Columnar Grain Boundaries Research Articles

Cracking in the Weld Heat-Affected Zone of COntinuously Cast Sheet and Ingot of Ni3Al

ABSTRACTWeldability is a key issue for successful applications of ductile Ni3Al in structural components. Here we report the results of an investigation of gas tungsten arc welding behaviors of continuously cast thin sheet and cast ingot of a Ni3Al alloy, known as IC-50, containing boron and zirconium. Cracking in heat-affected zone (HAZ) was prevalent in both types of specimens. Both in cast sheet and ingot, liquation cracking was observed to occur along the grain boundaries in HAZ. Microstructural studies showed that continuously cast thin sheet consisted of columnar grains along the thickness. Easy cracking along the boundaries of columnar grains made cast sheets more susceptible to hot cracking than ingots. The possibility of segregation of boron and zirconium, causing liquation cracking, will be discussed.

アルミニウム溶接金属におけるミクロ偏析

Bead-on-plate welding was performed on thin sheets of 99.96%-, 1050- and 1100- aluminum. Then, the microsegreagation was investigated in the weld metal of these materials with the aid of electron probe microanalyser. Morevoer, distribution of dislocations and compounds in the weld mead were examined by means of electorn microscope. Main conclusions obtained are as follows:1) Compounds of Al3Fe and AlFeSi were observed in the weld metal of 1050-aluminum.2) It was observed that Fe segregated in boundaries of subgrains. This was due to the phenomenon that compounds mentioned in (1) were developed in subboundaries.3) Content of compounds in boundaries of columnar grains was almost the same as in boundaries of subgrains.4) Boundaries of subgrains observed in 99.96%-aluminum consisted of dislocations parallel to the growth direction and tangled dislocations. In this case compounds were not observed.5) There existed compounds in boundaries of subgrains in the weld metal of commercially pure aluminum and dislocations tangled in the compounds. Besides, some boundaries consisted only of dislocations.

The role of oxide microstructure and growth stresses in the high-temperature scaling of nickel

The oxidation of nickel near 1000°C is accompanied by the generation of stresses parallel with the metal-oxide interface and of magnitude (~1500 psi) sufficient to elongate nickel rod, increase sheet area, and sharpen the angle of bend of ells and helices. A primary cause of this stress is identified with the formation of layers of new nickel oxide upon boundaries of columnar grains where nickel, diffusing through the oxide crystals, meets oxygen, diffusing along grain boundaries. Classical parabolic growth of the scale gives way to a slower quasilinear rate when the major site of new oxide formation is abruptly shifted to a system of grain boundaries lying close to the metal surface and created by recrystallization of the oxide under the influence of stress and high temperature. Another source of stress in the scale arises from the constantly changing area of the metal-oxide interface when oxidation is occurring upon curved metal surfaces. This stress reinforces that generated by deposition of material at oxide grain boundaries when the surface is convex and opposes it when the surface is concave.