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Role of Surfaces and Interfaces in Controlling the Mechanical Properties of Metallic Alloys† )
Won-Jong Lee,# Wen-Jui Chia,‡ Jinliu Wang,§ Yanfeng Chen, Semyon Vaynman,^ Morris E. Fine,^ and Yip-Wah Chung*,^
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# Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, Taejon 305-701, South Korea, ‡American Medical Systems, 3070 Orchard Drive, San Jose, California 95134, § National Science Foundation, 4201 Wilson Boulevard, Arlington, Virginia 22230, Western Digital Corporation, 44100 Osgood Road, Fremont, California 94539, and ^Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208
Received April 6, 2010. Revised Manuscript Received May 22, 2010 This article explores the subtle effects of surfaces and interfaces on the mechanical properties of bulk metallic alloys using three examples: environmental effects on fatigue life, hydrogen embrittlement effects on the ductility of intermetallics, and the role of coherent precipitates in the toughness of steels. It is demonstrated that the marked degradation of the fatigue life of metals is due to the strong chemisorption of adsorbates on exposed slip steps that are formed during fatigue deformation. These adsorbates reduce the reversibility of slip, thus accelerating fatigue damage in a chemically active gas environment. For certain intermetallic alloys such as Ni3Al and Ni3Fe, the ductility depends on the ambient gas composition and the atomic ordering in these alloys, both of which govern the complex surface chemical reactions taking place in the vicinity of crack tips. Finally, it is shown that local stresses at a coherent precipitate-matrix interface can activate dislocation motion at low temperatures, thus improving the fracture toughness of bulk alloys such as steels at cryogenic temperatures. These examples illustrate the complex interplay between surface chemistry and mechanics, often yielding unexpected results.
1. Introduction Because a material interacts with the outside world through its surfaces, it is easy to see the significance of surface science in today’s wide range of scientific and engineering disciplines, including catalysis, corrosion, thin-film growth, alloy design, microelectromechanical systems, tribology, semiconductors, and magnetic storage devices. As early as 1981, Somorjai1 recognized the highly interdisciplinary nature of surface science by stating that “because of the importance of surface studies in so many areas in addition to the chemical sciences and technologies, and because of the interdisciplinary nature of these studies, we refer to the field today more frequently as surface science than as surface chemistry or surface physics. Indeed, modern surface chemistry, which includes atomic-scale scrutiny of surface chemical phenomena, has its roots in solid-state physics and vacuum sciences.” Surfaces and interfaces not only affect chemical and electronic properties but also are important in mechanical properties of bulk systems. In this article, we will discuss one aspect of surface science that is not normally represented in the surface science literature;the impact of surfaces and interfaces on the mechanical properties of metallic alloys. Our intent is to present and review subtle effects of surfaces and interfaces that have unexpected consequences on bulk mechanical properties. In this article, we will focus on three aspects of mechanical properties of metallic alloys: (1) environmental effects on fatigue life, (2) hydrogen embrittlement effects on the ductility of intermetallics, and (3) the role of coherent precipitates in the toughness of steels. † Part of the Molecular Surface Chemistry and Its Applications special issue.
(1) Somorjai, G. A. Chemistry in Two Dimensions: Surfaces; Cornell University Press: Ithaca, NY, 1981.
16254 DOI: 10.1021/la101361n
2. Environmental Effects on Fatigue Life Fatigue is the progressive, localized, permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that have maximum values less than the static yield strength of the material.2 It culminates in the formation of cracks, eventually leading to fracture after a certain number of fluctuations or fatigue cycles known as the fatigue life. It has been known since the early work of Gough and Sopwith3,4 that the fatigue resistance of metals increases with decreasing ambient pressure. That the environment should have an effect on fatigue life can be rationalized as follows. During fatigue cycling, the shear stress causes a forward and backward motion of dislocations, resulting in the formation of slip steps. Under ambient conditions, the exposed slip steps will be covered with a monolayer of oxygen almost instantly. The adsorbed oxygen layer will likely interfere with dislocation motion, thus making slip less reversible (Figure 1). For a clean slip step subject to forward and reverse shear stress, one would expect the dislocation motion to be random and reversible, analogous to random walk in statistical mechanics. In this case, the average height of a slip step should increase as N1/2, where N is the number of fatigue cycles. However, if slip is only partially reversible because of oxygen adsorption, then the average height of a slip step should increase as NR, where R is between 0.5 and 1.0. 2.1. Environmental Effects on the Fatigue Life of NbMicroalloyed Steel. Fatigue test results obtained in vacuum can (2) Fine, M. E.; Chung, Y. W. Fatigue Failure in Metals. In ASM Handbook; Lampman, S. R., Ed.; ASM International: Materials Park, OH, 1996; Vol. 19, pp 63-72. (3) Gough H. J.; Sopwith, D. G. Atmospheric action as a factor in fatigue of metals. J. Inst. Met. 1932, 49, 93-107. (4) Gough H. J.; Sopwith, D. G. Some further experiments on atmospheric action in fatigue. J. Inst. Met. 1935, 52, 55-89.
Published on Web 06/08/2010
Langmuir 2010, 26(21), 16254–16260
Lee et al.
Article
Figure 1. Schematic illustration of the effect of adsorbates on dislocation reversibility during fatigue cycling of single-crystal Ag.13
be used as a reference to elucidate ambient effects on fatigue properties. However, the vacuum employed in most investigations was not lower than 1 10-4 Pa. At these pressures, an initially clean surface can be contaminated within a few seconds. Therefore, one needs to perform such experiments under ultrahigh vacuum conditions. Along these lines, we constructed an ultrahigh vacuum electrohydraulic fatigue apparatus5 and used it to measure the fatigue lives of 0.11 wt % Nb high-strength lowalloy steel at various plastic strain amplitudes in air and in ultrahigh vacuum (base pressure
