【作者】 李德辉；

【导师】 卢晨； 曾小勤；

【作者基本信息】 上海交通大学 ， 材料加工工程， 2007， 博士

【摘要】 镁合金由于强度低、变形能力弱和耐腐蚀性能差,至今未得到大规模的工业应用。最新研究表明,镁及镁合金中加入一定量的重稀土元素(Gd、Dy和Sm等)后具有显著的时效强化效应,可大幅度提升镁合金常规力学性能和高温耐热性能,应用前景广阔。本文将系统研究一个新的镁合金体系—Mg-Dy-Nd-(Gd)系合金不同加工状态下的组织、结构、力学性能和断裂行为,重点研究清楚合金的相形貌、结构、转变规律及强化机制,为研制高性能镁稀土合金提供实验和理论依据。Mg-Dy-Nd-(Gd)系合金铸态组织和性能研究结果表明,DN73K、DN103K、DN103K和DGN443K合金铸态组织均由初生基体相、存在于晶界的骨骼状Mg5RE(f.c.c., a=2.24nm)相、以及位于骨骼状Mg5RE相内部或者边缘的块状相RE17Mg3(f.c.c., a=0.526nm)组成。Mg-Dy-Nd合金中Dy含量增加或者用Gd替换部分Dy后,合金中骨骼状相均有所增加,拉伸强度增加,塑性降低,解理断裂特征更加明显。Mg-Dy-Nd-(Gd)系合金固溶态组织和性能研究结果表明,经固溶处理后,DN73K、DN103K、DN123K和DGN443K合金晶粒尺寸增加,晶界骨骼状Mg5RE相基本消失,沿晶界分布的块状RE17Mg3相有长大趋势。同时,合金的强度和塑性均较铸态合金有明显的提高,其断口解理面大部分由撕裂棱连接起来,表现为穿晶断裂。Mg-Dy-Nd-(Gd)系合金时效硬化曲线和时效析出行为研究结果表明,该系合金具有明显的时效硬化效应。Mg-Dy-Nd系合金中Dy含量较低的DN73K合金时效析出序列为β″、β1和β相;Dy含量较高的DN103K和DN123K时效析出序列依次为β″、β′、β1和β相;Gd代替部分Dy的DGN443K合金的时效析出序列依次为β″、β1和β相。其中,沿{ 112 0}面形成的β″相(DO19, a=2aMg=0.64nm,c=cMg=0.52nm)和椭球状β′相(b.c.o., a=0.640nm, b=2.223nm, c=0.521nm)均与基体完全共格,它们和基体的取向关系分别为:<0001>β″//<0001>α, { 11 00}β″//{ 11 00}α和<001>β′//<0001>α, {100}β′//{ 112 0}α;沿{ 11 00}面形成的板条状β1相(f.c.c., a=0.74nm)可形核于β′相,也可由β″相直接转变而来,其与基体的取向关系为:<110 >β1//< 0001>α, { 11 1}β1//{ 112 0}α;β1相经共格转变形成稳定的β相(f.c.c., a=2.223nm),β相为面心立方超结构,且与β1相具有相同的形貌及取向关系。Mg-Dy-Nd-(Gd)系合金时效态力学性能和强化机制研究结果表明,时效初期,合金的强度随着时效时间而增加,峰时效过后,合金的强度有迅速下降的趋势,高温长时间时效处理后,合金的强度随时效时间的变化不明显。峰时效态合金中的β″相及β′相为主要强化相,强化机制为共格强化;当位错遇到早期过时效态形成的β1相时,位错可穿过该相;当位错遇到过时效态形成的β相时,位错则绕过该相,从而形成Orowan强化。峰时效态合金经室温变形后形成大量的{ 101 2}型和一定的{ 101 1}型形变孪晶。随着拉伸温度的升高,流变应力的降低使得诱发孪晶的能力降低,从而使得合金形变孪晶的数量明显降低。高温变形过程中(高于250℃),β″和β′相的消失及β1和β相的形成使得位错更容易沿基面及非基面滑移,从而使得合金的拉伸强度明显降低,塑性增加。其中,合金峰时效态断口解理面光滑,解理断裂特征明显,早期过时效态和过时效态合金断口均为解理断裂,断口有一定撕裂棱存在;随着实验温度的升高,合金峰时效态断口由解理断裂向韧性断裂转变。DN123K合金挤压态组织与性能研究结果表明,未均匀化处理的合金经350℃挤压变形后,粗大的铸态骨骼状相得以破碎,合金的强度和塑性均有所提高;均匀化处理后的合金经350℃和450℃挤压后,发生了动态再结晶,过饱和固溶体内析出球状相,其结构与铸态骨骼状相的完全相同,合金的强度和塑性均得到明显的提高,实验范围内挤压温度对合金组织性能的影响并不明显。挤压态合金的断裂方式为韧性断裂。更多还原

【Abstract】 Magnesium alloys have not been widely used as a main structural material due to their low tensile strength, low deformation ability and bad corrosion resistance. Recent work has suggested that magnesium alloys with the addition of heavy rare earth elements (Gd, Dy, Sm, etc.) have obvious ageing strengthening response and better mechanical properties at room and elevated temperature than those of conventional magnesium alloys, thus maybe have better future application. In this study, the microstructures, mechanical properties and fracture behaviors of a new series of Mg-Dy-Nd-(Gd) alloys will be systemically researched, of which the morphology, microstructure, transformation law and strengthening mechanism of the precipitation phases are focused on. The aim of the present work is to provide experimental and theoretical results for the development and application of high performance magnesium rare earth alloys.Researches on the microstructures and mechanical properties of as cast Mg-Dy-Nd-(Gd) alloys indicate that the cast microstructures of DN73K, DN103K, DN123K and DGN443K alloys consist ofα-Mg phase, skeletal Mg5RE phase (f.c.c., a=2.24nm) along grain boundary and rectangular RE17Mg3 phase (f.c.c., a=0.526nm) at inner and fringe of the skeletal phases. The increasing amounts of eutectic phases in Mg-Dy-Nd alloy, with increasing content of Dy or replacing part of Dy by Gd, result in better tensile strength, lower elongation and obvious cleavage fracture.Researches on the microstructures and mechanical properties of solution treated Mg-Dy-Nd-(Gd) alloys indicate that the skeletal phases will be dispersed, and the grain size and rectangular RE17Mg3 phases will both grow up after being solution treated, which result in better mechanical property and toughness. The fracture fractographies are full of tearing ridge, indicating transgranular fracture.Researches on the age-hardening curves and ageing precipitation behaviors of Mg-Dy-Nd-(Gd) alloys indicate that these alloys have obvious age-hardening response, the precipitation sequence in DN73K alloy mainly involves formation ofβ″,β1 andβphases, the precipitation sequence in DN103K and DN123K alloys mainly involve formation ofβ″,β′,β1 andβphases, and the precipitation sequence in DGN443K alloy mainly involves formation ofβ″,β1 andβphases. Theβ" phases (DO19, a=2aMg=0.64nm, c=cMg=0.52nm), precipitated in { 112 0} planes, are fully coherent with matrix and the relationship betweenβ" and matrix is < 0001 >β″//< 0001 >αand { 11 00}β″//{ 11 00}α; the globularβ′phases (b.c.o., a=0.640nm, b=2.223nm, c=0.521nm) are fully coherent with matrix and the relationship betweenβ′and matrix is <001>β′//<0001>αand {100}β′//{ 112 0}α; the plane plateβ1 phases (f.c.c., a=0.74nm), precipitated in { 11 00} planes, may nucleate fromβ′andβ" phases, and the relationship betweenβ1 and matrix is <110>β1//<0001>αand { 11 1}β1//{ 112 0}α; the equilibriumβphase (f.c.c., a=2.223nm) can be nucleated fromβ1 phase by coherent transformation, and the relationship betweenβand matrix is identical to that ofβ1 and matrix.Researches on the mechanical properties and strengthening mechanisms of aged Mg-Dy-Nd-(Gd) alloys indicate that room temperature tensile strength of alloys increases with the increase of ageing time during earlier aging time, the strength decreases sharply with ageing time elongation after peak ageing time, and the high temperature strength don’t vary obviously after longer ageing time at elevated temperature. Theβ″andβ′, precipitated in peak aged alloys, are the main strengthening phases, which lead to coherent strengthening for their coherence with matrix. The dislocation can extend acrossβ1, however, the movement of dislocation may be hindered byβphases, and then the strengthening mechanism will be Orowan strengthening mechanism. Deformation twins can be formed in peak aged alloys after being deformed at room temperature, and the decreasing numbers of twins with increasing experimental temperature result from decreasing capability of inducing twins by decreasing flow stress. The disappearance ofβ″and/orβ′can accelerate basal and non-basal planes slipping after being deformed at elevated temperature, which result in lower tensile strength and higher plasticity. The peak aged, early over aged and over aged fracture fractographies show cleavage fractures, and the peak aged fracture fractographies transform from cleavage fracture to ductile fracture with increasing experimental temperature.Researches on the microstructures and mechanical properties of extruded DN123K alloys indicate that the as-cast coarse skeletal phases has been disintegrated after being extruded at 350℃, and the tensile strength and toughness are improved. After being uniformly heat treated and extruded at 350℃and 450℃, the crystal structures of these elliptical particles, precipitated during extrusion process, are identical to those of skeletal phases, and the strength and plasticity can be improved obviously, but in the range of this experiments the extrusion temperature have less effect on the microstructure and tensile strength of extruded alloys. The fracture fractographies of extruded alloys show ductile fracture.更多还原