本研究以真空感應爐熔煉37mm厚的Mg-9%Li-1%Zn (LZ91)合金，並以二重軋延機將其輥軋至2mm，以此板材進行材料特性之分析研究。此外，亦熔鑄直徑200mm之大尺寸鎂鋰鋅合金錠，包括Mg-6%Li-1%Zn (LZ61)、Mg-9%Li-1%Zn及Mg-10%Li-1%Zn (LZ101)等三種組成，並將圓錠材擠製及冷軋成為0.6mm的捲料薄板。本研究對LZ91鎂鋰鋅合金施以不同熱機處理（Thermo mechanical treatment）程序，並以TEM、SEM、XRD及微硬度計等設備，探討LZ91合金之微觀結構與時效析出（Precipitation）處理之關係。採用抗拉試驗及成型極限曲線（Forming limit curve）評估前述各材料之機械性質與成型性，亦由真應力－應變曲線探討其應變硬化指數（Strain hardening exponents, n values）。 研究結果顯示，鎂鋰鋅合金具有大量的冷加工能力，係因原HCP之鎂合金微結構中，出現延展性佳之BCC相所致。析出處理及冷加工對提升LZ91強度之效益均不顯著，然冷加工方式稍佳。LZ91合金中散佈微細Wurtzite結構的ZnO與立方晶之MgO顆粒；ZnO與鎂基地具有良好的整合性（Coherence），而MgO之整合性則不佳。時效析出處理之結果亦顯示，LZ91在100℃處理10小時可達尖峰時效（Peak aging）；而於50℃析出處理時，則須100小時才能得到最大硬度值。X光繞射分析則顯示，此合金經50℃/100小時與100℃/10小時的時效處理後，α(0002)面之主繞射峰旁均出現額外之小峰，此現象應為析出相或Spinodal相分解所造成。 不同鋰含量之鎂鋰鋅合金的拉伸試驗結果顯示，提高鋰含量可明顯增加材料之延展性，且LZ61、LZ91與LZ101的拉伸強度異向性（Anisotropy）不明顯，可能利於板材之沖壓成型製程。此外，LZ91與LZ101的延伸率均超過40％，與一般常溫沖壓鋼板及200℃之AZ31B鎂板的延展性相當。而應變硬化指數則隨合金之鋰含量的增加而減少，且含鋰大於9wt.％合金的n值小於0.05。LZ91與LZ101之曲線位於成型極限圖上方區塊；AZ31B於100℃及室溫的曲線則居於其下方部位，顯示LZ91與LZ101具有較佳之成型性。

An Mg-9%Li-1%Zn (LZ91) alloy was successfully cast into a 37mm thick ingot by vacuum induction melting technique, and then rolled into a thickness of 2mm. Greater size of 200 mm diameter billets with various compositions, which included Mg-6%Li-1%Zn (LZ61), Mg-9%Li-1%Zn and Mg-10%Li-1%Zn (LZ101), were also cast as well and extruded and subsequently rolled into 0.6-mm in thickness. The tests of tension and forming limit diagram (FLD) were conducted to investigate the mechanical behavior and formability of the magnesium-lithium alloys. The strain hardening exponents, n, of the alloys were also calculated through various true stress-true strain data sets. The preceding alloys have remarkable workability with respective to the rolling process, not demonstrated by other Mg alloys. This was attributed to the presence of a ductile β phase of BCC structure, despite the coexistence of the brittle HCP α phase. Thermo and mechanical treatments were performed, and the resultant microstructures were examined to elucidate the strengthening mechanisms associated with the alloys. A distinguishing feature of this research is the employment of XRD, SEM and TEM instruments to identify various phases involved. The experimental results indicated that neither age hardening nor cold rolling was effective in improving the strength of the LZ91 alloy, with cold rolling modestly better. Small number of fine particles was detected in the quenched state of the alloy, and their presence may contribute to the strength of this alloy. Furthermore, the LZ91 alloy had a dual phase structure with dispersed particles of ZnO and MgO oxides. The Wurtzite structure of ZnO was well oriented with respect to the Mg matrix, but the MgO was not. Peak aging hardness was obtained at a temperature of 100C for 10hrs. Alternatively, maximum hardness could also be reached at 50C for 100 hrs. In the XRD spectrum, the appearance of the extra bump next to the main peak of α(0002) after aging treatment at 50C/100 hrs and 100C/10 hrs, was believed to originate from a precipitate phase or a phase resulted from spinodal decomposition. The results of tensile tests revealed that the ductility of various magnesium-lithium alloys was much improved with a higher Li content. Furthermore, the tensile strength of Mg-Li sheets were found to show little anisotropy, which is expected to deliver great benefit to the press-forming process. The elongation of LZ91 and LZ101 were found to exceed 40％, which were comparable to those of conventional stamping steel and AZ31 at 200℃. The calculated n value demonstrated that (a) n decreased with the increasing lithium content in LZ alloys, and (b) as the lithium content was higher than 9wt%, all the n values were lower than 0.05. The present research found that the forming limit curves of LZ91 and LZ 101 were located at higher positions in the major strain versus minor strain plots than their counterparts of AZ31 at 100℃ and room temperature. This indicates a better formability of the alloys.

摘 要 I ABSTRACT III 謝 誌 VI CHAPTER 1 OVERVIEW 1 1.1 Scope 1 1.2 Problems and Issues 3 1.3 Research Objective 5 CHAPTER 2 LITERATURE SURVEY 7 2.1 Alloys Based on the Mg-Li System 7 2.2 Characterization of Mg-Li Alloys 15 2.3 Deformation Textures and Slip in HCP Metals 24 2.4 Corrosion Control of Mg-Li Alloys 28 2.5 Mg-Li Matrix Composites 32 CHAPTER 3 EXPERIMENTAL PROCEDURES 38 3.1 Material Preparation 38 3.2 Thermo-mechanical Processing 44 3.3 Microstructural Characterization 47 3.4 Mechanical Properties Test 47 3.5 Formability Evolution of Mg-Li Alloys 48 CHAPTER 4 RESULTS AND DISCUSSION 52 4.1 As-cast LZ91 Microstructures 52 4.2 As-rolled and Solution-treated LZ91 Microstructures 58 4.3 Effect of Aging Treatment on LZ91 64 4.4 Cold Work Hardening Phenomenon of LZ91 77 4.5 Softening of LZ91 by Annealing 80 4.6 Tensile Properties of LZ Alloys 82 4.7 Strain Hardening Exponent of Mg-Li Alloys (the n value) 92 4.8 Forming Limit Diagram (FLD) of Mg-Li Alloys 95 CHAPTER 5 CONCLUSIONS 104 FUTURE STUDIES 107 REFERENCES 108 LIST OF PUBLICATIONS 116 CURRICULUM VITAE 120

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