添加微量矽元素對於鎂鋰合金影響之研究

Abstract

在全球性輕量化及重環保之潮流下，鎂合金已成為新時代材料的寵兒，被廣泛地作為結構及非結構材料。鎂合金之比強度、比剛性佳，為目前結構用金屬最高者，且具備良好的電磁遮蔽效應、散熱快及吸震耐摔等特性，因而大受3C 產業之青睞。於鎂合金中添加鋰元素，除可降低鎂合金之密度，符合輕量化之要求外，亦可大幅改善鎂合金之加工性質。 當鎂合金添加鋰達到5.7％以上時，會引起鎂合金晶體結構由原本之六方最密堆積(HCP)部分變為體心立方堆積(BCC)，而體心立方結構的滑移系統較六方最密堆積多，使其在常溫下有較好的冷加工性質與低溫成形可塑性，因此達到增進加工性質的目的。若添加鋰的含量達到11％以上時，鎂合金晶體幾乎轉變為體心立方堆積(BCC)。 但是鎂鋰合金之機械強度與加工硬化尚嫌不足，結構之應用受到限制，因此，本實驗選用Mg-9 wt% Li（LZ91）合金作為鎂鋰合金之基本材料，產生30％α與70％β之雙相結構，並透過添加1.2％的Si於Mg-9 wt% Li（LZ91）合金中，探討對於原材Mg-9 wt% Li（LZ91）合金性質所造成的影響，包括顯微組織、機械性質及腐蝕行為。並將Mg-9 wt% Li-1.2Si（LZS911）合金鑄錠原材經過擠製、輥軋、ECAE等製程，研究其機械性質與顯微組織之關係，尋找出最有效的加工方式，並藉由TEM進一步去觀察及分析其析出相之結構。 添加矽於LZ91合金中，由於固溶度的關係，大部分的矽元素皆與基材內的鎂原子形成鎂矽介金屬化合物(Mg2Si)，而此析出物較為硬脆，經由冷滾軋70％後，可以使LZ91合金強化40MPa外，仍然維持一定的延展性。此外，鎂矽析出物於腐蝕溶液中，形成氧化矽(SiO2)將有助於基材內氧化鎂(MgO)與氫氧化鎂(Mg(OH)2)薄膜更加緻密減少從空孔發生腐蝕的行為，使得腐蝕電位上升，腐蝕電流下降，進而提升LZ91合金抗腐蝕性。 LZS911合金擠製原材經退火輥軋後，原本重新溶回母相之α相析出，可以有效的讓α相更為細化，在加工強化與析出強化的雙重作用下，相較於原材提升了約40MPa左右。而經由不同溫度、道次、角度ECAE製程後，溫度下降對於材料晶粒細化效果提升，最佳的擠製溫度大約在100℃。而隨著道次跟角度的增加，數量更多的α相分斷及細化於β相中，阻止β相中之差排移動，以達到分散強化之目的。顯微組織中也顯示出隨著道次與角度上升，更多的再析出α相形成，因此對於晶粒細化的效果更為顯著。機械性質以90∘-100-Bc8之抗拉強度最高，約為200MPa，比室溫滾軋70％LZS911提升約20MPa以上。並經由TEM的分析可以確定此析出的顆粒即為α相。

Magnesium and magnesium alloys are used in a wide variety of structural and nonstructural applications. Because of their high specific strength and specific stiffness, great damping properties, they are applied to 3C products. It is commonly recognized that magnesium possesses poor formability because of its hexagonal close-packed structure. To make up for this shortcoming and further reduce weight, alloying magnesium with lithium of extremely low density, 0.534 g/cm3, can achieve both goals. Mg–Li alloys exhibit two phase structures between 5.7 and11 mass% Li contents consisting of the Li-rich BCC-structured phase and the Mg-rich HCP-structured phase at room temperature. The single phase structure could exist for Li contents greater than 11 mass%. As the amount of Li added to the Mg–Li alloy increases, the phase still possesses HCP structure, but the crystal lattice axes ratio, c/a decreases such that slip between crystal planes become less difficult, the co-existence of the phase makes the Mg–Li alloy possible to be cold-worked. Because of poor mechanical properties and work hardening of Mg–Li alloy, the structural applications of Mg–Li alloy are confined. Therefore, the thesis aims to investigate how alloying 1.2Si% into Mg-9 wt% Li(LZ91), influences the properties of the alloys, including the microstructure, the mechanical properties and the corrosion property. And using different process of work hardening such as extrusion, rolling, ECAP to investigate the relationship between the microstructure and mechanical properties, and determine the optimistic working process. Furthermore, using the TEM instrument to observe and analyze the structure of the precipitate. Alloying the silicon elements to LZ91 alloy, because of the low solution ratio, the most silicon elements would precipitate as compound Mg2Si in the matrix. By 70％ cold-rolling, LZ91 alloy can improve their tensile strength as 40MPa and keep its ductility. In addition, the Mg2Si in the 3.5％NaCl solution would dissolve into SiO2 covering the MgO/ Mg(OH)2 thin film and reduces the corrosion rate of LZ91 alloy. After annealing and rolling process of the LZS911extrusion, the α phase would precipitate thinner particles from the matrix and improve the ultimate tensile strength about 40 MPa. After different temperature, process and extrude angle of ECAE process, the grain refining would become more obvious as temperature decreasing, and the optimistic temperature of the extrusion is about 100℃.By increasing the angle and process of ECAE, α phase would separate to the discontinuous structure and the precipitate α phase would be denser. 90˚-100℃-8 would have the maximum of the UTS, about 200MPa.