Yuji Muramatsu

Gas Contamination Due to Milling Atmospheres of Mechanical Alloying and Its Effect on Impact Strength

This study seeks to clarify the effects of gas contamination from milling atmospheres of mechanical alloying (MA) on mechanical properties. An iron-based dispersion alloy of Fe-13Cr-3W-0.5Ti-0.5Y2O3 (mass%) was selected as the experimental material. We prepared MA powders with the same composition as the dispersion alloy by milling mixed powders in various atmospheres such as argon, helium, hydrogen, nitrogen and vacuum, and then made bulk alloys by grooved rolling the MA powders. For the MA powders, we examined included atmospheric elements and their releasing processes with heat treatments under vacuum. For the bulk alloys, we also examined high-temperature behaviors of residual atmospheric elements and their effect on impact strength as a function of heat treatment time at 923 K.Experimental results showed that MA powder included some amount of atmospheric element that was unexpectedly difficult to remove with heat treatment. The contents of widely used argon and helium in MA powders, for example, were 130 and 5.3 mass ppm, and they were hard to remove even with treatments at 1323 K, which is considered the maximum allowable temperature. Therefore, most of these elements were introduced into bulk alloys as gas contamination. In contrast, nitrogen was effectively reduced with the treatment at 1323 K, while hydrogen could not be sufficiently removed. Residual argon and helium in bulk alloys formed bubbles at high temperatures and caused density decrease (swelling). On the other hand, hydrogen and nitrogen caused neither the formation of bubbles nor the swelling. Impact strength decreased with increased treatment time, and more remarkable decreases were observed in the alloys containing argon and hydrogen. We concluded that a proper milling atmosphere in MA for producing iron-based dispersion alloys applied to high-temperature materials was nitrogen.

The Effect of Stress Induced Transforming and Fabricating Residual Stresses on the Fracture Toughness of Mo-Zr02 Sintered Composites

To study the influence of residual stress on fracture toughness of Mo-ZrC2 sintered composites,the fracture toughness and residual stress under material fabrication process were examined experimentally. On based of these experimental results,the effect of initial residual stress,and of size and shape of transformation zone of ZrO2 dispersing secondary particles on toughness enhancement was analyzed numerically by finite element method.The toughnes increment of this material may be explained by the contribution of the effect of t-m stress induced phase transformation of ZrQ2 particle and surface residual stress.