JOURNAL OF MATERIALS SCIENCE 27 (1992) 1599-1607 Order hardening in nickel-molybdenum and nickel-tungsten alloys N. S. MISHRA, C. D. SINGH Research and Development Centre for Iron and Steel, Steel Authority of India Ltd, Ranchi 834 002, India S. RANGANATHAN Centre for Advanced Study, Department of Metallurgy, Indian Institute of Science, Bangalore 560012, India Order hardening characteristics were studied in Ni-20% Me, Ni-25% Me and Ni-20% W. They were processed through quench and reheat cycles and the progress of order hardening was monitored through microhardness measurements. A correlation with the microstructure as observed with polarized light microscopy and transmission electron microscopy was established. Among the various factors, such as degree of order, domain size, coherency strains and dislocations, which can contribute to strengthening, domain size seems to play a major role. 1. Introduction The increase in strength of alloys with the progress of ordering is an attractive concept in the development of alloys. While its full potential is yet to be realized in practice, there are several significant contributions in this area. The topic has been reviewed by leading researchers in this field [1-4]. In his considerations of strength in isostructurally ordered alloys, Cottrell [5] established the importance of a critical domain size for peak strength. Hence maximum strength can be expected at an intermediate stage of ordering treatment. In disordered alloys, dis- locations move singly, while interactions of super- dislocations with matrix dislocations and antiphase boundaries (APBs) will lead to an increase in flow stress. In the 1930s, hardening in neostructurally ordered alloys was a subject of intensive study. There had been a resurgence of interest in this area, after Arunachalam and Cahn [6] reported their careful work on CuAu. There is nearly a doubling of strength in this alloy. The extra hardening may be attributed to the change in lattice symmetry, resulting in substantial misfit stresses. The early studies on order hardening of Ni4Mo are due to Dubrovina and Umansky [7]. At 500 and 600~ no increase in hardness was observed. The increase observed on ordering at 700 and 800 ~ was ascribed to the growth of domains and an increase in the degree of order within them. Snyder and Brooks [8] established correlations between strength and domain size. Chakravarti et al. [9] extended this work. They observed a decrease in ductility and intergranu- lar fracture of the ordered phase. They attributed these to the inhibition of cross-slip and the pile up of coplanar dislocation arrays. Ling and Starke [10] 0022-2461 9 1992 Chapman & Hall have suggested that the high strength is due to the high density of dislocations which are generated to relieve misfit stresses at the interface. Irani et al. [11] suggested that order twinning and intergranular frac- ture operated to relieve stresses in the overaged condi- tion. Chen et al. [12] studied the influence of an applied stress on ordering of Ni4Mo and noted that the number of domain variants within any one grain was affected with a consequent change in mechanical properties. Recently, Kao [13] measured the strength of ordered Ni4Mo alloys via the tensile mode. Theirs is perhaps the first and only study relating yield strength and elongation with ageing time at various temper- ature up to and well beyond the critical temperature of ordering ( ~ 868 ~ Two initial domain sizes of 24 and 3000 nm were obtained by prior ordering treatment. As expected, a 50% increase in the strength was exhibited by the specimen with smaller domain size. Kao [14], however, ascribed this increase in strength to the degree of ordering rather than domain size. Their studies further revealed remarkable con- stancy of strength with test temperatures. However, the ductility remained inadequate (~ 1%). A steep rise in elongation values was observed when the sam- ples were tested at temperatures well above the ordering temperature with attendant loss in strength by about 50%. However, there is only one early study of order hardening in Ni4W by Epermian and Harker [15]. It may be relevant here to note a few significant recent contributions to hardening of CuPt. Irani and Cahn [16] ascribe hardening in CuPt to lattice mis- match and domain size. This paper is also noteworthy for its comparative study of hardening in several noncubic ordered alloys including Ni4Mo and Ni4W. 1599