Creep at low stresses: An evaluation of diffusion creep and Harper-Dorn creep as viable creep mechanisms
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TORICAL BACKGROUND
CREEP deformation refers to the unrecoverable plastic strain occurring in a material when it is subjected to a constant applied stress (or a constant applied load) over an extended period of time. Creep processes are diffusioncontrolled, and they become of particular importance in materials experiencing extensive periods of time at elevated temperatures, where these high temperatures are generally above ⬃0.4 Tm , where Tm is the absolute melting temperature of the material. The small-scale industries of the 19th century tended to operate at relatively low temperatures so that the occurrence of any creep deformation in mechanical parts was generally neither appreciated nor of significant magnitude to seriously impair the industrial operation. However, this situation began to change in the very early days of the 20th century when there was a concerted effort to increase the operating temperatures, and therefore the overall efficiency, of conventional working plants, such as steam boilers. It is not surprising, therefore, that the first scientific publication dealing exclusively with creep deformation should appear almost exactly 100 years ago in the classic report by Phillips[1] on the creep deformation occurring, as a function of time, in materials as diverse as Indiarubber, glass, and metal wires. This early article was followed initially by other limited publications on creep, most notably the early report by Andrade[2,3] claiming a t1/3 law in which the creep strain increases with time, t, raised to the third power, until ultimately, within the last 30 years, there appear annually a plethora of reports describing creep deformation in a very wide range of metallic and nonmetallic materials. An important change in direction appeared in the 1950s with the classic work of Dorn and his colleagues, where a phenomenological approach was developed and careful
experiments were undertaken to determine the precise functional relationship between the steady-state creep rate and external experimental parameters such as stress and temperature. This latter approach had two very significant advantages over the earlier attempts to develop constitutive relationships as in the classic Andrade t1/3 law. First, the approach, when combined with theory, permitted an assessment of the precise atomistic processes occurring during creep deformation, and thus it led to the concept of specific and well-defined rate-controlling creep mechanisms. Second, the approach provided, for the first time and over at least a reasonable range of experimental conditions, a predictive capability of the effect of changes in the operating stresses and temperatures. The many publications of this era are well documented in the creep literature, and they culminated in an extended and comprehensive overview of the creep behavior of a wide range of metals and metallic alloys.[4] This article follows on from this more recent approach, and it is concerned specifically with the significance of steady-state creep and the interpretation of rate-con
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