Creep Behavior of a Directionally Solidified Ni 3 Al Alloy
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CREEP BEHAVIOR OF A DIRECTIONALLY SOLIDIFIED NihAl ALLOY Yun Zhang, Dongliang Lin(T.L.Lin) Department of Materials Science & Engineering, Shanghai Jiao Tong University, Shanghai 200030, P.R.China ABSTRACT The creep behavior of a directionally solidified multicomponent NisAl alloy was investigated in a temperature range from 923 to 1173 K. The dislocation structure during secondary stage creep has been examined by transmission electron microscopy. At lower temperatures from 923 to 1023K under a stress of 500 MPa, there exist a number of dense three-dimensional dislocation networks in the NisAI creep specimen,while at a higher temperature of 1173 K under a stress of 200 MPa the dislocation structure degenerates to regular two-dimensional dislocation networks.Climb of dislocations occurs in the overall test temperture range. The stress dependence and temperature dependence of creep rates for the NisAl alloy were also determined.It was found that power law creep is obeyed with the stress exponent equal to 4.7 and the activation energy equal to 326.6 kJ/mol.The climb of dislocations was suggested to be the rate controlling factor for the secondary stage creep rate. INTRODUCTION The intermetallic Ni3Al alloy is considered for high temperature structural application,due to an anomalous increase of yield stress with temperature.Therefore, the study of creep behavior of NisAl alloy becomes important to its future high temperature service. Although the short - time mechanical properties of Ni 3A1 alloys are well documented, the understanding of their longtime creep properties is far less complete. Up to now there have been only a few investigations in which secondary stage creep in Ni3AI alloy has been reported[1 -4]. Therefore, extensive work is needed to understand the creep mechanisms operating in Ni 3Al alloy. In the present invetigation, the secondary stage creep characteristics of a directionally solidified (DS) nickel-rich off-stoichiometric NisAl alloy containing Zr, Hf, Ti, Nb, Ta, W, Mo, Cr, Mg, and B were determined in the temperature range from 923 to 1173K by analyzing the stress dependence of creep rates, the activation energy, and the dislocation structure during secondary stage creep. EXPERIMENTAL The chemical composition of the NisAI alloy used for testing is as follows (at%): Zr 0.20, Hf 0.80, Ti 0.40, Nb 0.20, Ta 0.60, W 0.20, Mo 0.60, Cr 8.53, Al 16.55, Mg 0.02, B 0.15, Ni balance. The elemental additions were within their solubility limits in NisAl, as transmission electron microscopy(TEM) results showed that the alloyed Ni3AI maintained the single phase state. Using the starting materials, which were electrolytic nickel (99.9wt% purity) and aluminum (99.9wt% purity) and other high purity materials as available, the alloys were prepared by induction melting and directional solidifying in argon gas.It should be noted that the boron was added by an intermediatealloy Ni-B with a boron content of 18.57wt%. The resulting DS Ni3Al alloy plates (170rmmX 70mmX 15mm) were homogenized in air at 1473K, 5h/furnace c
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