In-Situ Observation of Grain Boundary Behavior in Ni 3 Al Alloys During Tensile Deformation in Sem
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IN-SITU OBSERVATION OF GRAIN BOUNDARY BEHAVIOR IN NiAl ALLOYS DURING TENSILE DEFORMATION IN SEM Dongliang Lin (T.L.Lin) and Da Chen Shanghai Jiao Tong University, Shanghai, China ABSTRACT with various chemical The deformation behavior of NisAl alloys compositions and subjected to different heat treatments were in-situ observed in SEN. Moreover, in-situ observations of slip trace are supplemented by the In boron-doped NiyAl direct observation of dislocation arrangements by TEM. alloys it is shown that close to the grain boundary there exists a thin slip transition region, within which slip lines are reoriented or other slip systems are operated to produce a local strain accommodation and to relax the However, boron-enhenced ductility stress concentration at grain boundaries. is seriously affected by alloy stoichiometry, the addition of a tertiary alloy element and heat treatment, etc. INTRODUCTION
I
The intermetallic compound Ni3A1, which forms an L12 ordered crystal structure, has unique properties for structural use at elevated temperatures. is its The major difficulty with NiyAl as an engineering material, however, tendency for low ductility and brittle fracture in polycrystalline forms. and Considerable effect has been devoted to improving the ductility By now significant progress has fabricability of the Ni3 Al alloys [l]-[3]. Aoki and Izumi [4] first discovered been achieved through microalloying. that small boron additions substantially improved the ductility of NisAl. Liu and Koch [5] reported a tensile elongation exceeding 50% by control of boron The concentration, alloy stoichiometry, and thermo-mechanical treatment. beneficial effect of boron was also comfirmed by Taub, Huang, and Chang [6]. Meanwhile, a great effort has also been devoted to elucidating mechanisms However, there is no consensus. responsible for boron-enhenced ductility. One suggestion is that boron increases the cohesive strength of grain boundaries [7], which is supported by the intergranular segregation of boron. An alternative suggestion [8]-[10] is that boron appears to increase the mobility of grain boundary dislocations and, thereby, to ease the grain boundary accommodation slip, which makes propagation of slip across grain boundaries easier. Besides, there exists the interpretation [11] that boron lowers the stress for dislocation generation from sources along the boundary. The present study was therefore undertaken to obtain direct observations of the slip process, microcrack propagation and dislocation/grain-boundary interactions, and the results obtained are compared for boron-doped and The behavior of deformation slip bands, microcrack undoped Ni3 Al alloys. propagation, and their interactions with grain boundaries, were in-situ Furthermore, dislocation arrangements near the grain observed in an SEM. boundary were also observed in TEM. II
EXPERIMENTALS
The nominal Table I shows the chemical composition of the specimens. composition of the aluminides were calculated according to the formula The aluminides were (Ni3Al)b-x Bx
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