Dislocation density in Ni 3 (Al,Hf) T. Kruml a,b , V. Paidar a,c, *, J.L. Martin a a De Âpartment de Physique, Ecole Polytechnique Fe Âde Ârale de Lausanne, 1015 Lausanne, Switzerland b Institute of Physics of Materials, Academy of Sciences, Z Ï iz Ïkova 22, 616 62 Brno, Czech Republic c Institute of Physics, Academy of Sciences, Na Slovance 2, 182 21 Praha 8, Czech Republic Received 22 November 1999; accepted 5 January 2000 Abstract The temperature dependence of dislocation density and its relationship with the anomalous growth of the ¯ow stress are pre- sented. A pronounced density increase of about one order of magnitude in a temperature range below the peak of anomalous ¯ow stress growth has been clearly established experimentally. The correlation with dierent components of the applied stress is exam- ined and it is shown that the dislocation density is related to the stress required for dislocation multiplication. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Nickel aluminides, based on Ni 3 Al; B. Mechanical properties at high temperatures; D. Defects: dislocation geometry and arrangement 1. Introduction In spite of the sustained attention paid to the L1 2 ordered alloys represented by the Ni 3 Al intermetallic compound [1±3], the mechanism of the anomalous ¯ow stress behaviour in some of these alloys has not been entirely explained yet. Combined experimental and the- oretical work is still needed to attain further progress. Our recent measurements of dislocation density after plastic deformation in the temperature domain of ¯ow stress anomaly in Ni 22 at%Al 3 at% Hf are reported in this paper. Moreover, it is shown that the increase of dislocation density can be correlated with the stresses increasing with temperature that were measured on the same alloy before [4±6]. For comparison, the dislocation density determined experimentally by Baluc for Ni 24 at% Al 1 at% Ta is also discussed [7]. 2. Experiment Single-crystalline rods of the tested material had a chemical composition of Ni 74.8 Al 21.9 Hf 3.3 (at %). This ternary alloy belongs to a wider series of nickel base alloys with the L1 2 structure carefully investigated in our laboratory. It was studied in more detail as the eect of Hf on the stress is very pronounced. The specimens were cut from the rods by spark erosion and the surface of the specimens was then mechanically and electrochemically polished. The ®nal specimens had a square cross-section with sides of 2.9 mm and were 6.5 mm in length. The compression axis was chosen to be the [1 - 23] single slip orientation which leads to large values of the Schmid factor (m) for the [1 - 01] (111) primary octahedral slip system (m=0.47) and its [1 - 01] (010) cube cross-slip sys- tem (m=0.40), while the Schmid factor for the [1 - 01] (11 - 1) octahedral cross-slip system is zero. The specimens were compressed by P. SpaÈtig [4] in constant strain-rate tests over a wide temperature range from 84 K to 780 K, at a nominal strain rate of 510 5 /s and a shear strain mostly of about 10% on a Schenck RMC 100 machine. Details of this procedure can be found in [8]. The thin foils for transmission electron microscopy (TEM) observations were prepared from the specimens by a standard technique, consisting of spark-cutting, grinding and electrolytic double-jet thinning. Two dif- ferent electrolytes were used for thinning: a 10% perchloric acid in methanol at 10  C and 30 volts, b 45% acetic acid+45% butoxyethanol+10% per- chloric acid at 10  C and 15 volts, with slightly better performance of the second one. 0966-9795/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0966-9795(00)00004-2 Intermetallics 8 (2000) 729±736 * Corresponding author at Institute of Physics, Academy of Sci- ences, Na Slovance 2, 182 21 Praha 8, Czech Republic.