Study of the fracture behavior in soft and hard oriented NiAl single crystals by AFM F. Thome, M. GoÈken*, H. Veho University of Saarland, Department of Material Science, Bldg. 43B, PO Box 151150, D-66041 Saarbru Ècken, Germany Received 28 April 1998; revised 23 July 1998; accepted 1 September 1998 Abstract In order to understand the fracture behavior and room temperature brittleness of NiAl single crystals, atomic force microscopy (AFM) investigations were performed on in-situ loaded 4-point-bend specimens of hard and soft oriented crystals. A signi®cant amount of dislocations are emitted from the crack tip on dierent slip systems. {1 0 0} slip planes are activated in soft crystals, whereas in hard crystals also slip planes, which are identi®ed as {1 1 2} or {1 2 3} are observed. From evaluations of the AFM images, dislocation distributions ahead of crack tips and the plastic crack opening displacement (COD) from blunted crack tips are examined. The fracture toughness is calculated from the measured plastic COD and compared with the results obtained from load- displacement curves. First in-situ experiments on NiAl specimens heated up to 400 K are described, where a signi®cant increase in plasticity is observed. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: NiAl; Atomic force microscopy; Fracture toughness; Brittle-to-ductile transition; Slip systems 1. Introduction Intermetallic compounds, in particular NiAl, are promising candidates for high temperature applications. NiAl may oer the opportunity to develop a new class of alloys with unusually high melting points, low den- sities and good oxidation resistances. But still the room temperature brittleness prevents the application as a structural material. The physical and mechanical prop- erties of NiAl were reviewed by dierent authors [1,2]. The reason for the low fracture toughness is still in discussion. In¯uences from point defects and impurities probably are important, since special heat treatments were partially successful for enhancing the fracture toughness of NiAl [3]. Therefore, it is necessary to understand the local deformation behavior at crack tips to improve the toughness for future high temperature applications. Previous investigations of the fracture behavior were already performed with in situ bending experiments in an atomic force microscope (AFM) at room tempera- ture on soft single crystals [4,5]. In these studies it could be shown, that dislocation activity is still available at room temperature and quasistable crack growth is possible, too. The high magni®cation capability of the AFM allows the examination of dislocation emission processes and crack tip blunting. Even, small elastic deformations in the immediate crack tip region were observed with the AFM and were used to calculate the local stress intensity factor at the crack tip. No thin ®lms eects like in a transmission electron microscope (TEM) impede the interpretation of the experiments. Another study on the crack tip deformation behavior of NiAl uses the electron channeling contrast imaging technique for examining near surface defects in a scan- ning electron microscope [6]. In this work, the fracture behavior of NiAl single crystals is investigated in dierent orientations and temperatures by 4-point-bending experiments. Single crystals tested along h100i specimen axis, called hard crystals, and tested along h110i axis, called soft crys- tals, are loaded in-situ in an AFM (see Fig. 1). It is the objective of this study, to examine the dislocation emis- sion characteristics and the dislocation distribution around loaded crack tips. The operative slip systems of NiAl are known from previous work for dierent tem- perature regimes. In soft single crystals {1 1 0} and {1 0 0} slip planes with h100i Burgers vectors should be active at room temperature, whereas in hard orientated Intermetallics 7 (1999) 491±499 0966-9795/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(98)00120-4 * Corresponding author. E-mail: m.goeken@matsci.uni-sb.de