ultimate 
tensile strength of about 667.+-.30 MPa, an elongation of about 
9.3.+-.1.9%, and a relative density of about 97.07.+-.0.06% referring to 
the theoretic density. The specimen being subjected to five cycles of two 
additional cold rolling processes of about 8.5% and 7% reduction in area 
and two annealing processes at about 1200.degree. C. for about 2 hours 
after experiencing the normal sintering, named as Specimen V1, has a 
relative density of about 98.60.+-.0.03%. The final product of Specimen V1 
is tested at both room temperature and 800.degree. C. The obtained 
mechanical properties thereof such as the yield strength (YS), the 
ultimate tensile strength (UTS), the Young's modulus (E), and the maximum 
strain, are listed in Table 2. 
TABLE 2 
______________________________________ 
Mechanical properties of Specimen 
V1 at room temperature and 800.degree. C. 
Temperature Max. 
(.degree.C.) 
YS (MPa) UTS (MPa) E (Gpa) 
Strain (%) 
______________________________________ 
25.degree. C. 
433 .+-. 5 
773 .+-. 33 
182 .+-. 4 
16.1 .+-. 1.6 
800.degree. C. 
631 .+-. 21 
631 .+-. 21 
161 .+-. 3 
0.37 .+-. 0.02 
______________________________________ 
EXAMPLE 2 
Prepare a replacing Ni-plating solution and a electroless plating solution 
respectively as listed in Table 3. 
TABLE 3 
______________________________________ 
Compositions and related values of prepared Ni-plating solution 
Replacing Ni-plating Electroless Plating 
solution Value solution Value 
______________________________________ 
Nickel chloride (gm/l) 
30 Nickel chloride (gm/l) 
30 
Sodium citrate (gm/l) 
20 Dimethylaminoborane 
3.5 
(gm/l) 
Ammonium chloride 
7 Malonic acid (gm/l) 
40 
(gm/l) 
Sodium fluoride (gm/l) 
0.5 Thiourea (ppm) 1-4 
pH value 8-9 pH value 6-7 
Reaction temperature 
25 Reaction temperature 
70 
(.degree.C.) (.degree.C.) 
______________________________________ 
Immerse 14.5 gm of Al powders having a size of about 20 .mu.m into the 
replacing Ni-plating solution at room temperature for about 2 hours. The 
Al powders are taken out from the replacing Ni-plating solution and 
water-washed to the neutral state. The pretreated Al powders are subjected 
to a electroless plating process in the electroless plating solution with 
the assistance of a magnetic agitator to mix the Al powders and the 
solution sufficiently. After 20 minutes, add Ni powders of 78.00 gm into 
the solution to adjust the Ni and B contents in the Ni--Al intermetallic 
powders to be finally produced. After the reaction terminates, the 
obtained powders are water-washed and dried. Through a consecutive 
processes, such as canning, sealing, shaping, preliminary sintering, 
rolling, normal sintering, etc., similar to steps (B) to (D) described in 
Example 1, the obtained high-density composite product has a composition 
of Al being 23.89 at %, B being 0.1 wt %, and Ni with other elements being 
the remaining, as listed in Table 4. 
TABLE 4 
______________________________________ 
Composition analysis of the product 
in Example 2 through ICP-AES. 
Ni (at %) 
Al (at %) B (wt %) S (ppm) 
Fe (ppm) 
Cu (ppm) 
______________________________________ 
bal. 23.89 0.13 &lt;10 56 &lt;3 
______________________________________ 
The specimen obtained after the normal sintering process, named as Specimen 
S2, has an ultimate tensile strength of about 724.+-.35 MPa, an elongation 
of about 8.5.+-.0.5%, and a relative density of about 97.8% referring to 
the theoretic density. The specimen being further subjected to a first 
cold rolling with a reduction of about 15% in area and a first annealing 
at about 1200.degree. C. for about 2 hours after the normal sintering, 
named as Specimen T2, has an ultimate tensile strength of about 769.+-.30 
MPa, an elongation of about 14.8.+-.1.95%, and a relative density of about 
98.8% referring to the theoretic density. 
EXAMPLE 3 
Follow a similar procedure as described in Example 1, while pure B powders 
are added during the intermetallic powder preparation step instead. The 
amounts of the starting materials are: 78.60 gm of Ni powders, 11.40 gm of 
Al powders, and 0.108 gm of B powders having a purity of above 99.5% and 
an average diameter less than 60 .mu.m. 
EXAMPLE 4 
Follow a similar procedure described in Example 1, while the amount of Ni 
powders is 72.85 gm and that of Al powders is 15.39 gm during the 
preparation process. The final product of Ni--Al intermetallic compound 
has an Al composition of about 31.0 at % and has a high-density Ni.sub.3 
Al and NiAl dual phase. 
EXAMPLE 5 
Follow a similar procedure described in Example 1, while the amount of Ni 
is 69.14 gm and that of Al is 29.10 gm. The final product of Ni--Al 
intermetallic compound has an Al composition of about 37.0 at % and has a 
high-density Ni.sub.3 Al and NiAl dual phase. The NiAl content of the 
final product is higher than that in Example 4. 
The specimen in Example 5 obtained after the normal sintering process, 
named as Specimen S5, has an ultimate tensile strength of about 431.+-.32 
MPa, an elongation of about 1.63.+-.002%, and a density of about 
6.52.+-.0.01 g/cm.sup.3. The specimen, being further subjected to a first 
cold rolling process with a reduction of about 10% in area and an 
annealing process at about 1200.degree. C. for about 2 hours after the 
normal sintering process, has an ultimate tensile strength of about 
422.+-.21 MPa, an elongation of about 1.38.+-.0.12%, and a density of 
about 6.65.+-.0.02 g/cm.sup.3. These mechanical properties of specimens 
having Ni.sub.3 Al and NiAl dual phases have not been found in any 
disclosure. 
While the invention has been described in terms of what are presently 
considered to be the most practical and preferred embodiments, it is to be 
understood that the invention need not be limited to the disclosed 
embodiment. On the contrary, it is intended to cover various modifications 
and similar arrangements included within the spirit and scope of the 
appended claims which are to be accorded with the broadest interpretation 
so as to encompass all such modifications and similar structures.