鈹、鈧在鎂鋰合金強化與室溫時效軟化現象之探討; Strengthening and Room Temperature Age-Softening of Mg-Li Alloys Containing Sc and Be

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Title:	鈹、鈧在鎂鋰合金強化與室溫時效軟化現象之探討;Strengthening and Room Temperature Age-Softening of Mg-Li Alloys Containing Sc and Be
Authors:	蔣志德;Chih-te Chiang
Contributors:	機械工程研究所
Keywords:	鎂鋰合金;熱機處理;固溶處理;軋延;時效軟化;age-softening.;rolling;solid-solution;Mg-Li alloys;Thermomechanical Treatments
Date:	2010-07-16
Issue Date:	2010-12-08 13:43:12 (UTC+8)
Publisher:	國立中央大學
Abstract:	鎂鋰合金是可應用結構材料中最輕的，依據比例計算 LAZ1110 合金 (Mg-11%Li-1%Al-0.5%Zn) 擁有極輕的密度 1.64g/cm3.分別加入Sc 與 Be 兩種微量元素擠製形成 LAZ1110、LAZ1110+Sc、LAZ1110+Be 與 LAZ1110+Sc、Be 等四種新型鎂合金板材，並且從其晶粒細化，進行研究。LAZ1110 合金添加 Sc 元素有晶粒細化之效果；但添加 Be 元素會造成晶粒粗大化且可以促使 α 相以 ” 費德曼組織 ” 方式均勻的析出在 β 母相晶粒中，來達到析出強化之效果。鎂鋰合金雖然擁有極佳的室溫塑性成型性，但強度方面略顯不足，故本研究製程設計經由不同程度的熱機處理而獲得不同的顯微組織；可以更清楚瞭解熱機處理對顯微結構之影響，尤其是不同壓縮應變量及溫度對析出物析出特性、晶粒細化及再結晶的影響機制。四種合金經固溶、然後室溫壓延下，其抗拉強度都可獲得顯著的提升，顯示固溶強化加上冷加工之效果，大量 α 相固溶在 β-matrix 內，造成自我擴散 (self-diffusion) 速率降低，回復與再結晶不易產生，相對地，加工硬化效果提高；其中，以LAZ1110+Be 合金的抗拉強度由 152.3MPa 強化為 242.2MPa 獲得 59% 的提升為最高。Mg-Li 系列合金採取一般機械性質的強化機制為固溶處理、軋延 90%，發現有 θ 相 (MgLi2Al) 的產生，但 θ 相因在常溫時效過程中，非常快速的在 40 個小時左右就達到尖峰時效，抗拉強度可提升到 240MPa 左右，其原因是由於固溶強化加上冷加工硬化雙重強化效果下，使得此製程強度提升效果較單一冷加工硬化效果佳。但是放置於室溫 6 個月會產生強度時效軟化現象，強度因為 α 相析出，之後就隨時間增長而過時效，而使材料強度大幅下降，從另一角度看起來，這也提供合金室溫時效軟化特性機制的一種研究路徑。 Mg-Li alloys are arguably among the lightest materials viable for structural application. For example, a LAZ1110 alloy (Mg-11%Li-1%Al-0.5%Zn) possesses a density of 1.64g/cm3 as calculated in accordance to the rule of mixture. Surprisingly, the actual density is even further significantly lower such as comparable with plastics. However, their elastic moduli can be at least ten times higher than those of plastics. Another merit is its ease to be cold-worked, making it desirable for sheet, plate, tube and bar structures. Conceptually, it might be a good candidate for making parts for aero vehicle such as skin of fuselage, wing and landing frame. While extremely light and highly formable Mg-Li alloys have been drawing research interest, their relatively low strength is discouraging, and thus, an issue to be addressed. In this paper, four Mg-Li alloys were processed and evaluated: the first, a basic alloy with a nominal composition of Mg-11%Li-1%Al-0.5%Zn; the second, an alloy with only Be added to the first; the third, an alloy with only Sc added to the first; and the fourth, with both Be and Sc added to the first. By processes of solid-solution treatment plus 90% heavy rolling the strengthening θ-phase did not appear immediately after quenching and required an incubation period prior to hardening. The required time for reaching the hardness maximum was about 40 h, a high strength of ~240MPa was achieved for these Mg-Li alloys. Alloy natural aging for 6 months, acicular α phase on grain boundary and inside of grain, cause of precipitation of aged at room temperature. However, subsequent natural aging process proceeded spontaneously and resulted in strength decay. On the other hand, this room temperature softening behavior is uncommon, thus offers a convenient route for studying aging characteristics of metallic alloys.
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