Part VIII – August 1968 - Papers - The Effect of Ferromagnetism upon Creep Deformation of Alpha Iron and Its Solid-Solution Alloys

Creep tests of a! iron and its solid-solution Fe-Mo, Fe-Co, and Fe-Si alloys with bcc structure were conducted under constant stresses in ferromagnetic and paramagnetic temperature regions above 0.5T,(T, is the absolute melting temperature). It was found their high-temperature creep behavior changed in the vicinity of the magnetic transformation temperature, that is Curie temperature, TC. In the ferromagnetic region below the steady-state creep rates were lower than those expected from the extrapolation of the data in the paramagnetic region, the amount of decrease being affected by solute addition. Also, activation energies for steady-state creep were different in the two temperature regions. The observed changes were concluded to be due to the effect of ferromagnetism on diffusion. It is well-known that plastic deformation of metals and alloys in the temperature region where mobility of atoms becomes high enough is controlled by atomic diffusion." Many experimental results394 on fcc and hcp materials have been reported to confirm this view. In bcc metals and alloys, not much basic investigation of creep deformation has been performed. In particular, experimental work on iron5-&apos; and its alloys, covering a quite wide temperature range is extremely scarce. On the other hand, the effect of ferromagnetism on diffusion has recently been demonstrated in a iron and its alloys;&apos; it has been made clear that diffusion rates in ferromagnetic temperatur~e region are significantly lower than those expected from diffusion measurement in the paramagnetic region. Therefore, it is concluded that the magnetic effect influences the high-temperature creep behavior in a! iron and in its alloys. The present authors have conducted creep experiments on a iron and its solid-solution alloys over a very wide temperature range, and have demonstrated the magnetic effect. A part of the result has already been reported elsewhere.26 In this paper, the experimental results will be described in detail. While the manuscript was in preparation, a similar magnetic effect on creep deformation of a iron was reported by Ishida . Though their experiments covered an extremely wide temperature range, the creep stresses varied from several thousand psi to several ten thousand psi with decreasing temperature. Consequently, it may be said that there remain some questions concerning their results. I) EXPERIMENTAL PROCEDURE The materials used in the experiments were pure iron and its alloys; they were prepared by vacuum melting (10"" mm Hg) electrolytic iron (99.9 pct), molybdenum powder (99.9 pct), ultrapure silicon, and cobalt pellet (99.5 pct) in alumina crucibles. The ingots were hot-forged to plates 10 mm thick and 50 mm wide, and then were hot-rolled at about 700°C into sheets 1 mm thick. Creep specimens, 5 mm in width and 15 mm in gage length, were machined from the sheets. They were annealed at temperatures which were well above their respective recrystallization temperatures. The chemical compositions of the metal and alloys are shown in Table I together with their heat treatments. High-temperature creep tests by the use of a lever-type creep testing machine were carried out in argon atmosphere under constant tensile stress. In order to keep &eep stress constant within about *0.5 pct, stress change due to specimen elongation was compensated by tensile force of a stainless-steel bellows which was inserted between the loading lever and lower specimen grip with the assumptions of a linear relationship between change in cross section and strain and uniform strain along the section. Beyond the limit of compensation by the bellows, small amounts of load were removed after appropriate strains. The testing temperatures were maintained to within i2"C of the reported values. Creep deformation was auto-graphically recorded at the upper (moving) specimen grip using a linear differential transformer which was held by a stainless-steel rod mechanically connected to the lower specimen grip. It was also directly measured with a dial-gage reading to -& mm. 11) EXPERIMENTAL RESULTS AND DISCUSSION 1) Creep Curves of a Iron and Its Alloys. Creep curves of a! iron, Fe-Si, Fe-Co, and Fe-Mo (<2 at. pct Mo) alloys usually consisted of three stages, that is transient (primary), steady-state (secondary), and accelerating (tertiary) stages. An example is illustrated in Fig. 1 for a! iron. However, in Fe-Mo alloys with more than 2 at. pct Mo, an inverse transient creepZ8 was observed as indicated in Fig. 2. The inverse behavior, which varied with the amount of molybdenum, may be due to the substructure* in the annealed specimen as was sug-