Monazite (monoclinic LaPO₄) slip systems at room temperature

Abstract

Polycrystalline monazite (monoclinic LaPO₄) was deformed by spherical indentation at room temperature. Slip systems were identified using TEM of thin sections prepared parallel and close to the indented surface. Dislocation Burgers vectors (b) were identified by Burgers circuit closure in high resolution TEM images, supplemented by diffraction contrast where possible. A total of 441 b determinations were made in 97 grains. The most common slip systems were [001]/(010), [100]/(010) and [010]/(100). Slip on (001) was less common. Many other less common slip systems and Burgers vectors were also identified, including b = [101], [101], [011], [110] and [111]. b = [101] dislocations dissociate into ½[101] partials, and b = [101] dislocations are inferred to dissociate to ½[101] partials, with a low energy stacking fault of ∼30 mJ/m². b = [100] dislocations may dissociate into ¼[210] + ¼[210] partials. b = [010] may sometimes dissociate to ½[010] + ½[010] partials. Other types of partial dislocations were also observed and discussed. All partial dislocations were climb dissociated. The line energies of monazite dislocations and their partials were calculated, and stacking fault structures for partial dislocations are analyzed. Satisfaction of the Von Mises criterion for full ductility most likely involves [101]/(111) and ⟨011⟩/{011} or {111} slip, but other combinations that require both b = [101] and ⟨011⟩ or ⟨110⟩ are possible. If deformation twinning is active, slip systems with b = ⟨011⟩ or ⟨110⟩ may not be necessary for full ductility.

Keywords:

• dislocations
• slip systems
• monazite
• monoclinic
• partial dislocations
• TEM

Acknowledgements

Support of this work from AFOSR Grant 03ML03COR is gratefully acknowledged. Indented monazite was supplied by J. B. Davis and D. B. Marshall at Teledyne Scientific. J. Douin at CEMES-CNRS supplied the DISDI program and made helpful modifications to it for monoclinic crystals. P. Mogilevsky made helpful suggestions during review of this paper.