材料期刊网

用金相方法以及合金全腐蚀分离出Si相,分级沉降,然后进行扫描电镜观察的方法,对Al-Si合金未变质,Na,Sr,La变质的Si相进行了观察。发现La变质Si枝条最短,有时呈根瘤状,Na变质Si最长,Sr次之,此现象与变质剂的价电荷有关。在对上述退火试样观察中发现共晶Si相普遍溶解,再结晶产生新的Si晶。其中以La变质Si的再结晶速度最快,依次为Sr变质,Na变质,未变质的Si晶。对过共晶Al-Si合金考查结果表明,初晶Si并无溶解再结晶现象。再结晶速度与Si晶的平衡状态有关。据此提出“不平衡度”概念作为衡量变质结构的准则。

A comparison study has been made of the modification of Al-Si eutectic alloys with modifiers Na, Sr and La by metallography and SEM. It has been found that the concentrations of just entire modification of the modifiers in eutectic alloys were aU about 0.03 at.-％. It shows that the modification is influenced directly by the number of modifier atoms rather than weight. In SEM study, the Si phase was extracted out from the eutectics with HCl+ H_2O_2 followed by dil. HCl wash. Detailed observation found that the appearance of Si fibres at entire modification were something different with various modifiers used, the shortest were the crystals modified with La, Sr the next, and that of Na-modified were longer. This phenomenon was regarded as by the influence of valence electron charges of different modifiers. The SEM observation of annealed Si extracted from eutectic bulks found that the eutectic Si phase gradually dissolved in α-Al phase while new ball form equilibrium crystals grew up. The dissolve speed of Si phase dicreased with the modifier order La, Sr, Na. No evidence was found about the dissolve of primary Si phase even annealed over 144 h. It has suggested that the modified Si phase is in unequilibrium, the higher the valence electron charges of the modifiers the more unequilibrium of the Si crystals will be. The primary Si closed to the equilibrium state which therefore can not be modified in general situation.