Hisafuji Watanabe1, Eiichiro Sato1, Norio Kono1 and Hiroshi Takamatsu2
1Department of Metallurgy, Chiba Institute of Technology, Narashino
The Zn-25%Mn-15%Cu alloy is one of the cast zinc alloys which have high tensile strength, endurance to abrasion and corrosion resistance. But the hardness and brittleness of this alloy increases remarkably when the alloy is heated for a long time in the range of comparative low temperatures, i.e. 150° ∼ 200°C. So the authors have investigated phase changes on several ternary Zn-Mn-Cu alloys and binary Zn-Mn alloys in order to clarify the cause of being hard and brittle. Te results obtained are as follows:
(1) Precipitation of a new phase ``B'' takes place from grain boundaries in the ternary alloys which have been brittle-hardened after heating at low temperature. This phase is the intermediate one whose structure is fcc and lattice parameter is 7.16 Å.
(2) As the eutectoid reaction of the binary Zn-Mn alloy proceeds very slowly, the phase which must be stable only at higher temperatures is carried to room temperature under such a slow cooling rate as 5°C per hour. Then, heating the alloy at about 200°C results in formation of an intermediate phase during the process of the eutectoid reaction. The intermediate phase is identical to the B phase which will cause brittle-hardening of the ternary alloy.
(3) The precipitated part of the intermediate phase is harder than that of the binary one because of the coexistence of the γ Cu phase which had been precipitated from the highly Cu-concentrated phase retained in this part.
(4) The phase changes of the Zn-25%Mn-15%Cu alloy through tempering at 300°C are indicated in the following equation:
ε →(ε+Intermediate phase+γ Cu)→(ε+Intermediate phase+γ Cu+α+β Mn)
→(ε+γ Cu+α+β Mn)→(ε+γ Cu)→ ε.
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