鋅-4%鋁-3%銅合金相變化研究

Abstract

本研究主要藉由微硬度機、電阻值量測儀、熱分析儀、熱膨脹儀、掃描式電子顯微鏡、電子微探測儀、穿透式電子顯微鏡與X光繞射儀等儀器研究Zn-4%Al-3%Cu合金在不同溫度固溶化處理及時效的相變化機構。 藉由α相從η樹枝狀晶中析出,鑄造後的Zn-4%Al-3%Cu合金的硬度在室溫下逐漸上升, 而在100小時之後到達穩定值。於95℃時效的時候,α相的析出與粗大化效應重疊,導致時效初期η樹枝狀晶硬度下降, 並於20小時後達到最低值, 接著析出ε相, 產生二次硬化效果。α相從η樹枝狀晶的析出, 造成約0.03%的尺寸收縮, 而ε相在時效第二階段的析出造成約0.15%的尺寸膨脹。藉由穿透式電子顯微鏡的繞射圖可以發現η樹枝狀晶中析出的α相有兩組方向,而α/η相的晶體方向關係為: [-1101]η//[1-10]α,(11-20)η//(111)α,而這個結晶方向關係與Zn-4%Al合金中的α相結晶方向關係不同。 □□□□當Zn-4%Al-3%Cu合金在不同溫度固溶化處理時, 其相變化可以分成三種類型。第一, 當熱處理溫度低於250℃時,□□樹枝狀晶中發現長度3μm,寬度0.5μm的平板狀ε相。而這個ε相於高於250℃熱處理時, 逐漸溶解回η樹枝狀晶中。這個實驗結果與Gebhard 與Murphy等人所提出的三元相圖不同。第二, 在這個研究□吾人發現四相反應, α+ε→T'+η,發生在250℃~310℃之間,而且具有菱形晶結構的T'相在ε相與η相的界面產生。第三種相變化為固溶化處理的溫度高於310℃時, 在□樹枝狀晶□面可以發現圓顆粒的β相產生。在一般的情況,β相是由α相與η相在288℃反應而成。 當Zn-4%Al-3%Cu 合金從240℃以上固溶化處理後淬水冷卻後, 在η樹枝狀晶中析出微細的ε相(直徑約0.15μm),導致η樹枝狀晶的微硬度急速上升, 而在40分鐘後達到穩定值。這個ε相與η樹枝狀晶的晶體方向關係為 : [-1011]η//[-1011]ε,(01-12)η//(01-12)ε□□ 在這個研究□, 同時也探討Zn-4%Al-3%Cu合金固溶化處理後的時效性質。經過240℃熱處理後的試片, 再分別於50℃~150℃之間進行時效, 吾人發現有兩個析出機構。在時效初期, □樹枝狀晶□面析出α相, 這個α相與□樹枝狀晶的晶體方向關係為: [0001]η//[111]α,(11-20)η//(1-10)α 在時效第二階段, η樹枝狀晶□面析出T'相, 這個T'相與□樹枝狀晶的晶體方向關係為[11-20]η//[111]T',(1-101)η//(-110)T'。藉由Arrhenius速率方程式的計算, T'相析出的活化能為66.4kJ/mole, 遠低於鋅原子的自擴散活化能。吾人提出一個α/η相界面的擴散模型,說明T'相的析出是藉由α相與η相的界面擴散。

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.