- Email: [email protected]
A mechanism for fracture of grain boundary segregated “phases” in the temper embrittlement problem
Scripta METALLURGICA
Vol. 12, DD. 111-112, 1978 Printed in the United States
Pergamon Press, Inc.
A MECHANISMFOR FRACTUREOF GRAIN BOUNDARYSEGREGATED"PHASES" IN THE TEMPEREMBRITTLEMENTPROBLEM E.S. Machlin Henry Krumb School of Mines Columbia University New York, New York 10027
~Received November 14, 1977)
Recently, Machlin and Gee (1) have shown that the s t a b i l i t y and l a t t i c e parameters of the D8b, sigma phase, structure can be quantitatively explained on the basis of a model in which the chains of atoms comprising the E type sites are occupied by easily "shearable" atoms belonging to an "exotic" group of elements. Subsequently, Machlin and Whang (2) found that the energy of formation and l a t t i c e parameters of the A15 structure calculated on the basis of crystal f i e l d modified pair potentials were inquantitativeagreement with observation. Again, i t was i m p l i c i t in this model that easily "shearable" atoms belonging to an "exotic" group of elements occupied the chain sites in this structure. Since the only elements known to appear on these chain sites belong to the T i , V, Cr or Mn columns in the periodic table--the l a t t e r by d e f i n i t i o n belong to the "exotic" group of elements. Machlin and Whang (2) searched for and found independent evidence for the easily "shearable" property of these "exotic" elements in the form of the elastic constant C44 in the A2 structure. They showed that the observed values of the ratio of C44 to the bulk modulus for those exotic "elements" that are stable in the A2 structure, were much lower than for other A2 (or B2) elements (or compounds). They showed in a separate paper (3) that for an i n f i n i t e resistance to the atomic "shear" distortion--which physically corresponds to a distortion of the Born repulsion shell of electrons from a sphere to that of an e l l i p s o i d in t h e i r model--the ratio of C44 to the bulk modulus would equal about unity while for zero resistance to such "shear" distortion this ratio would equal zero for the A2 structure. This ratio for the transition element type of "exotic" element is less than 0.5 and equals 0.17 for Nb. Further, they showed that this low "shear" resistance accounts for the near zero value of Cli-C12 in the A15 structure of Nb3Sn and V3Si. Earlier, Machlin had shown (4) that a similar crystal f i e l d effect operates in certain non-cubic structures containing Bi (L1O) or Sn (B81). The l a t t e r result is physically reasonable because removal of degeneracy for the p electrons can alter the shape of their electron cloud from a sphere to an e l l i p s o i d . Thus, i t is reasonable to expect, that the metalloids of Group 5 and Sn, and perhaps other atoms having u n f i l l e d p shells but some p electrons, and which are not in a covalent (non-spherical but directed bond) state, belong to the "exotic" element group. With this background to develop p l a u s i b i l i t y for the concept that the grain boundary segregated "phases" in temper embrittled steels (e.g. phases involving combinations of Ni and Sb or Cr and P or Ni and Sn, etc.) have at least one elastic constant that is nearly zero, we may proceed to the suggestion of a mechanism for embrittlement of steels containing such "phases". F i r s t , these "phases" are atomically thick ( i . e . 2 to 3 atom layers at the most.)(5) Secondly, they extend along grain boundaries occupying a major fraction of the area of such boundaries.(6) I f these "phases" have a much smaller elastic modulus than the bulk material, as suggested they should, they would then produce a stress copcentration of order (grain boundary diameter/thickness of grain boundary "phase") ~ , which can equal 100. This factor is s u f f i c i e n t to induce b r i t t l e fracture in such steels. Acknowledgement: The research providing the background for the concept suggested in this note has been supported by EPRI through Contract RP253-I and by the National Science Foundation by
Iii
112
THE TEMPER EMBRITTLEMENT
PROBLEM
Grant DMR-77-08667 with Columbia University. REFERENCES 1. 2. 3. 4. 5. 6.
E.S. Machlin and Pak T. Gee, J. Solid State Chemistry 24, no. 3 (1978). E.S. Machlin and S.H. Whang, J. Phys. & Chem. Solids, to be published. E.S. Machlin and S.H. Whang, Submitted to Solid State Chem. E.S. Machlin, Acta Met. 22, 1433 (1974). E.D. Hondros and M.P. Sea-h-, Met. Trans. 8A, 1363 (1977). D.F. Stein, Interfacial Segregation, 1977 ASM, Materials Science Seminar.
Vol.
12, No. 1
Vol. 12, DD. 111-112, 1978 Printed in the United States
Pergamon Press, Inc.
A MECHANISMFOR FRACTUREOF GRAIN BOUNDARYSEGREGATED"PHASES" IN THE TEMPEREMBRITTLEMENTPROBLEM E.S. Machlin Henry Krumb School of Mines Columbia University New York, New York 10027
~Received November 14, 1977)
Recently, Machlin and Gee (1) have shown that the s t a b i l i t y and l a t t i c e parameters of the D8b, sigma phase, structure can be quantitatively explained on the basis of a model in which the chains of atoms comprising the E type sites are occupied by easily "shearable" atoms belonging to an "exotic" group of elements. Subsequently, Machlin and Whang (2) found that the energy of formation and l a t t i c e parameters of the A15 structure calculated on the basis of crystal f i e l d modified pair potentials were inquantitativeagreement with observation. Again, i t was i m p l i c i t in this model that easily "shearable" atoms belonging to an "exotic" group of elements occupied the chain sites in this structure. Since the only elements known to appear on these chain sites belong to the T i , V, Cr or Mn columns in the periodic table--the l a t t e r by d e f i n i t i o n belong to the "exotic" group of elements. Machlin and Whang (2) searched for and found independent evidence for the easily "shearable" property of these "exotic" elements in the form of the elastic constant C44 in the A2 structure. They showed that the observed values of the ratio of C44 to the bulk modulus for those exotic "elements" that are stable in the A2 structure, were much lower than for other A2 (or B2) elements (or compounds). They showed in a separate paper (3) that for an i n f i n i t e resistance to the atomic "shear" distortion--which physically corresponds to a distortion of the Born repulsion shell of electrons from a sphere to that of an e l l i p s o i d in t h e i r model--the ratio of C44 to the bulk modulus would equal about unity while for zero resistance to such "shear" distortion this ratio would equal zero for the A2 structure. This ratio for the transition element type of "exotic" element is less than 0.5 and equals 0.17 for Nb. Further, they showed that this low "shear" resistance accounts for the near zero value of Cli-C12 in the A15 structure of Nb3Sn and V3Si. Earlier, Machlin had shown (4) that a similar crystal f i e l d effect operates in certain non-cubic structures containing Bi (L1O) or Sn (B81). The l a t t e r result is physically reasonable because removal of degeneracy for the p electrons can alter the shape of their electron cloud from a sphere to an e l l i p s o i d . Thus, i t is reasonable to expect, that the metalloids of Group 5 and Sn, and perhaps other atoms having u n f i l l e d p shells but some p electrons, and which are not in a covalent (non-spherical but directed bond) state, belong to the "exotic" element group. With this background to develop p l a u s i b i l i t y for the concept that the grain boundary segregated "phases" in temper embrittled steels (e.g. phases involving combinations of Ni and Sb or Cr and P or Ni and Sn, etc.) have at least one elastic constant that is nearly zero, we may proceed to the suggestion of a mechanism for embrittlement of steels containing such "phases". F i r s t , these "phases" are atomically thick ( i . e . 2 to 3 atom layers at the most.)(5) Secondly, they extend along grain boundaries occupying a major fraction of the area of such boundaries.(6) I f these "phases" have a much smaller elastic modulus than the bulk material, as suggested they should, they would then produce a stress copcentration of order (grain boundary diameter/thickness of grain boundary "phase") ~ , which can equal 100. This factor is s u f f i c i e n t to induce b r i t t l e fracture in such steels. Acknowledgement: The research providing the background for the concept suggested in this note has been supported by EPRI through Contract RP253-I and by the National Science Foundation by
Iii
112
THE TEMPER EMBRITTLEMENT
PROBLEM
Grant DMR-77-08667 with Columbia University. REFERENCES 1. 2. 3. 4. 5. 6.
E.S. Machlin and Pak T. Gee, J. Solid State Chemistry 24, no. 3 (1978). E.S. Machlin and S.H. Whang, J. Phys. & Chem. Solids, to be published. E.S. Machlin and S.H. Whang, Submitted to Solid State Chem. E.S. Machlin, Acta Met. 22, 1433 (1974). E.D. Hondros and M.P. Sea-h-, Met. Trans. 8A, 1363 (1977). D.F. Stein, Interfacial Segregation, 1977 ASM, Materials Science Seminar.
Vol.
12, No. 1