On fracture initiation mechanisms and dynamic recrystallization during hot deformation of pure nickel
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I.
INTRODUCTION
MOST metals
and alloys of commercial value are hot worked by rolling, forging, or by other methods to give the material a desired size and shape for use directly or for further metal working at low temperatures. For example, commercially pure nickel is hot rolled in the temperature range 925 to 1475 K with heavy work being carried out above 1175 K. ~ The strain rate is usually in the range 10 -4-10~s -~. But the temperature range for hot working of Inconel X-750, a nickel based superalloy, is drastically reduced (to 1250 to 1475 K) in comparison with that for nickel. 2 Even though it has been known to metallurgists for several decades that the safe hot working range (of temperature and strain rate) for an alloy gets narrower as the complexity of the alloy increases, no theoretical or empirical models were developed to describe the limits of the safe range. Recently, Raj 3 has described the temperature-strain rate boundaries for hot working of aluminum, based on fracture initiation mechanisms and dynamic processes. A safe hot working window was identified for aluminum in the temperature-strain rate diagram. Dynamic recrystallization was considered by RRj3 as an undesirable feature during hot working of complex shapes because localization of flow occurs. However, both dynamic recovery and recrystallization are desirable processes during hot working of a uniform billet, in order to keep the flow stresses and rates of work hardening very low. When these are properly taken into account, the safe hot working zone is well defined for nickel, as will become clear in this paper, and it may be contrasted with the map for aluminum. 3 In Section II, the micromechanisms of fracture initiation during hot deformation of pure nickel are described and modeled. Dynamic recrystallization, which is extensively reported in nickel, is considered in Section III. A map for nickel is described which is analogous to that of aluminum. 3 Since this map (see Figure 3) does not describe precisely the hot working range of nickel, a different approach was taken to construct a reliable map. The hot-ductility data for nickel 4 is carefully analyzed to yield the upper and lower bounds of strain rate for effective dynamic recrystallization. Finally, a hot working map for pure nickel is developed, which comCHIMATA GANDHI is Assistant Professor, Department of Chemical and Metallurgical Engineering, Wayne State University, Detroit, MI 48202. Manuscript submitted September 28, 1981 METALLURGICAL TRANSACTIONS A
bines the fracture initiation mechanisms and effective dynamic recrystallization. In Section IV, the effects of stress state and impurities on fracture initiation mechanisms and the limitations of hot working maps are discussed. A few conclusions are drawn in Section V. In this paper, the term 'hot-working' is extensively used to describe only continuous deformation. This should not be taken as an equivalent term for industrial hot-working processes which involve compressive stresses and discontinuous deformation. The map develo
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