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

近年来，人们发现金属材料的力学性能在很大程度上取决于试样的表面条件和试验环境。本研究的目的是获得金属和合金的内在力学性能，并获得环境脆性的信息。为此，研制了一种新的拉力试验机。配备了超高真空室和氩离子枪，通过蚀刻对试样表面进行清洁。利用该机器，通过尽可能以理想的方式控制试样的表面状态和试验气氛来进行拉伸试验。本研究使用的材料是五种纯金属，Cr, Mo, Ti, Ni， Al和两种合金，Ti-6%Al-4% v和Fe-3%Si。除T-6%Al-4%V外，其余均为单晶。用表面蚀刻和未表面蚀刻的试样进行了一系列的实验。另外，用dv - x - α分子轨道计算了cr中杂质元素H、O、N、C、B的电子态。(1)通过x射线光电子能谱分析，确定了Ni表面存在NiO、Al表面存在Al_2O_3、Fe-3%Si表面存在Fe_2O_3等氧化膜。大块金属和合金的屈服应力和拉应力对表面存在这种薄的氧化物或类氧化物薄膜很敏感。大多数金属和合金的固有屈服和拉伸应力都应该高于在空气中使用涂有氧化膜的试样进行常规力学试验所得到的报告值。(2)通过计算发现，B、C等杂质元素容易与Cr形成较强的共价键，影响金属Cr的力学性能。

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.