Effect of Thermal Residual Stress on the Mechanical Properties of NiAl-Based Composites
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neutron source, located at Los Alamos National Laboratory [9]. Since thermal neutrons penetrate deeply (3-4cm) into most engineering materials they allow bulk average measurements and neutron diffraction is an effective, non-destructive technique for measuring strain in polycrystalline materials, including metal matrix composites [10]. Lattice parameters for the NiAl, AIN and A120 3 were obtained by refining diffraction patterns using Rietveld analysis and mean phase strains were calculated from changes in their values. The composites under study contain randomly oriented particles and fibers and since neutron diffraction is a bulk averaging technique, we treated lattice parameter changes as volume-averaged mean phase strains and ignored the short range variations that occur around the particles and fibers. Assuming a volume-averaged hydrostatic situation, the strains were converted into stresses. Details on the neutron diffraction measurements will be presented elsewhere [11 ]. For the mechanical tests, rectangular cross-sectioned compression test specimens (3mmx3mmx6mm) were machined parallel to the hot pressing direction by wire electro-discharge machining from the monolithic material (NiAl-AIN) and by diamond grinding from the hybrid composites (NiAl-A1N-A120 3). Compression tests were carried out at 300K and 1300K in air at a nominal strain rate of 8.5x10 4sec' using a modified Instron universal testing machine [12]. The samples were heated to 1300K in about 1 hour and were held at that temperature for 30 minutes before the test. The temperature was kept within +/- 2K during the test. Load-displacement data were converted to true compressive stress-strain assuming conservation of sample volume. RESULTS AND DISCUSSION Microstructure
The microstructure of the composites has been previously reported [3,4] and only a summary is presented here. A typical optical micrograph, Fig. 1(a), shows a homogeneous distribution of randomly oriented A120 3 fibers in (NiAl-A1N). Fig. 1(b) is a typical transmission election micrograph showing AIN dispersoid particles dispersed homogeneously in the NiAl matrix. The NiAl grains are generally between 0.1 and 0.7 jgm, but a few large grains (1-3 gim) were also observed. The dispersoid particle size distribution is bimodal with small particles from 0.01 to 0.15jim and larger ones from 0.15 to 0.7gtm. The fibers have a grain size of 0.2-0.4jtm. Few dislocations were observed in either the NiA1 or the A120 3 phases in the as-fabricated composites and only a few large (1-3gm) NiAl grains showed dislocations. Chemical analysis [3] shows that the NiAl matrix has a composition close to stoichiometry (Ni/A1=49/5 1).
AI 0A
NiA14AIN
Figure 1. The microstructure of NiAI-3.5 vol.% AIN-30 vol.% A120 3 Composites. (a) Optical micrograph showing the A120 3 fiber distribution in (NiAI-AIN), (b) TEM micrograph showing AIN particles (labeled by arrows) distributed in the NiAI matrix (marked m).
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Residual Stress
The volume-averaged hydrostatic thermal residual stresses in the NiAI mat
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