Part VIII – August 1968 - Papers - The Microplastic Response of Partially Transformed Fe-31Ni

The effects of testing temperature, frorn 77" to 420" K, and volume fraction of martensite on the micro-plastic response of unaged Fe-31Ni martensite-austenite aggregates have been determined. The kinetics of the aging phenomena which lead to a decrease in the microplastic response were also characterized. These determinations, supplemented by other experimental results, show that at least two mechanisms of plastic deformation give rise to the apparent softness of the quenched structures. Only one of these mechanisms is fully discussed in this paper The transformation of retained austenite to martensite during the application of stress leads, in specified conditions, to large microplastic strains. This deformation behavior cannot be described by normal transformation plasticity theory but is shown to result from the fact that stress-assisted formation of martensite is a possible deformation mode. The present results and further considerations of previous work lead to the conclusion that it is unnecessary to postulate a special work-hardening mechanism to explain the mechanical properties of unaged martensite. It is now generally accepted that dislocation motion can occur in many solids at stresses very much below the L'macroscpic" yield stress, e.g., 0.1 pct offset. This phenomenon has been investigated by a variety of techniques including measurements of elastic limit,' effective static elastic modulus,~ and irreversible deformation following stressing at low levels.3"5 Of particular interest to the work to be described are exper -iments conducted on the deformation of martensite in attempts to decide whether freshly quenched ferrous martensites are "hard" or "soft".6 Muir, Averbach, and cohenl and McEvily, Ku, and ~ohnston' have shown that as-quenched ferrous rnartensites can be plastically deformed at relatively low stresses. A difference between these two sets of experiments exists in that some diffusion of carbon would take place before testing in the experiments of Muir et a1. because the plain Fe-C alloys which they have tested transform to martensite well above room temperature. McEvily et a1. examined Fe-Ni-C alloys with Ms of about -30"~ and tested the alloys directly after quenching to — 195" ~ —a technique which obviates any appreciable carbon diffusion. Unfortunately, a characteristic of the alloys which transform below room temperature is that they do not transform entirely to martensite. The results discussed below will show that, because the transformation of this retained austenite under stress leads to plastic deformation, one cannot investigate the properties of martensite by such experiments. The existence of a second deformation phenomenon, which is not caused by retained austenite, is also established in the present work. In line with a previous suggestion,' it is believed that the second microdefor-mation mode is principally due to the internal stresses generated by the formation of martensite. To avoid confusion in the present report, the evidence we have found for this interpretation will be discussed separately in a brief note.7 EXPERIMENTAL Materials. The alloys were induction-melted and cast under vacuum; the resulting compositions are given in Table I. The ingots were hot-swaged to 2-in.-diam bar and further cold-swaged and/or cold-rolled prior to specimen preparation. The standard tensile specimens were machined from sheet. The gage section was 0.05 by 0.2 by 5 in.; 0.75-in.-wide ends had 0.2 5-in. centered holes for pinloading. The three-point bend samples were 0.075-in. thick and 0.6 in. bide. The distance between the outer loading points was 5.5 in. In order to establish a standard starting condition, all specimens were quenched to 77°K prior to annealing in vacuo for austenitizing. The temperature of the austenitizing anneal was controlled to + 5"~. The testing and aging temperatures were maintained by various liquid baths (nitrogen, 77"~; freon, 130° to 200°K; acetone, 200 to 300°K; silicone oil, 300' to 450°K) to better than il°K. Strain Measurement. The results herein were derived from both uniaxial tension and three-point bending experiments. For bending tests, the stresses and strains reported are those corresponding to the maximum fiber values. Normally, because of the small strains involved, very sensitive strain measurements are necessary to make microplastic measurements. However, because of the magnitude of the dilatation and shear involved in the martensitic transformation, the requirements in the present experiments proved to be less rigorous. In most experiments the plastic strain was evaluated by measuring stress relaxation and modulus defects. In this method, specimens are loaded rapidly to some predetermined load on an In-