~':'~!:~':"':': ~11.,I ~ ............... ~ ~f'lce ~"::~!~ ELSEVIER Applied Surface Science 90 (1995) 95-105 On the structure and chemistry of Ni 3A1 on an atomic scale via atom-probe field-ion microscopy G.P.E.M. Van Bakel 1, K. Hariharan 2, D.N. Seidman * Department of Materials Science and Engineering and the Materials Research Center, Robert R. McCormick School of Engineering and Applied Sciences, Northwestern University, Evanston, IL 60208-3108, USA Received 20 February 1995; accepted for publication 28 April 1995 Abstract The atom-probe field-ion microscope (APFIM) is employed to study the structure and chemistry of boron-doped NiaAI on an atomic scale. In this study annealed melt-extracted wire specimens were analyzed using time-of-flight, mass spectroscopy along the (100) direction exposing the {100} fundamental and superlattice planes. Not only is the depth resolution equal to the interplanar spacing of 0.18 rim, but the transitions between these planes are unambiguously identified by characteristic changes in the field-evaporation rate. The identification of the plane transitions allows, for the first time, to precisely count the number of detected atoms per plane in this material. The extent of the interruption associated with the transition from a pure nickel plane to a mixed nickel-aluminum plane is not significantly different from the reverse transition. From the small number of AI atoms encountered in the supposed pure Ni planes and the symmetry of the cubic system, it is inferred that variations in the measured composition of the mixed planes are not a result of actual composition fluctuations in this alloy, as has been previously argued. 1. Introduction Atom-probe field-ion microscopy (APFIM) en- ables the collection of quantitative chemical and structural information on an atomic plane-by-plane basis. It serves as an excellent tool to investigate the bulk of a crystal as well as the interracial regions associated with grain boundaries (GBs), that play an important role in the plastic behavior of NiaAI , on an atomic scale. Boron is an especially beneficial * Corresponding author. 1 Now with the Delft University of Technology, Delft, The Netherlands. 2 Now with the Boston Consulting Group, Chicago, Illinois, USA. microalloying element in polycrystalline NiaAI after proper thermal treatments [1-4]. Segregation of boron to GBs is a necessary condition for ductilization of brittle polycrystalline Ni3AI. Several possible mech- anisms have been proposed to explain ductilization via microalloying with boron. First, boron may strengthen cohesion between adjacent grains by changing the electronic structure of the GB region [5,6]. Second, boron segregation may be a precursor to nickel cosegregation or to disordering the GB region [7]. The latter mechanisms effectively change the crystal structure from L12 to fee. Since an fee crystal has more active slip systems than the L12 crystal dislocation transmission is facilitated, thus minimizing pile-ups of dislocations at GBs. More recently, the effects of environmental parameters have been recognized. Ambient water vapor can 0169-4332/95/$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0169-4332(95)00061-5