Method for enhancement of grain boundary cohesion in crystalline materials and compositions of matter therefor

Abstract

A method for identifying additive components for polycrystalline metals and materials that enhance grain boundary cohesion and compositions of such materials comprises calculation of an empirical value Δ B A which is dependent upon a summation of various energy values associated with the matrix and additives and identifying the additives having a negative value. Formulations of alloys having improved physical properties and their processing steps are also disclosed.

What is claimed is:  
     
         1 . An alloy, comprising in combination: 
 a crystalline matrix of at least one metal selected from the group consisting of iron and nickel, wherein said matrix includes crystals with grain boundaries, and at least an alloying element selected from the group consisting of tungsten (W), rhenium (Re), osmium (Os), niobium (Nb), iridium (Ir), technetium (Te), ruthenium (Ru), platinum (Pt), tantalum (Ta), zirconium (Zr), hafnium (Hf), vanadium (V) and titanium (Ti) in an amount that enhances cohesion at said boundaries characterized by segregation of said alloying element to the grain boundaries and the term ΔE B   A  being negative, said alloy being processed by low temperature heat treatment to segregate said alloying element to a grain boundary.    
     
     
         2 . The alloy of  claim 1  wherein said matrix is a nickel compound and said alloying element is selected from the group consisting of osmium (Os), rhenium (Re), ruthenium (Ru), tungsten (W) and niobium (Nb).  
     
     
         3 . The alloy of  claim 1  wherein said matrix is an iron compound and said alloying element is selected from the group consisting of tungsten (W), rhenium (Re), niobium (Nb), and osmium (Os).  
     
     
         4 . A generally solution heat treated alloy composition consisting essentially of a formulation in weight percent of 
 0.18-0.40 C    15.0-20.0 Co    6.0-7.5 Ni    2.0-4.0 Cr    0-1.7 Mo    1-3.0 W    0-3.0 Re    Balance Fe    and one or more additives in weight percent selected from the group consisting of up to 0.05 % Ti, 0.010 % La, 0.02 % Zr, 10-20 ppm B, combinations thereof and impurities, said alloy solution heat treated in the range of 800° C. for at least one hour to form carbides characterized primarily as M 2 C and to effect migration of Mo, W and/or Re to grain boundaries.    
     
     
         5 . The alloy of  claim 4  subjected to tempering in the range of 450° C. to 550° C. for at least one hour to increase hardness.  
     
     
         6 . The alloy of  claim 4  having an M S  temperature in the range of about 225° C. to 400° C.  
     
     
         7 . The alloy of  claim 4  wherein the mole percent of M 2 C carbides is greater than 0.030.