MATERIALS AND STRUCTURES PHYSICAL METALLURGY PROGRAM

Major effort has been spent on the problem of the ductile-to-brittle transition in the body-centered-cubic (b.c.c.) refractory metals, particularly toward understanding the mechanisms responsible for the yield point in these materials and for the strong temperature dependence of the yield and flow stress. As a result of a new interpretation of the yield point was developed, and it was proposed that the tendency for brittleness in the refractory metals is due to the inherent resistance of the b.c.c. lattice to the motion of dislocations. The other area in which significant progress was made is brittle fracture of ceramics. Work on sapphire (alpha-Al2O3) crystals showed that plastic flow and twinning play an important role in the fracture of sapphire. It was found that the effect of temperature on the fracture stress in the range of 1000 - 1500 C is related to the amount of deformation a specimen undergoes prior to fracture, rather than a direct effect of temperature on the fracture stress. (Author)