Abstract:
Alloys, formed by a eutectic transformation of the type Fe25-35 Ni15-25 Mn30-40 Al10-20 MO-5, are disclosed. M is selected from chromium, molybdenum, carbon and combinations thereof. The alloys have high strength and ductility. The alloys are prepared from readily available transition metals, and can be used in applications where properties similar to steel are necessary or advantageous.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of priority to U.S. Provisional Patent Application Ser. No. 61/028,809, filed Feb. 14, 2008, which is incorporated by reference herein. 
     
    
     GOVERNMENT INTERESTS 
       [0002]    The United States Government has rights in this invention under Contract No. NSF-DMR-0505774 between the National Science Foundation (NSF) and Dartmouth College. 
     
    
     BACKGROUND 
       [0003]    1. Field of the Invention 
         [0004]    This invention relates to novel alloys and methods of producing the alloys. More specifically, the alloys are strong and ductile microstructured alloys having lamellar structures. 
         [0005]    2. Description of the Related Art 
         [0006]    Basic research on alloy materials seeks to find improved materials, such as those that are lighter, stronger or less expensive than conventional metals and alloys. In other contexts, improved materials may have increased resistance to weather, chemicals or friction in an intended environment of use. Equipment that incorporates these new materials in component parts may have a longer service life, require less maintenance or achieve an improved performance level. From a cost of manufacture standpoint, it is desirable for these new materials to be made from readily available and highly affordable natural resources. 
         [0007]    One technique that may be used to produce an alloy with enhanced strength and ductility is a eutectic transformation. A eutectic transformation occurs when components of an alloy crystallize simultaneously from a liquid solution. Products of a eutectic transformation can often be identified by their lamellar structure where spacing between lamellae is typically on the order of less than a micron to a few microns. Such structures are generally strong and ductile. For example, the most well known lamellar material is carbon steel. 
       SUMMARY 
       [0008]    Alloys of the present disclosure advance the art by providing materials with exceptional strength and ductility. 
         [0009]    In one embodiment, an intermetallic composition formed by a eutectic transformation in at least two distinct structural phases has an average composition comprising from 25% to 35% iron, 15% to 25% nickel, 30% to 40% manganese and 10% to 20% aluminum, where the composition is described in terms of atomic percentages. 
         [0010]    In one embodiment, an intermetallic composition formed by a eutectic transformation in at least two distinct structural phases has an average composition according to the formula: 
         [0000]      Fe a Ni b Mn c Al d M e , 
         [0000]    where (in atomic percent) a ranges from 25 to 35; b ranges from 15 to 25; c ranges from 30 to 40; d ranges from 10 to 20; e ranges from 0 to 5; and M is selected from the group consisting of Cr, Mo, C and combinations thereof. 
         [0011]    In one embodiment, a method of producing an intermetallic composition includes heating a mixture of metals, to create a homogenous solution, according to the formula: 
         [0000]      Fe a Ni b Mn c Al d M e , 
         [0000]    where (in atomic percent) a ranges from 25 to 35; b ranges from 15 to 25; c ranges from 30 to 40; d ranges from 10 to 20; e ranges from 0 to 5; and M is selected from the group consisting of Cr, Mo, C and combinations thereof; cooling the homogenous solution to obtain a homogeneous solid; reheating the solid to a eutectic transformation temperature; and holding the eutectic transformation temperature for a period of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an X-ray diffraction pattern of Fe 30 Ni 20 Mn 35 Al 15  as cast, according to an embodiment. 
           [0013]      FIG. 2  is a differential thermal analysis (DTA) curve showing thermal processes of Fe 30 Ni 20 Mn 35 Al 15 . 
           [0014]      FIG. 3  shows a plot of yield strength versus temperature for Fe 30 Ni 20 Mn 35 Al 15 . 
           [0015]      FIG. 4  shows a stress versus strain curve for Fe 30 Ni 20 Mn 35 Al 15 . 
           [0016]      FIG. 5  shows a comparison of tensile and yield strengths for Fe 30 Ni 20 Mn 35 Al 15  versus some known alloys. 
           [0017]      FIG. 6  shows a comparison of percent elongation for Fe 30 Ni 20 Mn 35 Al 15  versus some known alloys. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Alloys that are both strong and ductile at room temperature are disclosed, along with methods for making the alloys by way of a eutectic transformation. In some embodiments, observed tensile and yield strengths of the alloys are greater than those of typical stainless steels, and the alloys show ductility comparable to high-strength ferritic stainless steels. 
         [0019]    The terms “alloy”, “intermetallic compound” and “intermetallic composition” are used interchangeably herein. They refer to compounds containing at least two different metals. 
         [0020]    A “eutectic alloy” is an alloy that is formed when at least two different metals, as well as any non-metals, are present in suitable concentrations and held at a eutectic transformation temperature for a suitable period of time. At the transformation temperature, at least two phases simultaneously crystallize from a liquid solution to form lamellae of the two phases. 
         [0021]    The alloys disclosed herein may be used in the manufacture of machine, building and industrial parts. The alloys may be particularly suitable for applications requiring high-strength, wear resistant parts including but not limited to: engines, bearings, bushings, stators, washers, seals, rotors, fasteners, stamping plates, dies, valves, punches, automobile parts, aircraft parts, building materials, and drilling and mining parts. Further, the alloys can be used in any known application currently utilizing stainless steel or any high-strength, ductile alloy. 
         [0022]    In a particular embodiment, an alloy contains iron, nickel, manganese and aluminum to which may be added chromium, molybdenum, carbon and combinations thereof. Such an alloy is represented by a macroscopic average formula: 
         [0000]      Fe a Ni b Mn c Al d M e ,   Formula (1)
 
         [0000]    where M is an alloying addition of any element or combination of elements;
   a ranges from 25 to 35;   b ranges from 15 to 25;   c ranges from 30 to 40;   d ranges from 10 to 20;   e ranges from 0 to 5; and
 
where a-e are expressed on an atomic percent basis.
   
 
         [0028]    In one aspect, M may be a metal or combination of metals. For example, M may be chromium, molybdenum, carbon and combinations thereof In some embodiments, the portion of the alloy that is allocated to M may also range from 0.05 to 4% or in other aspects from 0.5% to 3%. 
         [0029]    A narrower formulation that is within the general scope of Formula (1) is: 
         [0000]      Fe x Ni 50-x Mn 50-y Al y ,   Formula (2)
 
         [0000]    wherein x ranges from 25 to 35 (atomic percent basis) and y ranges from 10 to 20 (atomic percent basis). 
         [0030]    In another aspect, the composition of Fe a Ni b Mn c Al d M e  may be within the ranges:
   a ranges from 27 to 33;   b ranges from 17 to 23;   c ranges from 32 to 38;   d ranges from 12 to 18; and   e ranges from 0 to 2.5;
 
where a-e are expressed on an atomic percent basis and M is an alloying addition of any element or combination of elements.
   
 
         [0036]    The alloy may be formed by a heat treatment process that results in a eutectic transformation leaving at least two intermetallic phases of different structure and stoichiometry. The macroscopic formulas above pertain to the overall composition, but the macroscopic composition has nanostructure or microstructure of localized phase variances in composition and ordering. The presence of two phases present as lamellae results in ductility along the planes of the lamellae. This ductility may be measured as percent elongation. 
         [0037]    The following examples set forth preferred materials and methods for use in making the disclosed alloys. These examples teach by way of illustration, not by limitation, and should not be interpreted as unduly narrow. 
       EXAMPLE 1 
     Preparation and Characterization of Fe 30 Ni 20 Mn 35 Al 15    
       [0038]    A quaternary alloy of Fe 30 Ni 20 Mn 35 Al 15  composition was prepared by well known arc melting and casting techniques. A quantity of material including 24 g Fe, 17 g Ni, 27 g Mn and 5 g Al was placed in a water-cooled copper mold and heated until molten using the arc melting technique. Ingots were flipped and melted a minimum of three times under argon to ensure mixing. Quenching was done by allowing the alloy to rapidly cool in the copper mold to a temperature of ˜30° C. in approximately 10 minutes. A eutectic transformation was carried out by holding the quenched ingots at about 1215° C. for about 30 minutes. For this composition, the eutectic transformation temperature, as shown in the differential thermal analysis curve, was between about 1210-1290° C., or between about 1212-1250° C. or between about 1214-1230° C. In some embodiments, a 5% excess of Mn may be added to the starting materials because Mn accounts for the majority of weight loss during casting, which results from brittle sharding and evaporation. 
         [0039]    The resulting alloy had microstructure in the form of lamellae formed as two intermetallic phases. One phase was a B2 (ordered body-centered cubic, b.c.c.) phase having a composition of Fe 7 Ni 47 Mn 18 Al 28  in terms of atomic percent. The other phase was a face-centered cubic (f.c.c.) phase having a composition of Fe 50 Ni 7 Mn 37 Al 6  in terms of atomic percent. The widths of the body-centered cubic and face-centered cubic phases were 200 nm and 500 nm, respectively. 
         [0040]    The alloy was characterized using analytical techniques that are well known in the art. For example, water displacement analysis was used to determine that the alloy had a density of about 7.02 g/cm 3 , and chemical composition was determined by energy dispersive spectroscopy (EDS). As discussed above, the overall composition, Fe 30 Ni 20 Mn 35 Al 15 , was based on microstructured phases of Fe 7 Ni 47 Mn 18 Al 28  and Fe 50 Ni 7 Mn 37 Al 6 . 
         [0041]    Structural data was obtained using a Siemens D5000 X-ray Diffractometer with a Kevex silicon detector in the range of 20-110° 2θ, using an instrument that was calibrated against an alumina standard purchased from the National Institute of Standards (NIST).  FIGS. 1 and 2  show X-ray diffraction patterns for as cast and quenched samples, respectively. The as cast sample has undergone a eutectic transformation, and displays peaks representative of both the B2 and f.c.c. phases, as shown in  FIG. 1 . 
         [0042]    Room temperature hardness of the two phase alloy, Fe 30 Ni 20 Mn 35 Al 15 , was determined by taking the average of five measurements from a Leitz Microhardness Indentor with a 200 g load. The average Vicker&#39;s hardness was 310 kg/mm 2 . 
         [0043]    Differential thermal analysis (DTA) was performed on a Perkin Elmer Pyris Diamond TG/DTA. A typical DTA curve is shown in  FIG. 2 . The differential thermal analysis suggests that a pre-existing B2 phase begins to disorder at about 1160° C. and a eutectic transformation begins at about 1215° C. This eutectic transformation forms the B2 (b.c.c.) and f.c.c. phases of the lamellar alloy. 
         [0044]    Transmission electron microscopy (TEM), performed on either a JEOL 2000FX or a Philips CM 200, indicated that, after formation, the eutectic microstructure was stable up to at least 760° C. Optical microscopy confirmed that no distinct differences were observed between samples that had been annealed for 30 minutes at temperatures between 327-727° C. and then subsequently quenched. 
         [0045]    Yield strength of the eutectic alloy was determined using a MTS 810 mechanical testing system. The two phase alloy was subjected to mechanical testing at temperatures shown in  FIG. 3 . The yield strength at 294 K (room temperature) was determined to be about 750 MPa. High strength was maintained to about 675 K (400° C.) after which the yield strength decreased but was still approximately 200 MPa at 975 K (700° C.). 
         [0046]    The stress versus strain curve of  FIG. 4  shows results of a tensile test performed with a MTS 810 mechanical testing system at room temperature. A 1.75 inch sample of Fe 30 Ni 20 Mn 35 Al 15  was deformed about 0.35 inches prior to fracturing in a ductile manner under an applied stress of about 1200 MPa. The observed percent elongation was about 20% at room temperature. 
         [0047]    As shown in  FIG. 5 , the tensile and yield strengths of Fe 30 Ni 20 Mn 35 Al 15  are significantly greater than those of many known alloys, including austenitic, ferritic and mild steel. The ductility, measured as percent elongation, is also comparable to ferritic and mild steel, as shown in  FIG. 6 . 
       EXAMPLE 2 
     Preparation and Characterization of Fe x Ni 50-x Mn 50-y Al y ±5% 
       [0048]    Various alloys are cast with a composition: 
         [0000]      Fe x Ni 50-x Mn 50-y Al y ,   Formula (2)
 
         [0000]    where x ranges from 25 to 35 atomic percent plus or minus 5%, and y ranges from 10 to 20 atomic percent plus or minus 5%. 
         [0049]    The alloys are cast using the aforementioned arc melting technique and heated to a eutectic transformation temperature range of between about 1210-1290° C., or between about 1212-1250° C. or between about 1214-1230° C. The alloys are expected to be strong and ductile with a range of mechanical properties that can be manipulated by composition variations within the disclosed range. 
       EXAMPLE 3 
     Characterization of a Phase Diagram Near a Eutectic Transformation 
       [0050]    A portion of a phase diagram near a eutectic transformation may be constructed by varying percentages of Fe, Ni, Mn, Al and M as described in the context of Formula (1), except the subscripts a, b, c, d, and e, may be any value. The constituents are processed as described in Examples 1 and 2 to ascertain the presence or absence of eutectic transformation products. The preferred metals include combinations of Fe, Ni, Mn, and Al, in which case the ranges for x and y shown in Formula (2) may be any value. When adjusting the respective subscripts a, b, c, d, e, x and/or y, it is suggested to increase or decrease the individual ranges or combinations of ranges in steps of five percent from the values shown in Formulas (1) and (2), at least until the resulting alloy does not show evidence of a eutectic transformation. For alloys that contain four or five constituents, it is routine in the art that several hundred castings are needed to fully characterize the phase diagram around a eutectic transformation. 
         [0051]    It is understood for purposes of this disclosure, that various changes and modifications may be made to the disclosed embodiments that are well within the scope of the present compositions and methods. Numerous changes may be made which will readily suggest themselves to those skilled in the art and which are encompassed in the spirit of the compositions and methods disclosed herein and as defined in the appended claims.