Journal of Nuclear Materials 141-143 (1986) 497-502 North-Holland, Amsterdam 497 THE EFFECTS OF STRAIN RATE AND LONG-TERM AGING ON THE PROPERTIES OF (Fe,Ni)sV LRO ALLOYS H.T. LIN, R.C. WILCOX and B.A. CHIN Department of Mechanical Engineering, Auburn University Auburn, AL 36830, USA This study investigated the effect of strain rate and long-term thermal aging on the mechafiical properties of the (Fe,Ni)3V long range ordered (LRO) alloys. Tensile tests covering strain rates from 10 -6 tO 10 0 S -I were performed on the ordered alloys at room temperature. The results indicate that there is little or no effect of strain rate on the tensile strength and ductility. The long-term thermal exposure at 600°C does not cause any significant change in the tensile properties. The deformation and fracture characteristics and with controlling mechanisms change as a function of strain rate and aging time. 1. Introduction Long-range-ordered (LRO) alloys offer potential advantages over conventional alloys for high-tempera- ture structural applications [1--4]. Atomic ordering pro- duces a pronounced increase in the rate of work harde- ning [4--8] and improves the fatigue resistance [9]. As a result of the stronger binding and closer packing of atoms, most kinetic processes such as creep and grain growth, are slower in the ordered structure [10]. Also, the unique dislocation dynamics associated with the or- dered lattices give the LRO alloys excellent high-tem- perature strength and fatigue resistance [11]. The main disadvantage of the LRO alloys is their tendency to be brittle in the ordered state. The newly developed LRO alloys based on the general composition (Fe,Ni)3(V,Ti) show an elongation in excess of 30% [12,13]. This has been achieved by controlling the orde- red lattice structure through use of the e/a ratio or ave- rage electron density per atom outside the inert gas shell. The excellent mechanical properties exhibited by these ordered alloys have led them to be considered as structural materials in fusion reactors and steam turbi- ne systems. Before these alloys can be used widely for high-temperature applications, the major challenge is to improve and maintain the ductility of LRO alloys in the ordered state. In an effort to clarify the deforma- tion and failure mechanisms, and to optimize the ducti- lity of the iron-base LRO alloys, this study investigates the effects of strain rate and long-term thermal aging on the room temperature tensile properties, structural stability and fracture mechanism of the LRO alloys. 2. Experimental procedure Two series of tensile specimens of iron-base LRO alloys, LRO-37 and LRO-42, were produced by Oak Ridge National Laboratory. The nominal chemical composition of LRO-37 is 37Fe-39.5Ni-22.4V-0.4Ti, while that of LRO-42 is 42Fe-39.5Ni-22.4V--0.4Ti- 0.1Ce. The specimens were solution-treated for 20 min at ll00°C, followed by an ordering treatment involving step cooling from 600°C to 500°C. The ordered speci- mens were aged at 600°C for three months and six months. Upon completion of the long-term age, all spe- cimens were electro-polished in a solution of 40% wa- ter, 40% nitric acid and 20% hydrofluoric acid. Room temperature tensile tests were performed with an MTS hydraulic testing machine at strain rates of 2.8 x 10 -6 to 2.8 x 10° s -I. To investigate deforma- tion and fracture mechanisms, selected specimens were examined in detail with an ISI-SS 40 scanning electron microscope operated at 20 kV. Additionally, the micro- structure of the LRO alloys before and after long-term thermal aging was examined using a Zeiss photomicro- scope. 3. Experimental results 3.1. Optical microscopy Fig. 1 shows optical micrographs of the LRO alloys in their ordered state before and after different aging treatments, Annealing twins are visible together with some inclusions which were analyzed to be VC parti- cles. The two LRO alloys, LRO-37 and LRO-42, were Control .*" 't~t -'~ t - " ~, -,. "*:~ Aged • ."-G-e 1 Fig. 1. Microstructure of long range ordered alloys. 0022-3115/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)