The mechanisms of crack initiation and crack propagation in metal-induced embrittlement of metals
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boundaries of the base metal have been embrittled by solid state diffusion of embrittler atoms some tens of atom diameters into and along base metal grain boundaries. (The potential role of grain boundary diffusion embrittlement was earlier suggested by Arkharov 8 and by Flegentova et al 9 as a possible effect supplementary to the major surfaceactive embrittlement effect.) None of these models has received universal acceptance, though the bond-breaking model has been most generally accepted because it seems to account for more of the experimental observations than the others. The paper presented here is a report on the first phase of an effort to help identify the correct MIE model by taking advantage of a littlestudied feature of MIE, namely, delayed failure. If a metal in contact with an embrittler is placed under a fixed tensile load of a magnitude less than that necessary to fracture it at once, it will, in most cases, undergo a form of static fatigue--delayed f a i l u r e - in which fracture takes place at constant load after a substantial length of t i m e - - u p to several hours, or even longer, depending on the load and the temperature. For a given embrittlement couple (the base metal-embrittler combination), we have found that the time to failure is both stress and temperature dependent. The phenomenon thus presents a clear opportunity to study the kinetics of the cracking process and consequently to obtain significant information on the underlying mechanisms. Until the present work, however, only a few delayed failure studies have been carried out (References 10 to 18) and of these only three published works 1~ used techniques capable of distinguishing the initiation from the propagation of the embrittlement crack. In addition, the temperature and stress dependencies have not been thoroughly studied. In the present work, pure indium (melting point 156 ~ has been used as the embrittler applied to smooth (unnotched) tensile samples of commercial 4140 steel quenched and tempered to a room temperature ultimate tensile strength of 1500 _ 12 MPa (218 ---2 ksi). The samples were tested under various fixed initial stresses ranging from about the proportional limit to just above the 0.2 pct offset yield strength at temperatures in the range 107 to 182 ~ Thus, for the first time in SMIE, the same embrittlement system has been studied extensively both in the SMIE and the LMIE
ISSN 0360-2133/82/0311-0457500.75/0 AMERICAN SOCIETY FOR METALS AND THE METALLURGICAL SOCIETY OF AIME
VOLUME 13A, MARCH 1982 - - 457
range. In each test, the electrical potential drop produced by a constant current over that portion of the sample gage length covered by indium was monitored. When a crack formed, the otherwise constant potential drop increased and continued to do so during crack propagation; the data thus provided a measurement of both the crack initiation time and the crack propagation time as a function of test temperature and initial stress, again for the first time in MIE studies. In the main body of this paper, the
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