Phase Transformations in an Zn-4Al-3Cu alloy

Autor: Bean-Jon Li, 李秉璋
Rok vydání: 1998
Druh dokumentu: 學位論文 ; thesis
Popis: 87
The phase transformations of Zn-4Al-3Cu alloy treated with various solution-treatment temperatures and aging temperatures were studied. The study was conducted by means of microhardness, electrical resistivity measurement, differential scanning calorimetry (DSC), thermomechanical analyzer (TMA), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and X-ray diffraction. Subsequent to casting, the hardness of Zn-4Al-3Cu alloy increases and reaches a stable value after 100 hours during natural aging. This phenomenon is attributable to the precipitation of rod-like α phase from the η dendrite obtained from solidification. At higher aging temperature (95℃), the precipitation process and the overaging process involving in α phase overlap. Consequently, a decrease of microhardness after 20 hours of aging is observed. Subsequent to this, a copper-rich ε phase is precipitated out from the η dendrite during the later stage of aging, producing a secondary hardening effect. The precipitation of α phase induces dimensional shrinkage of approximate 0.03 %. In contrast, the precipitation of ε phase in the later aging stage results in a dimensional expansion of approximate 0.15%. Two groups of parallel α phase plates form within the η dendrite during aging at 95 ℃. The orientation relationship between the rod-like αphase and ηdendrite is determined to be [-1101]η//[1-10]α and (11-20)η//(111)α,which different from that observed in Zn-4Al alloy. The phase transformations that take place during solution-treatment process in this study can be grouped into three categories. First, a plate-like ε phase of 3 μm length and 0.5 μm thickness is observable in the ηdendrite after heat-treating below 250℃. Above this temperature the gradual dissolution of the ε phase occurs. This result is opposite to what is claimed by E. Gebhardt and S. Murphy in their ternary phase diagrams. Secondly, a four-phase transformation, α+ε→ T''+η, occurs in the temperature range between 250℃ and 310℃. Here, T'' phase, having the rhombohedral structure, forms at the interface between ε platelet and ηdendrite. Thirdly, a spherical β phase can be observed in the ηdendrite when the solution-treatment temperature is increased to above 310℃. The β phase is supposed to form at the eutectoid temperature (288℃). It is concluded that the microhardness of ηdendrite will increase appreciably in 40 minutes subsequent to quenching from a solution- treatment temperature above 240℃. This microhardness increase is attributable to the precipitation of numerous fine ε phase of 0.15 μm in diameter. Orientation relationship between this fine εphase and η dendrite has been determined to be (-1011)η//(-1011)ε,(01-12)η//(01-12)ε. Further, the phase transformation of this subject alloy with various aging temperatures (50℃~150℃) after solution-treatment was also studied. It can be observed that precipitation and overaging of α phase take place almost concurrently during the first stage of aging for specimen solution-treated at 240℃. Orientation relationship between α phase and ηdendrite has been determined to be [0001]η//[111]αand (11-20)η//(1-10)α, which is the same as that observed in the cast-aged Zn-4Al alloy. The fact that the crystal orientation between α phase andη dendrite varies with copper content in theηdendrite has been established in this study. Subsequent to the precipitation of α phase, the T'' phase will be precipitated during the second stage aging for the specimen solution-treated at 240℃. The T'' phase has a rhombohedral structure, whose orientation relationship with η dendrite has been identified as [11-20]η// [111]T'' and (1-101)η// (-110)T''. The kinetics of the aging process can be described by the Arrhenius rate equation. The activation energy for formation of T'' phase is evaluated to be 66.4 kJ/mole, which is lower than that for the self-diffusion of zinc atoms in pure zinc (91.1 kJ/mole). It is postulated in a proposed diffusion model that the lower activation energy for formation of T'' phase is attributable to the high-diffusion path at the α/ηinterface.
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