Joining of NiAl to Nickel-Base Alloys by Transient Liquid Phase Bonding
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cooling to room temperature, followed by heating and holding at 725 'C for 16 hours and finally cooling to room temperature. Conversely, a single post-bond heat treatment of heating to and holding at 1000 'C for 10 hours was employed for bonds prepared using the wide-gap technique. Fixed heating and cooling rates of 2 'Cs-' and 1.5 'Cs- 1 were used. Samples for transmission and scanning electron microscope (TEM and SEM) characterization were prepared as described in [8].
Figure 1: Microstructure of NiAI/Cu/Ni bond held for 20 min at 1150 °C (a = NiAI substrate, b = y' precipitates, c = y/y', d =/3-NiAI, and e =Ni substrate).
Figure 2: Microstructure of NiAI/Cu/Ni bond held for 2 hr at 1150 0C (a = NiAl substrate, b = y' precipitates, c = y/y', d = /3-NiAI, and e = Ni substrate).
RESULTS AND DISCUSSION In general, the microstructural development in all systems examined could be divided into three stages: initial bond formation, isothermal solidification and the terminal microstructure (post-bond heat treatments). In the following sections, microstructural features observed during each stage and their stability are described. Initial joint formation Initial joint formation of the NiAL/Cu/Ni system (holding times less than 20 minutes at 1150 'C) was characterized by the presence of a y/y' two phase mixture at the bond centerline (FIG01). This two phase mixture was a noticeable distinguishing feature of the NiA1/Cu/Ni bonds, as compared to the other bond systems examined. Transfer of Al and Ni to the joint centerline was the main factor in forming the yly' two phase mixture. In this region, the y phase matrix formed grains of about 10-50 pm in diameter which bore no orientation relationship to either of the substrates. The y' phase precipitated within the ygrains and on rygrain boundaries with a cube-cube orientation relationship to the ygrains. Within the ygrains, the y' distribution was bimodal with coarse roughly cuboidal particles, around 500 nm in length, and fine spherical precipitates with diameters up to 10 nm. A thin layer of y', coating y-ygrain boundaries, was also observed. The lack of an orientation relationship between KK9.4.2
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yand the adjacent substrates suggests that the y grains were formed, from the liquid, during cooling from the bonding temperature. In contrast, formation of y' appeared to have taken place by solid state precipitation after the joint centerline liquid has solidified to Y. The formation of the y/y' two phase mixture might be the first stage of joint formation in all bond systems examined. However, this microstructure was not observed in the NiAl/MM247 bonds. It might be the case that, microstructural progression in the NiA1/MM247 bonds (which was much faster than for the NiA1/Cu/Ni bonds) was so rapid such that observation of an analogous
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