Effect of Heat Treatment on Fatigue Properties of Surface Nanocrystallized Commercially Pure Zirconium

Abstract:

Commercially?pure?zirconium?was?strengthened?by?using?the?surface?mechanical?attraction?treatment?(SMAT).?Grains?in?the?surface?have?been?refined?and?compressive?residual?stress?has?been?induced.?Then,?heat?treatment?was?conducted?to?release?the?residual?stress?while?the?refined?microstructure?remained.?surface?microstructures?were?examined?by?optical?microscope?(OM)?and?transmission?electron?microscope?(TEM).?Residual?stresses?at?different?depths?were?measured?by?X-ray?diffraction?method.?Fatigue?properties?were?evaluated?by?four-point?bending?fatigue?test.?Morphology?of?the?fracture?surface?was?observed?by?scanning?electron?microscope?(SEM).?And?influence?mechanism?of?fatigue?properties?was?analyzed.?Based?on?the?experiments,?it?can?be?found?that?the?deformation?layer?with?the?depth?of?150μmwere?formed?after?SMAT.?Average?grain?size?of?surface?was?refined?to?35?nm.?And?a?residual?stress?field?with?depth?of?334?μmand?the?maximum?stress?of?-695.5MPa?can?be?obtained.?After?heat?treatment,?the?depth?of?residual?stress?field?reduced?to?115?μmand?the?maximum?stress?turned?into?-148.8MPa.It?is?found?that?residual?compressive?stress?can?improve?the?fatigue?limit?and?affect?the?location?of?crack?source?significantly.?The?fatigue?limit?of?SMATed?samples?was?increased?by?23%?compared?with?untreated?samples.?After?heat?treatment,?the?fatigue?limit?of?was?increased?by?13%?compared?with?the?samples?before?SMAT.