Materials Science and Engineering A264 (1999) 108 – 121 Thermal residual stresses in NiAl–AlN–Al 2 O 3 composites measured by neutron diffraction Hahn Choo a, *, Mark Bourke b , Philip Nash a , Mark Daymond b , Ning Shi b a Mechanical, Materials and Aerospace Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA b Manuel Lujan Jr. Neutron Scattering Center, Los Alamos National Laboratory, Los Alamos, NM 87545, USA Received 16 March 1998; received in revised form 30 October 1998 Abstract Thermal residual stresses in NiAl – (AlN) Particle –(Al 2 O 3 ) Fiber composites were measured at room temperature for two Al 2 O 3 fiber volume fractions (5 and 30%) using neutron powder diffraction. Lattice parameters in each phase were obtained by refining diffraction patterns using Rietveld analysis and lattice strains were calculated from changes in their values. By assuming a volume averaged hydrostatic situation, in conjunction with an FE model that was used to infer the initial stresses between NiAl and AlN, the strains were converted into stresses. The tensile residual stresses in the NiAl matrix were 0.2 and 0.6 GPa for the 5 and 30% Al 2 O 3 composites respectively. The compressive residual stress in the AlN dispersion particles decreased from -1.5 to -1.1 GPa as the fiber volume fraction increased from 5 to 30%. The compressive residual stresses in the Al 2 O 3 fibers were -1.7 and -1.3 GPa for the 5 and 30% Al 2 O 3 composites respectively. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Residual stress; Neutron diffraction; NiAl; Composite 1. Introduction NiAl and NiAl-based alloys have potential for use as high temperature structural materials because of their attractive properties such as low density, high melting temperature, high thermal conductivity and excellent oxidation resistance [1,2]. Recently developed AlN dis- persion strengthened NiAl, processed by mechanical alloying (MA), shows excellent high temperature strength [3–6] and is a potential replacement for exist- ing superalloys. However, it suffers from low room temperature fracture toughness and requires better creep resistance before it could be used in practice. One possible solution is to reinforce the NiAl with ceramic fibers such as Al 2 O 3 [7–10]. To achieve a balance between the room temperature fracture toughness and high temperature strength, a hybrid NiAl composite containing both AlN dispersion particles and Al 2 O 3 fibers has been developed [11,12]. As in most fiber-reinforced metal-matrix composites, there is a considerable difference in the coefficient of thermal expansion (CTE) between the matrix (NiAl) and the reinforcements (both AlN and Al 2 O 3 ). Thus on cooling from the fabrication temperature, thermal residual stresses develop as a result of differential ther- mal contractions of the three phases. These stresses influence mechanical properties such as yield strength, fracture toughness and fatigue resistance [13–19]. Therefore understanding their evolution and magnitude is important, especially in systems where the matrix has high strength both at ambient and high temperature but limited low temperature ductility [11,12]. Since thermal neutrons penetrate deeply (3 – 4 cm) into most engineer- ing materials and allow bulk average measurements, neutron diffraction is an effective, non-destructive tech- nique for measuring strain in polycrystalline materials, including metal matrix composites [20 – 23]. The work described in this paper characterizes the room temperature residual strains (and stresses) in each phase in NiAl–(AlN) Particle –(Al 2 O 3 ) Fiber composites for two Al 2 O 3 fiber volume fractions. Neutron measure- ments were made at the Manuel Lujan Jr. Neutron Scattering Center (MLNSC), a spallation neutron source, located at Los Alamos National Laboratory. * Corresponding author. Present address: Los Alamos National Laboratory, Los Alamos, NM 87545, USA. Tel.: +1-505-665-0839; fax: +1-505-665-2676. E-mail address: choo@lanl.gov (H. Choo) 0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII:S0921-5093(98)01116-2