【作者】 吕滨江；

【导师】 彭建；

【作者基本信息】 重庆大学 ， 材料科学与工程， 2014， 博士

【摘要】 变形镁合金因其缺陷少以及综合力学性能优异的特点能够满足不同领域的特殊用途，成为镁合金研究开发的一个重要方向。Mg-Zn-Zr系镁合金具有较高的强度成为目前应用最为广泛的变形镁合金之一，然而其塑性和热成形性差的缺点使其不能得到大规模的工业应用。本文在低Zn含量的Mg-2.0Zn-0.3Zr镁合金的基础上，添加不同含量的稀土元素Y，通过半连续铸造方法获得Mg-2.0Zn-0.3Zr-x(wt.%)Y合金以提高合金的综合力学性能，并改善合金的热加工性能。研究结果表明，随着Y添加量的增大，挤压态Mg-2.0Zn-0.3Zr-x(wt.%)Y合金的晶粒逐渐得到细化；当Y含量为0.9wt.%时，Mg-2.0Zn-0.3Zr-0.9Y合金的抗拉强度达到254±1.0MPa，延伸率达到24.3±0.4%，合金表现出较好的综合力学性能；当Y含量为5.8wt.%时，Mg-2.0Zn-0.3Zr-5.8Y合金的平均晶粒尺寸达到4.5±1.2μm，合金的抗拉强度和延伸率分别达到281±2MPa和30.1±0.7%，达到所有试验合金最优的综合力学性能。通过水冷冷却增大熔体过冷度的方法获得更大体积分数I-相的Mg-Zn-Zr-Y合金。由于Mg的层错能低，热变形过程中合金更容易发生动态再结晶。因此，动态再结晶是镁合金细化晶粒和提高综合力学性能的一种重要手段。为了研究第二相对变形镁合金在热变形过程中动态再结晶演变和热加工性的影响，通过热压缩实验，分别研究了半连续铸造获得的Mg-2.0Zn-0.3Zr-x(wt.%)Y合金中W-相、18RLPSO相、14H LPSO相和I-相对合金热变形行为、动态再结晶和热加工性能的影响机制；在大量实验数据的基础上，基于Avrami方程，本文提出一种新的动态再结晶动力学模型：并在不同成分合金的热模拟实验中进行验证；提出并采用结合显微组织分析、动态再结晶动力学模型和动态材料模型(DMM)塑性加工图计算的方法系统优化合金热加工工艺参数，指导工业生产。研究结果表明：1). Mg-2.0Zn-0.3Zr合金的动态再结晶激活能Q的平均值为169.8KJ/mol；在实验条件范围内没有出现加工失稳区。2). W-相的生成使Mg-2.0Zn-0.3Zr-0.9Y合金的再结晶激活能增大到236.2KJ/mol；当应变速率在0.001-0.01s-1之间，变形温度在250-400℃之间，Mg-2.0Zn-0.3Zr-0.9Y合金的动态再结晶机制以连续动态再结晶(CDRX)机制为主，伴随有晶界弓出形核的不连续动态再结晶(DDRX)机制发生，动态再结晶体积分数随着变形温度的升高而逐渐增大当应变速率在0.1-1s-1之间，在较低温度范围(250-300℃)，动态再结晶主要以孪生-动态再结晶(TDRX)机制为主，当变形温度升高，孪晶密度减小，动态再结晶机制又以连续动态再结晶(CDRX)机制为主，同时伴随有晶界弓出形核的不连续动态再结晶(DDRX)机制的发生，动态再结晶体积分数随着变形温度的升高先增大后降低；3).含18R LPSO相的Mg-2.0Zn-0.3Zr-5.8Y合金的动态再结晶激活能Q的平均值为293.0KJ/mol；18R LPSO相和层错(SFs)会阻碍和延迟动态再结晶的发生，合金在300-350℃和0.001-0.01s-1以及300-450℃和0.1-1s-1变形参数范围内发生不完全动态再结晶；热压缩变形过程中，以CDRX机制为主，伴随有晶界弓出形核的DDRX机制。4).含14H LPSO相的Mg-2.0Zn-0.3Zr-5.8Y合金的动态再结晶激活能（Q值）平均值为348.4KJ/mol；14H LPSO相的动态再结晶机制与18R LPSO相基本相同，以原始晶界附近和LPSO相与Mg基体界面附近亚晶形核长大的CDRX机制为主，伴有少量晶界弓出形核的DDRX机制发生；在相同的变形条件下，经过500℃×20h均匀化退火热处理后，在Mg-2.0Zn-0.3Zr-5.8Y合金基体中析出的片层状14H LPSO相以及变形过程中形成的扭折带延迟动态再结晶和阻碍动态再结晶晶粒的长大的作用要大于18R LPSO相；14H LPSO相的生成增大了Mg-2.0Zn-0.3Zr-5.8Y合金的失稳区，降低了合金在较低加工温度下的热加工性能。6).通过水冷冷却制备含I-相的Mg-2.0Zn-0.3Zr-0.2Y合金的平均激活能Q=277.8KJ/mol；在低应变速率下(0.001-0.01s-1)，热压缩试样主要在原始晶界附近发生的CDRX机制为主，动态再结晶体积分数随着变形温度的升高而增大；当应变速率达到0.1s-1，在较低变形温度，晶粒内部开始局部发生TDRX，动态再结晶体积分数随着变形温度的升高逐渐增大；当应变速率达到1s-1，变形温度在350-350℃之间，动态再结晶主要以TDRX机制为主，随着变形温度的升高，孪晶密度不断减小，原始晶界附近的CDRX机制逐渐开始占主导地位，动态再结晶体积分数先减小后增大。更多还原

【Abstract】 Wroughtmagnesium alloy takes lots of advantages, such as excellentmechanicalproperties and fewer defects,which makes the development of improved wroughtalloysbecome an active research field. Mg-Zn-Zr alloys with high strength are one ofthe most widely usedwroughtmagnesium alloy, but their poor plasticity and workabilitylimits their application to major extent. In this paper, various Y contents were added intoMg-2.0Zn-0.3Zr alloy to improve its mechanical properties and hot workability. Theresults show: when Ycontent reached0.9wt.%,Mg-2.0Zn-0.3Zr-0.9Y alloy performed agood mechanical propertieswith the tensile strengthof254±1.0MPa and the elongationof24.3±0.4%. The grains of as-extruded Mg-2.0Zn-0.3Zr-x(wt.%)Y alloys wererefined with the increaseing addition of Y element; when the content reached5.8wt.%,the average grain size of Mg-2.0Zn-0.3Zr-5.8Y reached4.5±1.2μm, and the tensilestrength and elongation reached281±2MPa and30.1±0.7%, respectively.Mg-Zn-Zr-Yalloys containing quasicrystal phase were prepared by as-cast method withwater-cooling, and the effects of different Zn and Y content (same Zn/Y ratio) onmicrostructure and micro-hardness were also discussed.Because of the low stacking fault energy of Mg, non-basal slip is activated hardlyby means of the broad extended dislocation, which can cause dynamic recrystallization(DRX) much more easily. Thus, the DRX becomes one of the factors to improve themechanical properties and refine grains of magnesium alloys. Base on Avrami equation,an new kinetics model of DRX evolution is proposed in this paper as:X1exp[k(ε-εcε*) nDRX], and the accuracy of the model was verified in thecompression test of different alloys. The effects of second phases (W-phase,18R LPSOphase,14H LPSO phaseand I-phase) on hot deformation behavior, DRX and hotworkability of Mg-2.0Zn-0.3Zr-x(wt.%)Y alloys were studied by compression test；theeffect of on hot deformation behavior, DRX and hot workabilityofMg-2.0Zn-0.3Zr-0.2Y alloy prepared by as-cast method with water-cooling wasstudied; a method for optimizing deformation techniques by combining microstructureobservation and kinetics model of DRX with dynamic materials model (DMM)processing map was proposed. The results show that:1). By means of regressionanalysis for the Arrhenius type equation of flow behavior, the average activation energy Q value of Mg-2.0Zn-0.3Zr alloy was determined as169.8KJ/mol; the instable regiondoes not appear under experimental conditions;2). the average activation energy Qvalue of Mg-2.0Zn-0.3Zr-0.9Y alloy increased to236.2KJ/mol for the formation of Wphase; in temperature range of250-400℃and strain rate range of0.001-0.01s-1, themain DRX mechanism was continuous dynamic recrystallization (CDRX),accompanied the discontinuous dynamic recrystallization (DDRX)--the nucleation bybulging of local grain boundaries, and the volume fraction of DRX grains increasedwith the increasing of deformation temperature; in temperature range of250-300℃andstrain rate range of0.001-0.01s-1; the main DRX mechanism was TDRX; when thetemperature in the range of300-400℃, under the strain rate range of0.001-0.01s-1, thedensity of twins decreased with the increasing of deformation temperature, CDRXmechanism became the main DRX mechanism, accompanied the DDRX―thenucleation by bulging of local grain boundaries, as the deformation temperatureincreased, the volume fraction of DRX grainsfirst increased and then decreased.3). theaverage activation energy Q value of Mg-2.0Zn-0.3Zr-5.8Y alloy containing18R LPSOphase increased to293.0KJ/mol;18R LPSO phase can delayed the DRX, un-completedDRX occurred in the deformation temperature range of300-350℃and the strain raterange of0.001-0.01s-1, and at the deformation temperature range of300-450℃and thestrain rate range of0.1-1s-1; the main DRX mechanism was CDRX, accompaniedDDRX―the nucleation by bulging of local grain boundaries.4) the average activationenergy Q value of Mg-2.0Zn-0.3Zr-5.8Y alloy containing14H LPSO phase was348.4KJ/mol; the DRX mechanism of the alloy containing14H LPSO phase is almostsimilar to that of the18R LPSO phase; compared with18R LPSO phase,lamellar14HLPSO phase distributed in Mg matrix and deformation kinks can delay the DRX andhinder the growth of DRX grains significantly at same deformation condition; theformation of14H LPSO phase after homogenization treatment at500℃for20hincreased the instability region and reduced the workability of Mg-2.0Zn-0.3Zr-5.8Y atlow deformation temperatures.6). the average activation energy Q value ofMg-2.0Zn-0.3Zr-0.2Y alloy containing icosahedral quasicrystal phase prepared by watercooling was277.8KJ/mol; under a strain rate range (0.001-0.01s-1), CDRX occurrednear local grain boundaries was the main DRX mechanism, accompanied the DDRXwith nucleation by bulging of local grain boundaries, and the volume fraction of DRXgrains increased with the increasing of deformation temperature; when the strain ratereached0.1s-1, some twins formed at a lower deformation temperature, the volume fraction of DRX grains also increased with the increasing of deformation temperature;in the deformation temperature range of250-350℃and the strain rate range of1s-1, themain DRX mechanism was TDRX, and the volume fraction of DRX grains increasedwith the increasing of deformation temperature; in the deformation temperature range of350-450℃and the strain rate range of1s-1, the density of twins decreased with theincreasing of deformation temperature, CDRX occurred near local grain boundariesgraduallybecame the main DRX mechanism, accompanied the DDRX with nucleationby bulging of local grain boundaries, and the volume fraction of DRX grainsfirstincreased and then decreased.更多还原