AZ31鎂合金經ECAE後晶粒細化與超塑性之研究

Abstract

本實驗係以AZ31( Mg-Al-Zn )鎂合金經ECAE（Route C）後，分別對晶粒細化、常溫機械性質、退火之晶粒成長與超塑性進行研究。 實驗結果顯示：在晶粒細化方面，經ECAE N＝6之後，大部分的晶粒均被均勻細化，晶粒由原先之12.6 μm細化至1.4 μm，而細化機制屬於動態再結晶所新生成的細小晶粒。因細化的效應使得在室溫下的UTS由N＝0時之251 MPa增加至N=6之350 MPa，這符合Hall-Petch 的理論。在晶粒成長方面，低於210 ℃退火1小時，晶粒仍在再結晶階段，退火溫度在220 ℃以上時，晶粒已大幅成長至6 μm以上；而隨著晶粒的成長，硬度值也從原先N＝6之HV 83下降至HV60以下。高溫超塑性方面，隨著擠型次數增加，細晶粒所佔的體積也增加，超塑性拉伸時所須之應力減少，伸長量增加。利用180 ℃ ECAE 擠型6次之試片進行超塑性拉伸時，在400 ℃，應變速率為10-3 s-1的環境中伸長量最佳為317 ％。而高溫拉伸破裂的機制為晶界間孔洞的形成，孔洞隨伸長量的增加而成長、破裂。

In this research, the fine grain, mechanical properties, annealing and superplasticity of AZ31 magnesium alloy through ECAE ( Route C ) process were studied. Most of grains were fined by six extrusion passes. The initial grains were 12.6mm in size and the final grains were 1.4mm after six extrusion passes. The UTS of orignal specimen were promoted from 251 MPa up to 350 MPa by six extrusion passes, and the result was in agreement with Hall Petch Theory. For the grain growth, the grains were recrystallized stage at annealing temperature below 210 ℃ for 1 hour. However, the grains remarkably grew up to 6 mm in size, as the annealing temperature higher than 220 ℃. Comparatively, the hardness was decreased from HV 83 to HV 60with the grain growth. The optimum elongation was 317 % by superplastic tensile test at the operating conditions, temperature 400 ℃ and strain rate 10-3 s-1. The fracture mechanism was voids formed at grain boundaries, and an increase in voids and void coalescences were raised with elongation till to fracture.