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The ratio of surface area to volume increases as a material gets smaller. For this reason, the mechanical properties of a micro sized material are strongly influenced by the surface conditions. To accurately study how the mechanical properties are influenced by the size, we need to test specimens with negligible work affected layers. However, conventional microfabrication methods are poorly suited for the manufacture of specimens that meet this condition. In order to overcome these problems, we developed a novel method for manufacturing micro sized tensile specimens with negligible work affected layers using a precision electrolytic polishing apparatus. A polycrystalline austenitic stainless steel (SUS304) wire with a diameter of 0.5 mm was selected as the material for study. The sample wire (anode) was vertically set at the center of a ring shaped counter electrode and gripped by a collet chuck. Once the sample was set, a rectangular wave voltage for electrolytic polishing was applied in a flowing electrolytic solution. The apparatus was used to manufacture a tensile specimen with a diameter of 51 mm, i.e., a diameter smaller than the average grain size of the sample. Optical, laser, and scanning electron microscope observations revealed a smooth and mirrored specimen surface. Tensile testing of the specimen by a testing machine for micro sized materials revealed a clear yield point on a stress displacement curve. The yield and fracture strength of the specimen were 100 and 180 MPa, respectively, or about the same as those of a single crystal. The specimen deformation observed by scanning electron microscope mainly occurred at single crystal regions placed between grain boundaries.
The ratio of surface area to volume increases as a material gets smaller. For this reason, the mechanical properties of a micro sized material are strongly influenced by the surface conditions. To accurately study how the mechanical properties are influenced by the size, we need to test specimens with negligible work affected layers. However, conventional microfabrication methods are poorly suited for the manufacture of specimens that meet this condition. In order to overcome these problems, we developed a novel method for manufacturing micro sized tensile specimens with negligible work affected layers using a precision electrolytic polishing apparatus. A polycrystalline austenitic stainless steel (SUS304) wire with a diameter of 0.5 mm was selected as the material for study. The sample wire (anode) was vertically set at the center of a ring shaped counter electrode and gripped by a collet chuck. Once the sample was set, a rectangular wave voltage for electrolytic polishing was applied in a flowing electrolytic solution. The apparatus was used to manufacture a tensile specimen with a diameter of 51 mm, i.e., a diameter smaller than the average grain size of the sample. Optical, laser, and scanning electron microscope observations revealed a smooth and mirrored specimen surface. Tensile testing of the specimen by a testing machine for micro sized materials revealed a clear yield point on a stress displacement curve. The yield and fracture strength of the specimen were 100 and 180 MPa, respectively, or about the same as those of a single crystal. The specimen deformation observed by scanning electron microscope mainly occurred at single crystal regions placed between grain boundaries.
A micro-sized fracture toughness testing technique has been applied to investigate the fracture properties of colonies in a fully lamellar Ti-46Al-5Nb-1W alloy. Micro-sized cantilever specimens with a size ≈ 10 × 20 × 50μm 3 were prepared by focused ion beam machining. Notches with a width of 0.5μm and a depth of 5μm were also introduced into these specimens by focused ion beam machining. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness values obtained were in the range 1.4 − 6.9MPam 1/2. Fracture surface observations indicate that these variations are due to differences in local lamellar orientations ahead of the notch. These fracture toughness values are also lower than those having been previously reported in conventional samples. This may be due to the absence of significant extrinsic toughening mechanisms in these micro-sized specimens. Fracture mechanisms of these alloys are also considered on the micrometer scale. The results obtained in this investigation give important and fundamental information on the development of TiAl based alloys with high fracture toughness.
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