Kinetics for intermetallics growth in transient liquid phase bonding (TLPB) for Ni/Al diffusion couples

S. TUMMINELLO; S. SOMMADOSSI; A. URRUTIA; S. ARICÓ

Wels

Workshop; 2nd IIW European-South American school of Weldig and Correlated Process; 2012

TU-Graz, IWW

Although there are many works dealing with properties of Al-Ni system, the basic atomic mechanisms which control a number of important phenomena like diffusion-reaction are still not fully understood. The aim of this work is to analyze the phase stability of the intermetallics formed in the Ni/Al interaction during the Transient Liquid Phase bonding process (TLPB), i.e. during isothermal phase transformation in diffusion-reaction couples between 800 and 1170ºC. Optical micrographs and SEM-WDS analyses showed the presence of 5 intermetallic layers (Ni3Al, C4, C3, Ni2Al3, NiAl3) in the bonding zone between Ni/Al, despite the phase diagram only predicts 4 phases. Two layers with chemical compositions close to the NiAl existence range edges were observed between 800 and 1170ºC, named here C3 and C4. The C4 layer showed evidence of two morphology types: smooth close to C3 and a dashed martensitic-like close to Ni3Al. The C3 layer may be related to Ni3Al4 or NiAl_Ni-poor, while C4 with NiAl_Ni-rich. The kinetic study showed a diffusion controlled growth for C3 and interface control for C4 at 1170ºC. The micro-hardness tests indicated that C3 (617 HV0,05) was harder than C4 layer, both smooth (450 HV0,05) and martensitic type (388 HV0,05) regions. The bonds have stable interfaces, without defects and cracks, even after several thermal cycling with abrupt steps from 1170 to 0ºC.