微量鈹與鈧對Al-7Si-0.6Mg合金:微結構、熱穩定性、與淬火敏感性之影響;Effect of trace Be and Sc on microstructure, thermal stability, and quench sensitivity of Al-7Si-0.6Mg alloys

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Title:	微量鈹與鈧對Al-7Si-0.6Mg合金:微結構、熱穩定性、與淬火敏感性之影響;Effect of trace Be and Sc on microstructure, thermal stability, and quench sensitivity of Al-7Si-0.6Mg alloys
Authors:	曾有志;Tzeng,Yu-Chih
Contributors:	機械工程學系
Keywords:	Al-7Si-0.6Mg合金;熱穩定性;淬火敏感性;介金屬化合物
Date:	2016-07-22
Issue Date:	2016-10-13 14:52:39 (UTC+8)
Publisher:	國立中央大學
Abstract:	本研究探討微量鈹(0.05wt%)與鈧(0.04wt%)對Al-7Si-0.6Mg合金微結構、熱穩定性與淬火敏感性之影響。結果顯示，微量鈹與鈧均可以大幅降低富鐵相與鑄件縮孔之含量，而鈹可使鑄態合金中針狀與文字形富鐵相轉變為對延性較無害之節球狀Al-Fe-Si富鐵相，增加固溶於基地內的Mg含量，促使更多非平衡Mg2Si強化相的析出，有效提昇合金時效熱處理後的機械性質，但非平衡Mg2Si強化相在250℃熱暴露的過程中成長粗化成平衡β-Mg2Si相，導致合金機械強度的下降。而含鈧之鑄態合金對於非平衡Mg2Si相之析出量並無顯著影響，但富鐵相則為節球狀Al12Si6Fe2(Mg,Sc)5富鐵相，且可大幅提昇鑄件之緻密性，並在250℃熱暴露過程中析出Al3Sc相，有效抑制了晶粒成長與阻礙差排移動，導致含鈧合金不論在T6態或熱暴露後其機械性質（強度與延性）均較不含鈧或鈹之合金為高。本研究並以Jominy End Quench終端淬火實驗法，探討微量鈹(0.05wt%)與鈧(0.04wt%)對Al-7Si-0.6Mg合金淬火敏感性的影響，結果顯示，微量鈹可大幅降低Al-7Si-0.6Mg合金中富鐵相之含量與略為細化共晶矽，並可增加合金中非平衡Mg2Si強化相的析出量，提升時效強化效果，使合金之淬火敏感性改善約10%。而添加鈧亦可降低合金中富鐵相的含量並可使合金在淬火過程中析出Al3Sc相，抑制粗大的β′-Mg2Si相的析出，提高後續時效處理過程中細小且密集的強化相析出量，進而有效改善合金之淬火敏感性達60%以上。研究也發現鑄態不含鈹與鈧之Al-7Si-0.6Mg合金之富鐵相以粗針狀β-Al5FeSi相及含鎂之文字形兩種型態存在，淬火速率的快慢並無法改變合金中共晶矽與富鐵相之型態，但快速淬火可促使合金之鋁基地成過飽和狀態，致使時效處理後之合金能獲取較高之機械性質，而慢速淬火則會在淬火過程中析出較粗大的β′-Mg2Si相及產生無析出區，因而降低合金時效處理後之機械性質，使合金具有較高淬火敏感性。 ;In the present study, the effects of trace amounts of beryllium (Be, 0.05 wt%) and scandium (Sc, 0.04 wt%) addition on the microstructures, thermal stability and quench sensitivity of Al-7Si-0.6Mg alloys were investigated. The results show that traces of Be and Sc significantly reduce the amount of the iron-bearing phase and the interdendritic shrinkage. Be transformed the acicular iron-bearing phases into the nodular Al-Fe-Si iron-bearing phase, which is less harmful to ductility. Moreover, the addition of Be increased the Mg content of the solid solution within the matrix, prompting greater precipitation of the metastable Mg2Si phase after T6 heat treatment and effectively enhancing the mechanical properties of the alloy. However, during the following thermal exposure at 250oC for 100 h, the metastable Mg2Si phase grew into the coarse β-Mg2Si equilibrium phase, resulting in a decrease in the mechanical strength of the alloy. Meanwhile, the addition of Sc had insignificant effect on the amount of metastable Mg2Si phase that precipitated. However, here, the iron-bearing phase was a nodular Al12Si6Fe2(Mg,Sc)5 phase, which significantly enhanced the density of the castings. After the same thermal exposure procedure, it was remarkably found that the precipitation of fine Al3Sc particles effectively inhibited grain growth and hindered the movement of dislocations. These factors led to the Sc-containing alloy having the best mechanical properties (strength and ductility) than the alloys without Sc or with Be during the following thermal exposure at 250oC. In this study, the Jominy end quench test was adopted to investigate the effect of trace beryllium (Be, 0.05 wt%) and scandium (Sc, 0.04 wt%) on the quench sensitivity of Al-7Si-0.6Mg alloys. The results show that Be addition suppressed the formation of the brittle plate-like β-Al5FeSi iron-bearing phase, and prompt greater precipitation of the metastable Mg2Si strengthening phase to enhance age-hardening effect, ultimately improving the quench sensitivity of an Al-7Si-0.6Mg alloy by about 10%. Similarly, adding Sc can also change the plate-like structure of the iron-bearing phase to a comparatively harmless nodular form. In addition, at a lower quenching rate, the preferred precipitation of the Al3Sc phase inhibits the formation of a coarse β′-Mg2Si phase, promoting the nucleation of finely dispersed metastable Mg2Si precipitates in the subsequent aging treatment. The quench sensitivity of the alloy was effectively improved by more than 60%. The quenching rate does not change the morphologies of the eutectic silicon and iron-bearing phases, but rapid quenching rates can cause the Al matrix of the alloy to be in a supersaturated state, resulting in enhanced mechanical properties after aging treatment. In contrast, slow quenching rates precipitates the relatively coarse β′-Mg2Si phase and also produces a precipitate free zone during the quenching process, thereby diminishing the mechanical properties of the alloy after the aging treatment, so that the Al-7Si-0.6Mg alloy possesses higher quench sensitivity.
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