Nanostructured Mn-Al Permanent Magnets And Methods of Producing Same

Abstract

A bulky consolidated nanostructured manganese aluminum alloy includes at least about 80% of a magnetic τ phase and having a macroscopic composition of MnXAlYDoZ, where Do is a dopant, X ranges from 52-58 atomic %, Y ranges from 42-48 atomic %, and Z ranges from 0 to 3 atomic %. A method for producing a bulky nanocrystalline solid is provided. The method includes melting a mixture of metals to form a substantially homogenous solution. The method also includes casting the solution to form ingots, measuring compositions of the ingots; crushing the ingots to form crushed powders, and milling the crushed powders to form nanocrystalline powders. The method further includes verifying the presence of τ phase and determining the amount of the τ phase, and simultaneously consolidating the nanocrystalline powders into a bulky nanocrystalline solid and undergoing phase transformation from ε phase to at least 80% τ phase, β and γ2 phases.

1 . A bulky consolidated nanostructured manganese aluminum alloy comprising at least about 80% of a magnetic τ phase and having a macroscopic composition of Mn X Al Y Do Z , wherein
 Do is a dopant, 
 X ranges from 52-58 atomic %, 
 Y ranges from 42-48 atomic %, and 
 Z ranges from 0 to 3 atomic %. 
 
     
     
         2 . The bulky consolidated nanostructured manganese aluminum alloy of  claim 1 , wherein the manganese aluminum alloy further comprising carbon having a macroscopic composition of 51 atomic % manganese, 46 atomic % aluminum and 3 atomic % carbon. 
     
     
         3 . The bulky consolidated nanostructured manganese aluminum alloy of  claim 2 , wherein the permanent magnetic properties comprise coercive forces of about 5.2 kOe. 
     
     
         4 . The bulky consolidated nanostructured manganese aluminum alloy of  claim 1 , wherein the manganese aluminum alloy has a macroscopic composition of 54 atomic % manganese, 46 atomic % aluminum. 
     
     
         5 . The bulky consolidated nanostructured manganese aluminum alloy of  claim 4 , wherein the permanent magnetic properties comprise coercive forces of about 4.8 kOe. 
     
     
         6 . A method for producing a bulky nanocrystalline solid comprising
 melting a mixture of metals comprising between 52-58 atomic % manganese and between 42-48 atomic % aluminum to form a substantially homogenous solution;   casting the solution to form ingots;   measuring compositions of the ingots;   crushing the ingots to form crushed powders;   milling the crushed powders to form nanocrystalline powders;   verifying the presence of τ phase and determining the amount of the τ phase; and   simultaneously consolidating the nanocrystalline powders into a bulky nanocrystalline solid and undergoing phase transformation from ε phase to at least 80% τ phase, β and γ 2  phases.   
     
     
         7 . The method of  claim 6 , further comprising characterizing microstructure of the bulky nanocrystalline solid and measuring magnetic properties of the bulky nanocrystalline solid. 
     
     
         8 . The method of  claim 6 , further comprising annealing the nanocrystalline powders to determine conditions for consolidating the nanocrystalline powders. 
     
     
         9 . The method of  claim 8 , further comprising annealing at temperatures between 200° C. and 600° C. to maximize the amount of the magnetic metastable τ phase transformed from a milled nanocrystalline unstable high-temperature ε phase, thereby minimizing the presence of non-magnetic equilibrium β and γ 2  phases. 
     
     
         10 . The method of  claim 8 , wherein the annealing time is shorten for higher annealing temperature to avoid decomposition of the τ-phase into the γ 2  and β phases. 
     
     
         11 . The method of  claim 6 , the step of consolidating the milled powders comprising backpressure assisted Equal channel angular extrusion (ECAE). 
     
     
         12 . The method of  claim 11 , further comprising increasing backpressure to consolidate the nanocrystalline powders. 
     
     
         13 . The method of  claim 11 , further comprising controlling a temperature of the nanocrystalline powders within 200° C. to 600° C. during the ECAE. 
     
     
         14 . The method of  claim 11 , further comprising decreasing rate of extrusion with increasing temperature to shorten annealing time at higher temperature to avoid decomposition of the τ-phase into the γ 2  and β phases. 
     
     
         15 . The method of  claim 11 , wherein the bulky nanocrystalline solid is in a form of rod shapes. 
     
     
         16 . The method of  claim 15 , wherein the bulky nanocrystalline solid has a cross-section in one of square, rectangular, and circular shape. 
     
     
         17 . The method of  claim 6 , further comprising repeating the step of consolidating the nanocrystalline powders until the bulky nanocrystalline solid having minimum defects. 
     
     
         18 . The method of  claim 6 , wherein the bulky nanocrystalline solid is machinable. 
     
     
         19 . The method of  claim 6 , wherein the mixture of metals further comprises a dopant comprising at least one of carbon and boron. 
     
     
         20 . The method of  claim 6 , wherein the mixture of metals comprises 54 atomic % manganese, and 46 atomic % aluminum. 
     
     
         21 . The method of  claim 6 , wherein the mixture of metals comprises 51 atomic % manganese, 46 atomic % aluminum and 3 atomic % carbon.