1Hitachi Engineering Works, Ltd., Hitachi Central Laboratory, Tokyo
The electron beam melting and brittle fracture behaviour of metal molybdenum has been studied using a 60 KW electron beam melting furnace, a Tensilon testing machine, a Charpy impact testing machine and the X-ray Laue back reflection procedure. The crystal grains of electron beam melted molybdenum were extraordinally large in size and thick, and about 100 mm long columnar grains were observed in a 30 mm dia ingot. These grains, which grew columnar-like, had a certain degree of orientation preference.
The metal molybdenum in the as cast state exhibited a typical brittleness and the fractured surfaces were principally grain boundaries except cleavage fractures of (100) planes as shown by the X-ray back reflection results.
According to the spectrographic analysis, the volatile metallic impurities were volatilized preferentially as expected from their vapour pressure data. On the other hand, those metallic impurities, which have relatively higher melting points and lower vapour pressures, were rather rich in melted molybdenum. Although C,O and N were not analysed, the tensile stress-strain-diagram of an electron beam melted molybdenum wire showed almost no yielding behaviour. Therefore, the present author assumed that the electron beam melted molybdenum had the least amount of interstitial impurities. The brittle fracture strength was remarkably increased with frequency of the melting operation.
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