The physics of high temperature creep in metals

Abstract

A consequence of the growing success in producing alloys that deform very slowly even at high stress and temperature is that the problem of creep fracture has loomed larger. There is a fracture mechanism quite distinctive to creep, in which tiny holes nucleate and grow by some means until they are so large, or sufficiently linked together, that the metal breaks. The speed of this fracture process increases with temperature and stress, and evidently also depends on composition in a complicated way, which has made possible a certain degree of manufacturing control if not of understanding. In essence, the formation and growth of the holes is a phase change in which stress provides the driving free energy since the holes enlarge the overall dimensions. Both the nucleation and growth rates help to determine the time to fracture, which is what really matters. There are several obscure points such as the nature of many of the nucleating sites and the strong influence of deformation rate during nucleation and growth. Both theoretically and practically, understanding of the youthful problem of creep fracture is less mature than that of the older problem of creep deformation.' Creep ' is the name given to the slow deformation and slow fracture that takes place in metals under sustained load, particularly when hot. I n the light of figure 1 it is no surprise that the real study of creep commenced shortly after the turn of the century with two papers by Andrade (1910, 1914), followed by one by Dickenson (1922). These three papers formed, as it were, the head-waters of a stream which swelled as it ran through time, until now it is a very large river of publications. The purpose of this article is to describe the state of physical understanding of creep processes which this river of information has produced.The river has two main channels: that concerned with creep deformation and that concerned with the special type of fracture peculiar to creep.

Influence of Temperature on Sticking Behavior of Iron Powder in Fluidized Bed

“…With increasing temperature in the range from room temperature to 1 073 K, the DTA curve is a reposeful straight line without endothermic peak or exothermic peak. The result shows that any liquid phase is not appear in iron powder sample from room temperature to 1 073 K. Studies by McLean and Murty et al 14,15) that most metals such as iron would have logarithmic creep under stress at the temperature 0.3Tm (Tm=1 536°C) which was far below its melting point as most metals, and it even showed the property similar to the liquid around 0.5Tm. According to their conclusion, it can be found that the iron particles would become soft around the temperature Ts (0.3Tm, 461°C or 734 K), beginning to present some viscosity.…”

Examining Stress Relaxation in a Dissimilar Metal Weld Subjected to Postweld Heat Treatment

“…The residual stress relaxation was simulated through the reduction of yield strength with temperature during the heating stage and creep relaxation during the holding stage. The holding temperature was set to 760 • C for a duration of 3 h. The creep properties for the weld metal were generated from cross-weld creep tests conducted on PWHT specimens at 650 • C at various stresses ranging from 80 -120 MPa and extrapolated to the holding temperature (760 • C) using Arrhenius equation for creep as given in Eq.1 [32]. The Norton power law definition for creep can be represented as Eq.2 [33].…”

Creep as a Mechanism for Sealing Amalgams

“…The mechanical property creep is defined as the deformation of a metal (or other material) under a load that is below the proportional limit (McLean, 1966;Williams & Hedge, 1985). The rate of creep depends on applied stress, temperature and time (McLean, 1966).…”

“…The mechanical property creep is defined as the deformation of a metal (or other material) under a load that is below the proportional limit (McLean, 1966;Williams & Hedge, 1985). The rate of creep depends on applied stress, temperature and time (McLean, 1966). An essential mechanical condition required to produce creep is a continual application of monotonic, non-hydrostatic stress (Osborne, Winchell & Phillips, 1978b), and higher creep rates of metals occur at temperatures closer to their melting point (Honeycombe, 1984).…”