Fracture toughness tests were carried out using different Zr composition in Zr Cu Al based bulk metallic glasses (BMGs) and heat treated those just under the glass transition temperature (T g ) in order to examine the effects of Zr composition and heat treatment on the fracture toughness (K Q ; fracture toughness including the values under unsatisfying the plane strain condition). Zr 50 Cu 40 Al 10 , Zr 55 Cu 35 Al 10 and Zr 60 Cu 30 Al 10 BMGs fabricated by a tilt ark casting method were used. Heat treatments were done for 90 minutes just under the T g s of each BMG. Tests were conducted based on ASTM E399. Compact type specimen was used (thickness=2.3, 2.0 mm, thickness : width=1 : 4). Fatigue pre crack was induced under DK less than 6 MPa・m 1/2 by a servohydraulic fatigue machine and the fracture toughness test was done by using the same machine. The K Q dominantly increased with increasing the Zr composition. The K Q in heat treated BMGs decreased in comparison with that in unheat treated BMGs in all Zr composition. In the BMG of the high Zr composition, however, the decrease was small after heat treatment, and the K Q showed enough high values. In heat treated BMGs, the dispersion of K Q s did not become small, but fatigue pre cracks were induced more vertical against the loading axis and less bifurcation than those in unheat treated BMGs.
Fatigue tests were carried out on nanocrystalline Tiand Cu based bulk metallic glasses (BMGs) for which there are still no reports on fatigue strength. Test alloy rods with a diameter of 2 mm were prepared in nanocrystalline Ti 41.5 Zr 2.5 Hf 5 Cu 42.5 Ni 7.5 Si 1 at and Cu 60 Zr 30 Ti 10 at systems by copper mold casting. Nanocrystals were dispersed in the metallic glassy phase in both BMGs. The tensile strength (s B ) in the nanocrystalline Tiand Cu based BMGs were 2.04 GPa and 2.00 GPa, and Young's modulus were 95 GPa and 114 GPa, respectively. The test specimens were machined to hourglass shape type (the radius in axial direction; 4.45 mm, the minimum diameter; 0.9 mm), and after machining the specimen surfaces were electropolished by 50~ 100 mm. The specimens were tested at a stress ratio of 0.1 and a frequency of 5 8 Hz. The fatigue limit (s w ) (half a total stress amplitude) and fatigue ratio (s w /s B ) in the nanocrystalline Tiand Cu based BMGs showed very large values (Ti based alloy; 806 MPa and 0.40, Cu based alloy; 490 MPa and 0.25). This experimental result indicated a possibility that the nanocrystal dispersed BMGs have higher fatigue strength than the high strength crystalline alloys with high fatigue strength (e.g. Cr Mo steel, s w 500 MPa). (Received February 16, 2006; Accepted August 7, 2006) Keywords: fatigue, fatigue limit, bulk metallic glass, bulk glassy alloy, amorphous, nanocrystal
Fracture toughness of the Cu Zr Ti bulk metallic glass (Former Cu Zr BMG) under plane strain condition could not be evaluated because the BMG with a sufficient thickness for the plane strain condition had not been fabricated. However, as the fracture morphology was similar to that in the Zr based BMGs having the large plane strain fracture toughness (K IC ), e.g. about 50 MPa・m 1/2 , it was presumed that the Former Cu Zr BMG has a large K IC . Recently, in Cu Zr Ag Al BMG (New Cu Zr BMG), a large sample of 25 mm in diameter became possible to be made. Therefore, in this study, K IC of the New Cu Zr BMG was investigated. The New Cu Zr BMG plate sample of Cu 36 Zr 48 Ag 8 Al 8 was made by a copper mold casting method. Tensile strength was 1850 MPa and Young's modulus was 115 GPa. Compact type specimen with 2.3 mm in thickness and 9.2 mm in width was used. Fatigue pre crack was induced under stress intensity factor range (DK) less than 6 MPa・m 1/2 , and the fracture toughness test was conducted at a loading rate of 1 MPa・m 1/2 ・s -1 . The new Cu Zr BMG exhibited a K IC of 17 MPa・m 1/2 . The former Cu Zr BMG that could not satisfy the plane strain condition exhibited a fracture toughness of 62 MPa・m 1/2 . The stretched zone width along the inner fatigue crack front and vein pattern size on the unstable fracture surface in the new Cu Zr BMG were smaller than those of the former Cu Zr BMG. Therefore, it seems that the K IC of the new Cu Zr BMG is smaller than that of the former Cu Zr BMG.
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