Work strengthening by a deformation-induced phase transformation in “MP alloys”

Work strengthening and microstructure were investigated for a class of alloys, designated “MP Alloys”, containing 20 pct Cr, 10 pct Mo, and the remainder cobalt and nickel in proportions ranging from 60Co∶10Ni to 30Co∶40Ni. These alloys, in the fully annealed, homogenized condition, have a fcc structure with yield strengths ranging from about 45 to 60 ksi. Deformation at room temperature rapidly increases the yield strength of the alloys to about 250 ksi. Structural analyses by X-ray and electron diffraction techniques indicate that this marked increase in strength is associated with a deformation-induced martensitic transformation forming a network of extremely thin hcp platelets within the fcc grains. The nature of this martensitic transformation was studied as a function of alloy composition, deformation temperature, and structural variables, such as the platelet size, thec/a ratio of the hcp phase, and twinning.