The influence of aluminium and iron additions on copper-manganese-zinc alloys

Abstract

Alloys approximating to 70Cu-20Mn-10Zn are white in colour, are a phase (F.C.C) and possess mechanical properties and corrosion resistance comparable with the nickel silvers and alpha brasses. The present investigation has examined the effects of aluminium and iron as separate additions on the mechanical properties and corrosion resistance of these basic alloys. These elements have been used to replace zinc in order to retain the characteristics white colour of the alloys. With additions of upto 4.18% Al the alloys were a but at 4.28% Al the b type phase (B.C.C) appeared. A 7.3% Al alloy was wholly b. The a alloys were extremely malleable and 80% cold work was easily achieved. Solid solution hardening of the a alloys in the annealed condition raised the U.T.S. from 26 to 28 tons/in2 and the yield point from 9 to 10 tons/in2 with a fall in ductility from 49 to 40% EI. The onset of the b phase gave a marked increase in tensile strength but ductility was further reduced. In the 80% cold worked condition, the increase in U.T.S. in the a range was from 48 to 54 tons/in2. The b alloys were not very malleable. Suitable annealing treatments reduced the hardness level of all the a alloys to 80-90 HV; a level acceptable for cold stamping and coining operations. For iron additions it appeared that the alloys remained a upto approximately 0.70%. Beyond this amount an iron-rich phase was precipitated giving increased strength and hardness. In the annealed a range, the U.T.S. increased from 26 to 30 tons/in2 the yield point from 9 to 16 tons/in2 and the ductility dropped from 49% to 30% EI. In the 80% cold worked condition the U.T.S. was raised from 48 to 52 tons/in2. The onset of the iron-rich phase gave an added hardening effect and increased resistance to softening during annealing. Corrosion resistance, as measured by weight-loss in static immersion and salt-spray tests was increased 3 fold for the a type alloys containing maximum aluminium. Iron additions reduced the corrosion resistance, while the onset of the iron-rich phase caused a more rapid decrease in the static immersion test but increased resistance in the salt-spray test. (HK Lloyd is now Prof. of Metallurgy, University College of South Wales and Monmouthshire, Cardiff; and CH Ogle is now with the Royal Armament Research and Development Establishment, Fort Holstered, Sevenoaks, Kent)