Microstructure characteristics of Ni-43Ti-4Al-2Nb-2Hf alloy prepared by conventional casting and directional solidification

To further investigate the microstructure characteristic and solidification mechanism,so as to provide knowledge for the microstructure control of a NiTi-Al based high-temperature structural material,the microstructure of Ni-43Ti-4Al-2Nb-2Hf(at.%)alloy ingots prepared by conventional casting(arc-melting)and directional solidification (DS)at various drawing velocities(2 mm·min -1 ,18 mm·min -1 ,30 mm·min -1 and 60 mm·min -1 ,respectively)was investigated by means of electron probe microanalyses.Experimental results reveal that the microstructures are composed of NiTi matrix phase,β-Nb phase and Ti 2 Ni phase for samples obtained by both conventional casting and DS.Conventional casting has an equiaxial structure,while DS has a slender and acicular cellular structure which grows along the[001]orientation preferentially.Small amounts of whiteβ-Nb phase and black Ti 2 Ni phase co-exist at the grain boundaries or intercellular regions.With an increase in drawing velocity,the NiTi matrix phase is inclined to grow along(100)and(200)crystallographic planes,and the cellular arm spacing reduce gradually, but the directionality of the solidified structure weakens significantly.The homogeneous dispersion ofβ-Nb phase and the decrease of Ti2Ni phase in DS samples are beneficial to improving the mechanical properties.Solidification mechanism analysis indicates that the dark grey NiTi matrix phase initially precipitates from the liquid phase,and then the divorced eutectic reaction takes place,which produces the light gray matrix phase andβ-Nb phase.Finally, the peritectic reaction happens,which generates the black Ti2Ni phase.The complete solidified path of the alloy is L→NiTi+L→NiTi+β-Nb+L→NiTi+β-Nb+Ti 2 Ni.

Experimental and Simulation Studies on Fabricating GCr15/40Cr Bimetallic Compound Rollers Using Electroslag Surfacing with Liquid Metal Method

Electroslag surfacing with liquid metal （ESSLM） is an excellent method for producing high quality bimetallic compound rollers. The quality of each compound roller is primarily determined by the metallurgical quality of the combined interface. A GCrl5/40Cr compound roller is produced using an ESSLM non-consumable electrode electro- slag heating method. The temperature and electric fields produced by the ESSLM system are calculated. As the roller core moves downward in the mold, it passes through five sections., the preheating section, the rapid heating section, the temperature homogenizing section, the bimetal fusing section and the cooling section which listed from the top to bottom of the mold, respectively. The temperature distribution and the degree of the surface temperature fluctuation in the roller core are different for each section. Near the combined interface, four layers are found from the roller core to the cladding layer= the remelting layer, the fusion layer, the interface solidification layer and the chilling layer, re spectively. Among these, the fusion and interface solidification layers are the key transition zones that greatly influ- ence the combination quality. The surface temperature of the roller core prior to cladding is mainly determined by the drawing velocity, and the thickness of the transition layer increases as the drawing velocity decreases. A transition layer that is too thick or too thin will reduce the mechanical properties at the combined interface. Therefore, the drawing velocity should be limited to a moderate range to produce a satisfactory bimetallic Compound roller.