時效對高強度Al-Zn-Mg-Sc-Zr合金之顯微組織、機械性質與應力腐蝕的影響

Influence of Aging on the Microstructure, Mechanical Properties and Stress Corrosion Cracking of High Strength Al-Zn-Mg-Sc-Zr Alloy

吳凌玫(Ling-Mei Wu)

指導教授 ： 王文雄

國立臺灣大學/工學院/機械工程學研究所/博士(2008年)

https://doi.org/10.6342/NTU.2008.00100

摘要

本研究探討均質化溫度對Al-Zn-Mg-Sc-Zr合金之再結晶行為和分散粒子分佈的影響；同時以不同時效處理探討機械性質、顯微組織與抗應力腐蝕破壞(SCC)的影響。以上之研究主要是透過微硬度試驗(Microhardness test)、拉伸試驗(tensile test)、應力腐蝕試驗(SCC test)、示差掃描熱量(DSC)分析、光學顯微鏡(OM)、掃描式電子顯微鏡(SEM)、穿透式電子顯微鏡(TEM)以及高解析穿透式電子顯微鏡(HRTEM)等方法來進行。另外，也藉由小角度X光散射(SAXS)和穿透式電子顯微鏡一起探討不同恆溫時效後析出物大小及形貌的演變。
實驗結果顯示，經最佳均質化條件處理後獲得細小且緻密的Al3(Sc,Zr)分散粒子分佈，在熱輥軋及固溶處理後抑制再結晶的能力最大，再結晶分率為最低。而較高的再結晶分率導致不整合的界面增加，提供有效異質成核位置，進而使平衡η相容易成核在分散粒子上，導致基地上溶質過飽和減少，降低合金的析出硬化能力和強度。在機械性質方面，熱擠製材的強度比輥軋材來的高，對所有不同時效處理的熱擠製材有較佳機械性質是因晶粒細化的結果；另一方面尖峰時效(T6)的條件呈現最大的抗拉強度和最小的伸長率，經RRA處理之抗拉強度相似於T6處理，而經Two-Step處理其強度則為最低值，因為寬的析出缺乏區(PFZ)和粗大的晶界析出物導致Two-Step處理的強度損失。然而T7(過時效)處理比T6、RRA和Two-Step等處理有較寬的析出缺乏區和較粗大的晶界析出物使得強度降低。
此合金的應力腐蝕敏感性以T6處理最為嚴重，其次為RRA及Two-Step處理，T7處理為最小，在T7處理中，基地析出物和晶界析出物的尺寸遠大於T6、RRA和Two-Step等處理，導致差排的平面滑移更加困難，此結果減少集中在晶界上的應力，大大地改善抗應力腐蝕性，但是強度會損失。因此增加基地析出物和晶界析出物的尺寸，導致差排滑移型態改變從平面滑移到均質的滑移模式，提高了合金的抗應力腐蝕性。另外，添加Sc和Zr產生Al3(Sc,Zr)分散粒子大大地細化晶粒與抑制再結晶，進而增加合金的強度和抗應力腐蝕敏感性，由於Al3(Sc,Zr)分散粒子抑制差排的平面滑移，明顯地降低晶界上的應力集中現象，進而減少腐蝕的敏感性。
合金在120°C人工時效後，發現硬度值隨著時效時間的增加而增加，在48小時時效後達到最大的硬度值，最後隨時間的增長硬度隨之下降。最初硬度的增加歸因於基地內有大量的GP zones和介穩定η’相，而過時效T7處理由於η’相轉變成穩定相η相導致硬度值下降。此合金的結構特徵根據TEM和HRTEM的觀察，在水淬後小溶質聚集快速地形成，在早期的時效階段，小溶質聚集成長至球狀GPI zones，GPI zones為圓球狀與基地整合，然而合金從480°C固溶溫度淬火後時效於120°C，並未觀察到GPII zones存在，此外，Zr的存在會抑制GPII zones的形成。隨著時效時間的增加，GPI zones逐漸地溶解且η’相同時地析出，HRTEM的結果顯示較小η’析出物邊緣躺在{111}Al 上且非常接近球狀的GPI zone，然而48小時時效後，介穩定η’相以板片狀析出在{111}Al上，其尺寸大約7nm，在96小時時效以後，顯微組織包含主要的η1和 η2析出物，少量的η4析出物和微量的η’析出物，其η1-plates躺在{100}Al , η2-plates躺在{111}Al 以及η4-rods沿著 <110>Al 析出。
另外，分別在100°C, 120°C 和140°C進行不同時間之恆溫時效，時效硬化曲線顯示正常的形狀；100°C時效，硬度隨時效時間持續地增加；120°C時效，在過時效之前達到最大的硬度；140°C時效由於擴散速度較快，所以在較短的時效時間提前達到尖峰的硬度值，且其尖峰硬度值較120°C者為小。根據TEM觀察和SAXS量測，可以得到不同時效處理的析出物尺寸和形貌之定量訊息。SAXS結果顯示，析出物尺寸在100°C時效處理期間成核半徑沒有明顯發展；在較高的溫度，析出物尺寸成長較快，140°C高溫時效時析出半徑已有幾個奈米，此與TEM的觀察是一致的。

關鍵字

Al-Zn-Mg-Sc-Zr合金均質化處理時效處理機械性質 ； 應力腐蝕 ； 顯微組織 ； Al3(Sc,Zr)分散粒子

並列摘要

In this study, the effect of homogenization temperature on the recrystallization behavior and dispersoid distribution; while the influence of aging on the mechanical properties, microstructures and resistance to stress corrosion cracking (SCC) of an Al-Zn-Mg-Sc-Zr alloy have been investigated. The mechanical and SCC properties were investigated through microhardness, tensile and SCC tests. The microstructural characterizations were analyzed by using differential scanning calorimetry (DSC), Optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy and high resolution transmission electron microscopy (TEM and HRTEM). In addition, the size and morphology of precipitates evolution after various isothermal aging have been studied together with small angle X-ray scattering (SAXS) and TEM.
The experimental results show that an optimum homogenization is obtained by precipitating a large number and uniformly distribution of small particles that is optimized with regard to inhibit recrystallization. This is observed to have a lower recrystallization fraction in hot rolled and solution treated plate. Hence, a higher recrystallization fraction led to an increase in incoherent interfaces which provided very effectively heterogeneous nucleation site, and then the equilibrium η (MgZn2) phase easily nucleated on the dispersoids. Due to a decrease of solute supersaturation of the matrix, it would reduce the capabilities of precipitation hardening and lower the strength of the alloy. For the mechanical properties, hot extruded specimens show the highest strength than that in as-rolled condition. The higher values of mechanical properties for all tempers of hot extruded condition are a result of grain refinement. On the other hand, the peakaged condition (T6) exhibited the highest ultimate strength and the smallest elongation. It can be clearly observed that the UTS value of RRA temper was similar to the T6 temper, while the Two-Step temper had the lowest value. The wide precipitatedfree zone (PFZ) and coarse grain boundary precipitates of the Two-Step temper lead to the loss of strength comparing with the T6 and RRA tempers. However, the T7 temper indicated an overaged condition and had the widest PFZ and largest grain boundary precipitates lead to severely reduce the strength comparing with the T6, RRA and Two-Step tempers.
The SCC susceptibility is the most severe for the T6 temper, intermediate for the RRA and Two-Step tempers, and minimal for the T7 temper. In T7 temper, the sizes of matrix precipitates and grain boundary precipitates are grater than those in the T6, RRA and Two-Step tempers, which make the planar slip more difficult. This results in decreasing of stress concentration at the grain boundary and greatly improving the SCC resistance, but loss of the strength. Thus, increasing the size of matrix precipitates and grain boundary precipitates can increase the SCC resistance of the alloys, because it can result in the change dislocation slip type from the planar slip to homogenous slip mode. In addition, the addition of Sc and Zr resulted in Al3(Sc,Zr) dispersoids that greatly refined grains and restrained recrystallization process, which enhanced the strength and SCC resistance. The concentrations of stress at the grain boundary may be reduced significantly due to the Al3(Sc,Zr) dispersoids suppress the planar slip of dislocation, which decreased the susceptibility to corrosion.
This alloy artificially aged at 120°C for various time. It can be found that the hardness value increased with increasing aging time, and then reached the maximum hardness after 48h aging (peakaged T6 temper), finally decreased gradually with longer aging times. The initially increase is attributed to the nucleation and growth of the large number of coherent GP zones and semicoherent metastable η’ phase in the matrix. Hence, the decrease of hardness value for the overaged T7 temper due to the precipitates transformed gradually from the metastable η’ phase to the stable η phase. The structural features of this alloy are based on TEM and HRTEM observations, smallclusters formed rapidly after water quenching. These smaller clusters grow to spherical GPI zones at the earliest stages of aging. However, GPII zones have not presented after quenching from a solution temperature of 480°C and aging at 120°C. Besides, the presence of zirconium suppresses the formation of GPII zones. With increasing aging times, the GPI zones dissolve gradually and η’ precipitates form simultaneously. HRTEM result shown that, small edge-on η’ precipitates lying on {111}Al matrix planes and are close to the spherical GPI zone. After 48h aging, it is proposed that metastable η’ particles are plate-like precipitates on {111}Al with a size of about 7 nm. After 96h aging, the microstructure contains mainly η1 and η2 precipitates, with smaller amount of the η4 type and minor quantities of η’ particles. The η1-plates lying on {100}Al , η2-plates on {111}Al and η4-rods along <110>Al.
Furthermore, the isothermal aging was performed at 100°C, 120°C and 140°C, respectively, for different periods of time. Aging hardness curves display the normal shapes: at 100°C aging, the hardness value increased with increasing aging time; at 120°C aging, a maximum hardness was reached before overaging; at 140°C aging, diffusion rates were faster and peak hardness was thereby achieved after shorter aging time. The peakhardness value is lower than that of 120°C aging. Both TEM observation and SAXS measurement obtain quantitative information about the size and morphology of precipitates at various aging conditions. SAXS results shown that, at 100°C aging, the size of precipitates have not show significant evolution from the nucleation radius during aging treatment. The size of precipitats increased rapidly at higher temperatures. The precipitates were only a few nanometers even at higher temperature (140°C). This is consistent with the TEM observations.

並列關鍵字

Al-Zn-Mg-Sc-Zr alloy ； Homogenization treatment ； Aging treatment ； Mechanical properties ； Stress corrosion cracking ； Microstructure ； Al3(Sc,Zr) dispersoids

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