Mn-Based alloy of nonequilibrium austenite phase

A Mn-based alloy is disclosed. The alloy is comprised of 4 to 30 atomic % of at least one element selected from the group consisting of Al, Ni, and Cr; 1 to 15 atomic % of C, 30 atomic % or less of at least one element selected from the group consisting of Co, Mo, W, Ta, Nb, V, Ti, and Zr; and the balance of alloy making up 100 atomic % being comprised substantially of Mn. The alloy has a nonequilibrium austenite phase. The alloy disclosed has high ductility and workability. The alloy is capable of being cold worked and has excellent tensile strength. The Mn-based alloy can be produced at substantially the same cost as any Fe-based alloy. The disclosed alloy is a nonmagnetic alloy which has been found to be very useful for nonmagnetic electromagnetic parts and composite materials.

FIELD OF THE INVENTION 
This invention relates to a Mn-based alloy of nonequilibrium austenite 
phase which possesses excellent tensile strength and high ductility. 
BACKGROUND OF THE INVENTION 
Conventional Mn-based alloy assumes an A-12 type .alpha.-Mn structure 
containing 58 atoms in the unit cell at room temperature. Therefore, the 
alloy is too brittle to be normally worked or formed. Therefore, an 
inexpensive Mn-Al powder alloy has been formed to have a small amount of 
utility as a material for a magnet. None of the Mn-based alloys existing 
today possess any appreciable degree of strength, elongation and high 
ductility. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a Mn-based alloy which is very 
rich in ductility and workability, capable of being cold worked, and 
excellent in mechanical properties including tensile strength. Another 
object of this invention is to provide a Mn-based alloy which is useful in 
a great variety of products such as nonmagnetic electromagnetic parts, 
composite materials, and textile materials. 
As a result of diligent efforts to meet the objects described above, the 
present inventors have found that a Mn-based alloy of a specific 
composition, when solidified by quenching, retains intact (even at room 
temperature) an austenite phase which is stable only at elevated 
temperatures. The Mn-based alloy having a nonequilibrium austenite phase 
at room temperature is very rich in ductility and workability and capable 
of being cold worked. 
More specifically, the invention is directed to providing a Mn-based alloy 
having a nonequilibrium austenite phase. The alloy is produced by 
solidifying by quenching an alloy comprising 4 to 30 atomic % of at least 
one element selected from the group consisting of Al, Ni, and Cr, 1 to 15 
atomic % of C, not more than 30 atomic % of at least one element selected 
from the group consisting of Co, Mo, W, Ta, Nb, V, Ti, and Zr. The balance 
of the alloy making up 100 atomic % is comprised substantially of Mn. The 
Mn-based alloy of a nonequilibrium austenite phase provided by this 
invention is very rich in ductility and workability. Furthermore, the 
alloy is capable of being cold worked, excellent in mechanical properties 
including tensile strength, inexpensive and, therefore, is highly used in 
nonmagnetic electromagnetic parts, composite materials, textile materials, 
etc. 
DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
The Mn-based alloy of a nonequilibrium austenite phase according to this 
invention is obtained by quenching and solidifying a molten alloy which 
comprises 4 to 30 atomic % of at least one element selected from the group 
consisting of Al, Ni, and Cr, 1 to 15 atomic % of C, not more than 30 
atomic % of at least one element selected from the group consisting of Co, 
Mo, W, Ta, Nb, V, Ti, and Zr, and the balance to make up 100 atomic % 
being comprised substantially of Mn. 
The Mn-based alloy of the invention will now be described. It contains at 
least one element selected from the group consisting of Al, Ni, and Cr in 
a concentration of 4 to 30 atomic %. The elements and percentages amounts 
described above represent the metallic elements and the amounts which are 
essential for the purpose of enabling a Mn-alloy in a molten state to be 
solidified by quenching into a tough Mn-based alloy, whereby the austenite 
phase which is stable only at elevated temperatures is supercooled intact 
to room temperature. If the atomic % amount of these elements is less than 
4 atomic %, the alloy produced will have very high brittleness because it 
can no longer be expected to enjoy the effect described above and further 
because the alloy suffers precipitation of an .alpha.-Mn phase. 
An alloy which does not contain the indicated atomic % amounts of metals 
cannot be used to produce continuous ribbons and thin wires of fixed 
shapes. If the concentration exceeds 30 atomic %, the alloy to be produced 
is rigid and brittle due to the precipitation of an intermetallic compound 
MnX (X denoting Ni, Al, or Cr). Thus, the alloy lacks practicability. The 
elongation of the alloy material is effected by the amounts of Al, Ni 
and/or Cr to be present. As the amount of these components increases, the 
elongation of the alloy decreases. The tensile strength is not affected 
much by the content of Al and is liable to increase with the content of Ni 
and/or Cr. The Mn-based alloy of this invention further contains C in a 
concentration in the range of 1 to 15 atomic %. The carbon must be present 
in this amount to enable the austenite phase to be cooled down intact to 
room temperature when the molten Mn-based alloy is solidified by 
quenching. If the concentration is less than 1 atomic %, the alloy 
obtained produces the nonequilibrium austenite phase at room temperature 
with difficulty and shows very high brittleness because the quenching does 
not easily manifest its effect. If the concentration exceeds 15 atomic %, 
the produced alloy is brittle due to the precipitation of a carbide 
Mn.sub.23 C.sub.6. 
When at least one element selected from the group consisting of Co, Mo, W, 
Ta, Nb, V, Ti, and Zr is contained in a concentration of not more than 30 
atomic %, the Mn in the austenite phase which is stable only at elevated 
temperatures improves the mechanical properties of the alloy such as 
tensile strength. This improvement is brought about without impeding the 
conversion of the austenite phase, by quenching, into the nonequilibrium 
austenite phase which is stable even at room temperature. However, if the 
concentration of the elements exceeds 30 atomic %, the Mn-based alloy will 
be too brittle to be useful due to the precipitation of a MnY (Y denoting 
Co, Mo, W, Ta, Nb, V, Ti, and/or Zr) type compound. Particularly in the 
aforementioned alloy composition, an alloy composed of 7 to 26 atomic % of 
at least one element selected from the group consisting of Al, Ni and Cr, 
3 to 10 atomic % of C, not more than 30 atomic % of at least one element 
selected from the group consisting of Co, Mo, W, Ta, Nb, V, Ti and Zr 
(providing that Co have concentration not exceeding 30 atomic %, Mo and/or 
W have concentration not exceeding 20 atomic % and the at least one 
element selected from the group consisting of Ta, Nb, V, Ti and Zr have 
concentration not exceeding 10 atomic %), and the balance to make up 100 
atomic % being comprised substantially of Mn (e.g., 25 to 95 atomic % of 
Mn), when converted from its molten state by quenching into a solid state, 
assumes a highly tough austenite phase. 
The Mn-based alloy of the present invention is highly desirable because 
uniform ribbons or thin wires of a circular cross section can be 
manufactured from this alloy. Moreover, the Mn-based alloy of this 
composition is capable of being cold rolled or cold drawn. Particularly 
with respect to wire drawing, it should be noted that the workability such 
as cold drawing of this alloy can be effected to more than 90% of 
reduction of area. The alloy is advantageous in that the tensile strength 
at fracture notably increases proportionally with respect to increases in 
the area of reduction. The alloy has been found to be highly suitable for 
economic production of nonferrous and nonmagnetic heavy-duty metal fibers 
having diameters not exceeding about 150 .mu.m, preferably 50 .mu.m or 
more. The reduction ratio of area is represented by the following 
equation: 
##EQU1## 
wherein D is a diameter of wire before the wire drawing and d is a 
diameter of the thin wire after repeatedly wire drawing. That is, it shows 
the reduction ratio of the cross section of thin wire to be reduced in 
accordance with subjecting to the drawing workability. 
An alloy of the present invention may include additional elements such as 
Si, B, P, Ge, Cu, and Hf in addition to the essential elements of this 
invention provided in that these additional elements are only present 
within a range in which the objects and effects of this invention are not 
impaired by their presence. 
The particular diameter of microcrystals in the nonequilibrium austenite 
phase varys with the alloy composition and cooling speed. However, it 
should be pointed out that it is the successful formation of the austenite 
phase and not the magnitude of the particle diameter of the crystals which 
is important. 
The alloy of this invention is produced by preparing a molten alloy in the 
aforementioned composition and quenching this molten alloy. Various 
methods are available for effecting this quenching. For example, the 
single roll method, the double roll method, and the submerged rotary 
spinning method which are liquid quenching methods are particularly 
effective. Plates of the alloy may be produced by the piston-anvil method, 
the splat etching method, etc. The aforementioned liquid quenching methods 
(single roll method, double room method, or submerged rotary spinning 
method) have a cooling speed in the range of about 10.sup.4 .degree. to 
10.sup.5 .degree. C./sec., while the piston-anvil method or the splat 
etching method has a cooling speed in the range of about 10.sup.5 .degree. 
to 10.sup.6 .degree. C./sec. By using some of these quenching methods, 
therefore, the quenching of the molten alloy can be efficiently carried 
out. The term "submerged rotary spinning method" refers to a method as 
disclosed in Japanese Patent Application (OPI) No. 64948/80 (The term 
"OPI" as used herein refers to a "published unexamined Japanese patent 
application.") The " submerged rotary spinning method" is a method for 
obtaining a thin wire of a circular cross section by placing water in a 
rotary drum in motion thereby centrifugally forming a film of water on the 
inner wall surface of the drum and extruding molten alloy through a 
spinning nozzle into the water film. To produce a continuous thin wire 
uniformly by this method, the peripheral speed of the rotary drum is 
preferably equal to or greater than the speed of the flow of molten alloy 
being thrown out of the spinning nozzle. Particularly, the peripheral 
speed of the rotary drum is preferably 5 to 30% high than the speed of the 
flow of molten alloy extruded through the spinning nozzle. The angle 
formed between the flow of molten metal extruded through the spinning 
nozzle and the water film formed on the inner wall surface of the rotary 
drum is preferably greater than 20.degree.. 
The Mn-based alloy of this invention has a wider range of equilibrium 
austenite phase at elevated temperatures than the Fe-based alloy. It, 
therefore, acquires the nonequilibrium austenite phase at room temperature 
over a wide range of alloy compositions. At the same time, it enjoys 
stability because it is capable of keeping the austenite phase from 
converting into martensite. Particularly, the alloy acquires a large thick 
austenite phase as compared with the Fe-X-C (X denoting Cr, Mo, W, or Al) 
alloy. This fact is profoundly significant from the industrial point of 
view. When the Mn-based alloy incorporates Cr, among other elements of the 
same group, it becomes highly resistant to corrosion. Therefore, the alloy 
may be used in nonmagnetic corrosion-proofing materials. 
Moreover, the Mn-based alloy of this invention is capable of being cold 
worked continuously. For the production of thin wires, for example, this 
Mn-based alloy can be cold drawn to economically form wires of high 
tensile strength having diameters in the range of 1 to 200 .mu.m. It is 
noteworthy that the tensile strength of the Mn-based alloy can be improved 
to even more than 150 Kg/mm.sup.2. Such strength has not been previously 
attained using any of the nonferrous materials developed to date. 
Because the Mn-based alloy of this invention has the quality and structure 
described above, it can be readily used in the production of a variety of 
products including, nonmagnetic high resistance materials, nonmagnetic 
springs, nonmagnetic switch relays, belts, tires and other rubber 
reinforcements, plastics, concretes and other similar composite materials, 
and knit and woven fabrics such as fine mesh filters.

The present invention will now be described more specifically below with 
reference to working examples. However, the present invention is not 
limited to be following examples. 
EXAMPLE 1 
An alloy composed of 85 atomic % of Mn, 10 atomic % of Al, and 5 atomic % 
of C and prepared in a molten state was extruded, under argon gas pressure 
of 2.5 kg/mm.sup.2, through a spinning nozzle of a varying diameter of 0.1 
to 1 mm. The extrusion was made onto the surface of a steel roll having a 
diameter of 20 cm, rotating at a varying speed of 1000 to 5000 rpm, cooled 
and solidified to produce ribbons of 10 to 500 .mu.m in thickness. 
The ribbons thus obtained were noted to trend toward gradual loss of 
toughness in proportion to growth in thickness. Up to about 500 .mu.m of 
thickness, however, the ribbons were capable of being bent by 180.degree. 
and folded fast over themselves without fracture. When the ribbons were 
tested for texture by observation through an optical microscope, an X-ray 
diffraction meter and a transmission electron microscope, they were found 
to be composed of microcrystals of nonequilibrium austenite phase in the 
structure of a face centered cubic lattice. The crystals measured about 1 
to 5 .mu.m. The crystals showed a trend toward gradual growth in particle 
diameter in proportion to growth in ribbon thickness. 
The tough alloy ribbon having a nonequilibrium austenite phase and 
measuring 200 .mu.m in thickness was tested for tensile strength by an 
Instron tension tester over a test distance of 2.0 cm at a strain rate of 
4.17.times.10.sup.-4 /sec. The ribbon was found to be a very tough 
material having tensile strength of 35 kg/mm.sup.2, yield strength of 15 
kg/mm.sup.2, and elongation of 22%. 
EXAMPLE 2 
An alloy composed of 75 atomic % of Mn, 18 atomic % of Al, and 7 atomic % 
of C was melted. The molten alloy was extruded, under argon gas pressure 
of 3.0 kg/cm.sup.2, through a spinning nozzle 150 .mu.m in orifice 
diameter into a cooling water bath 2.5 cm in depth formed centrifugally 
(350 rpm) within a rotary cylinder 50 cm in diameter, there to be cooled 
and solidified with the rotating body of cooling water. As the thin wire 
of alloy of a circular cross section was cooled and solidified, it was 
continuously wound up on the inner wall of the rotary cylinder. (At this 
time, the speed of the cooling water (V.sub.W) inside the rotary cylinder 
and the speed of the flow of molten alloy (V.sub.J) extruded through the 
spinning nozzle were adjusted, thus V.sub.W /V.sub.J =1.15.) 
This operation produced a continuous thin wire of a substantially circular 
cross section having a uniform diameter of 130 .mu.m. 
When this thin wire was tested for texture in the same way as in Example 1, 
it was found to have a nonequilibrium austenite phase in the structure of 
a fcc. The particle diameter of the crystals was about 3 .mu.m. 
This thin wire was found to be a highly tough material having tensile 
strength of 40 kg/mm.sup.2, yield strength of 25 kg/mm.sup.2 and 
elongation of 4%. 
This thin wire was cold drawn, without any process annealing, through a 
commercially available diamond die up to 79% reduction of area. During the 
cold drawing, the wire sustained no damage of any sort. Thus, the cold 
drawing produced a very strong thin wire having a highly uniform tensile 
strength of 160 kg/mm.sup.2, a yield strength of 135 kg/mm.sup.2, and an 
elongation of 1.1%. 
EXAMPLE 3 
By following the procedure of Example 2, a very tough continuous thin wire 
having a circular cross section 130 .mu.m in diameter was obtained from an 
alloy composed of 62 atomic % of Mn, 18 atomic % of Al, 8 atomic % of Cr, 
7 atomic % of C, and 5 atomic % of Ta. 
When this thin wire was tested for texture by observation through an X-ray 
diffraction meter and a transmission electron microscope, it was found to 
have a nonequilibrium austenite phase of crystals 2 to 3 .mu.m in particle 
diameter. The thin wire had tensile strength of 50 kg/mm.sup.2, yield 
strength of 30 kg/mm.sup.2, and elongation of 3.8%. These test results 
indicate that the addition of Cr and Ta enabled the produced alloy to 
acquire enhanced toughness and improved tensile strength. 
This thin wire was cold drawn, without any process annealing, through a 
commercially available diamond die to 79% of reduction of area. When the 
cold drawn wire was tested for tensile strength under the same conditions, 
it was found to be a heavy-duty wire having tensile strength of 190 
kg/mm.sup.2, yield strength of 140 kg/mm.sup.2, and elongation of 0.8%. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.