Measurement of Intrinsic Mechanical Properties of Brittle Metallic Materials with Ideally Clean Surface and Approach to Environmental Brittlement

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Recently, it has been revealed that mechanical properties of metallic materials are strongly dependent on the surface conditions of specimens and on the test environments as well. The purposes of this research are to get intrinsic mechanical properties of metals and alloys and also to get information of environmental brittlement. For these purposes a new tensile test machine was made. It was equipped with an ultra-high vacuum chamber and an Ar ion gun to clean the specimen surface by etching. With this machine, tensile tests were carried out by controlling the specimen surface-state and the test atmosphere in an ideal manner as much as possible.Materials used in this study were five pure metals, Cr, Mo, Ti, Ni, Al, and two alloys, Ti-6%Al-4%V and Fe-3%Si. Except for T-6%Al-4%V,all the specimens were single crystals. A series of experiments was carried out using the specimens with and without surface etching. In addition, the DV-Xalpha molecular orbital calculation was performed to simulate the electronic states of impurity elements, H,O,N,C and B in Cr.The results obtained in this study were summarized as follows ;(1) The existence of oxide films such as NiO on Ni, Al_2O_3 on Al and Fe_2O_3 on Fe-3%Si, was identified by the X-ray photoelectron spectroscopy. The yield stress and tensile stress of bulk metals and alloys were sensitive to the existence of such thin oxide or oxide-like films on the surface. Both the intrinsic yield and tensile stresses of most metals and alloys were supposed to be higher than the reported values which have been obtained by a conventional mechanical test in air using the specimens coated with some oxide films.(2) It was found from the calculation that impurity elements such as B and C tended to make the strong covalent bond with Cr and to affect the mechanical properties of Cr metal.