Holding time influence on creep behavior of transient liquid phase bonded joints of Hastelloy X

Transient liquid phase (TLP) bonding using various holding times (5, 80, 320, and 640 min) was conducted on Hastelloy X using an Ni-Cr-B-Si-Fe filler metal at 1070 degrees C. High temperature stress rupture (creep) tests were performed on the TLP bonded joints at 816 degrees C under a stress of 110 MPa. The 320 min holding time sample had the highest rupture life (95.9 ks). The presence of uniformly distributed borides along the joint grain boundaries was determined to be the primary reason for improved creep properties. These particles were observed using electron backscattered diffraction (EBSD). The large values of creep strain observed in the 640 min holding time sample were due to a decrease in the number of grain boundary borides present in the joint area. An investigation on the creep fracture surfaces revealed a transition of the failure mechanism from transgranular to intergranular with increasing holding time. The fracture surface and elemental map were studied using electron probe microanalysis (EPMA); results showed that cracks initiated at the interface of eutectic components in samples with lower holding times (5 and 80 min), while the hard and brittle grain boundary borides available in the joint were the preferred sites for crack initiation in the 320 min sample.