onic microscope (TEM) and electron diffraction, etc. The test results indicated that obviously a diffusion transition zone forms near the interface of FeaAI/Q235 under the condition of heating temperature 1050～1100℃, holding time 60 rain and pressure 9.8MPa, which indicated that the diffusion interface of Fe3AI/Q235 was combined well. The diffusion transition zone consisted of Fe3AI and α-Fe(AI) solid solution. Microhardness near the diffusion transition zone was HM 480-540. There was not brittle phase of high hardness in the interface transition zone. This is favorable to enhance toughness of Fe3AI/Q235 diffusion joint.

The microstructure and phase constitution near the diffusion bonding interface of Mg/Al dissimilar materials are studied using a scanning electron microscope (SEM), X-ray diffraction (XRD) and transmission electron microscope (TEM). The test results indicated that an obvious diffusion zone was formed near the Mg/Al interface during the vacuum diffusion bonding. The diffusion transition zone near the interface consists of various MgxAly phases. The transition region on the Mg side mainly consists of Mg crystals, and the new phase formed was the Mg3Al2 phase having a face-centred cubic lattice. This is favorable for improving the combined strength of Mg substrate and diffusion transition zone.

Microstructure, precipitates and fracture morphology in the coarse grained heat-affected zone (CGHAZ) of a new high-purity 0Cr18Mo-Ti ferritic stainless steel were studied by means of optical metallography, SEM, TEM, X-ray diffractometer, etc. Experimental results indicated that grain coarsening resulted in brittle fracture in the CGHAZ of 0Cr18Mo-Ti steel. The reduction of impact toughness in the CGHAZ due to change of cooling rate can be attributed to the increase of nitrides (TiN, Cr-N, etc). These nitrides in the CGHAZ promote initiation and propagation of brittle cracks. The precipitated Cr2N nitrides in the grain boundaries decrease impact toughness in the CGHAZ of 0Cr18Mo-Ti steel by promoting crack initiation. In practical applications, the welding heat input (E) should be as low as possible to prevent toughness reduction in the CGHAZ.

The characteristics of phase constitution near the interface of Fe3Al/Q235 diffusion bonding are researched by means of scanning electron microscope, X-ray diffraction and transmission electron microscope, etc. The test results indicated that an obvious diffusion transition zone forms near the Fe3Al/Q235 interface as a result of vacuum diffusion bonding (heating temperature 1050-1080℃, holding time 60min and pressure 9.8MPa). The maximum value of the Al content 13 in the transition zone was 16.6wt.% (about 28.5 at.%). The micro-hardness in the diffusion transition zone was HM 14 200-400. The transition zone consists of Fe3Al and a-Fe(Al) solid solution. There was no brittle phase of high hardness 15 near the diffusion interface. This is favorable to the enhancing of the toughness of Fe3Al/Q235 diffusion joint.

Fe3Al intermetallic and 18-8 stainless steel were welded by means of tungsten inert gas (TIG) arc welding. The microstructure performance of the welding zone was analysed using a metalloscope and a scanning electron microscope. The test results indicated that microstructure of the welded metals consists of austenite, proeutectoid ferrite, acicular ferrite, carbide free bainite and lath martensite distributed on the austenitic boundaries as well as grains inside. The microhardness of the fusion zone was lower than that of Fe3Al base metal. The NiAl in the fusion zone was favourable to improve toughness and avoid the welding cracks.