鑄態鎂-(2，5，8)wt.%錫合金經不同熱處理後其磨耗性質之研究

Abstract

本篇論文主要利用掃描式電子顯微鏡(SEM)與X光繞射分析儀(XRD)來探討(a)鑄態(b)由鑄態經300oC、1小時均質化處理後(c)由鑄態經520 oC、15小時、爐冷後(d)由鑄態經520 oC、15小時固溶處理後(e)由鑄態經固溶時效處理(1~3天，250oC)後，五種條件鎂錫合金之顯微結構，並使用Pin-on-disc磨耗試驗機測試其磨耗性質。 鑄態Mg-(2，5，8)wt.%Sn金相結構中有明顯的樹枝晶與大量Sn原子固溶在晶界周遭的偏析現象；由鑄態經過300oC、1小時均質化處理後之Mg-(2，5，8)wt.%Sn，原先大量固溶在晶界周遭的Sn原子在經過均質化處理後析出更多之Mg2Sn，Sn原子在晶界周遭的偏析現象因此減少；由鑄態經過520 oC、15小時、爐冷後之Mg-(2，5，8)wt.%Sn，Sn原子均勻固溶於Mg基地之中，偏析情況完全消失。因為爐中冷卻之關係，有少量的Mg2Sn存在；由鑄態經過520 oC、15小時固溶處理後之Mg-(2，5，8)wt.%Sn，其Mg2Sn皆完全回溶於Mg基地之中，且偏析情況完全消失；由鑄態經固溶時效處理(1~3天，250 oC)之Mg-8wt.%Sn，隨著時效時間增加，Mg2Sn顆粒之大小與數量皆隨之粗化與增加。 下面討論將由磨耗率低至高來進行。磨耗率最低的為固溶處理後之Mg-(2，5，8)wt.%Sn，Sn原子均勻固溶於Mg基地中所提供之壓縮內應力能夠阻止裂縫生成與擴展，此外固溶處理後Mg2Sn完全消失，減少了裂縫成核處與析出物剝落刮損表面之情況，降低Delamination & Abrasive wear發生機率，因此固溶處理後之Mg-(2，5，8)wt.%Sn磨耗率最低；再來為由鑄態經過520 oC、15小時、爐冷後之Mg-(2，5，8)wt.%Sn，Sn原子均勻固溶在Mg基地中所提供之壓縮內應力能夠阻止裂縫生成與擴展。因為爐中冷卻之關係，有少量Mg2Sn析出，增加裂縫之成核處與析出物剝落刮損表面之情況，因此磨耗率高於固溶處理後之Mg-(2，5，8)wt.%Sn；鑄態之Mg-(2，5，8)wt.%Sn因為有明顯偏析現象，晶粒中較軟的樹枝晶處容易發生Adhesive wear情況，且晶界上有許多Mg2Sn存在，容易成為裂縫成核之處而發生Delamination wear；磨耗率最高的為由鑄態經過300oC、1小時均質化處理後之Mg-(2，5，8)wt.%Sn，均質化處理後使得原先固溶在Mg基地中的Sn原子析出成為Mg2Sn，使Sn原子固溶減少與Mg2Sn數量變多，產生更多質軟之Mg基地與裂縫成核處，此兩項因素會使Adhesive & Delamination wear發生機率上升，因此磨耗率為最高。固溶時效熱處理之Mg-8wt.%Sn，因為有許多細小顆粒狀Mg2Sn析出，造成其容易發生Delamination wear與受到自身Mg2Sn剝落後造成Abrasive wear之情況，且發生機率隨著時效時間增加而上升，因此磨耗率也隨著時效時間而上升。

The microstructure of (a)as-cast (b)300oC,1hr homogenization (c)520oC,15hrs annealing (d)520oC,15hrs solution heat treatment(SHT) (e)solution treatment and aging (1~3days,at 250oC),were investigated by using scanning electron microscopy (SEM) and X-ray diffraction meter (XRD).Following, these samples were tested with Pin-on-disc wear tester. From SEM observation of as-cast Mg-(2,5,8)wt.%Sn, the segregation phenomenon and primary Mg dendrite were found. Besides, Mg2Sn precipitation mainly distributed along the grain boundary. After the homogenization heat treatment (300oC,1hr),the solid-solute Sn atom around the grain-boundary precipitated more Mg2Sn at grain boundary .Thus, the degree of segregation decreased and Mg2Sn amount increased after making homogenization heat treatment. Because the 520oC temperature was high enough to attain the single-Mg phase region in Mg-Sn phase diagram, the Sn atom distributed uniformly in Mg matrix after 520oC 15hrs annealing heat treatment. Because of the furnace-cooling, there was a little amount of Mg2Sn observed. After solution heat treatment, not only Sn atom distributed uniformly in Mg matrix, but also Mg2Sn precipitation completely disappeared. After solution treatment and aging, the precipitation size & amount increase with aging time increase from 1 day to 3 days. The discussion followed the order of wear rate from low to high. Sn atoms uniformly distributed in Mg matrix, provided the compressive stress which could prohibit from forming and propagating of crack. Besides, the elimination of Mg2Sn decreased the initiation of crack & occurred-probability of abrasive wear. Therefore, 520 oC 15hrs, solution heat treatment Mg-(2,5,8)wt.%Sn alloys had the lowest wear rate. Annealing at 520oC 15hrs Mg-(2,5,8)wt.%Sn had second lower wear rate. Sn atoms uniformly distributed in Mg matrix, provided the compressive stress which could prohibit from forming and propagating of crack. Because of furnace-cooling method, there were a little amount of Mg2Sn observed at grain boundary, those precipitates increased the initiation of crack & occurred-probability of abrasive wear. The as-cast Mg-(2,5,8)wt.%Sn alloys had the third lower wear rate since the existence of soft primary dendrite increased the adhesive wear occurred-probability and there were many Mg2Sn precipitates which increased the delamination wear occurred-probability. After homogenization treatment, the solute amount of Sn atom in Mg matrix decreased and more Mg2Sn precipitated at grain boundary. The two phenomenon mentioned above increased the delamination wear occurred-probability. The wear rate of solution and aging treatment Mg-8wt.%Sn increased with aging time. The more Mg2Sn precipitated, the more nuclei-site of crack was. Besides, the excess amount of Mg2Sn was found to abrade the sample surface, resulted in increasing wear rate. In addition, the wear rate decreased with increasing hardness in all conditions except the aging condition. The wear rate increased with increasing hardness in aging treatment, because there was excess amount of precipitates which was found to abrade the surface.