镁合金专业英语作业.doc

Hot deformation behavior of Mg3Gd1Zn (GZ31) magnesium alloy was studied by hot compression tests over the temperature range of 300500 C under strain rates of 0.00010.1 s-1. This material exhibited typical broad single-peak dynamic recrystallization behavior followed by a gradual drop towards the steady state stress.The constitutive behavior of the tested alloy was studied by the power,exponential,and hyperbolic sine laws.The stress multiplier and the hyperbolic sine exponent were calculated as 0.024 MPa-1 and 3.42,respectively.The deformation activation energy was found to be about 173.2 kJ/mol,which is higher than the lattice self-diffusion activation energy of magnesium (135 kJ/mol).The latter can be ascribed to the presence of gadolinium, which shows the importance of rare earth elements in increasing the deformation resistance at high temperatures.在300500温度范围内，0.00010.1 s-1的应变速率下进行热压缩试验研究Mg3Gd1Zn (GZ31)镁合金的热变形行为。该材料具有典型的宽单峰动态再结晶行为其次是对稳定状态的压力逐渐下降。用动力，指数，双曲正弦定律研究了被测合金的本构行为。应力乘子为0.024 MPa-1，双曲正弦指数为3.42。变形激活能为173.2kJ/mol，高于镁的晶格自扩散激活能（135kJ/mol）。后者可以归因于钆的存在，说明了稀土元素在增加高温下变形抗力中的重要性。Due to their excellent combination of properties,magnesium alloys have attracted a considerable attention from automobile,aviation,electronic and communication industries 1. Currently,some of the best heat-resistant magnesium alloys,with good mechanical properties, at both ambient and elevated temperature,are those based on MgREZn system 2,3.The addition of gadolinium (Gd) can remarkably improve the heat resistance and high strength of magnesium alloys due to solution hardening and precipitation strengthening 4.由于其优异的综合性能，镁合金已经在汽车，航空，电子和通信行业引起相当的重视。目前，一部分最优异的在环境温度和更高的的温度下具有良好机械性能的耐热镁合金，是基于MGREZn系统的。由于固溶强化和析出强化，钆（Gd）的加入能显著提高镁合金的耐热性和强度。The activation of additional slip systems of the hexagonal closed packed (hcp) crystal structure at elevated temperatures normally increases the workability of Mg alloys, and hence, hot deformation processing can be considered as a viable route for shaping of these alloys 5,6.Moreover,hot working can significantly alter the as-cast microstructures by dynamic recrystallization (DRX) during deformation and static restoration processes between deformation passes,which in turn enhance the final properties of the material7,8.密排六方结构的额外滑移系统在升高温度下的激活通常会提高镁合金的工艺性能，因此热变形处理被看做是一种可行的镁合金塑形方式。此外，热处理可以在变形和静态恢复之间的过渡过程中通过动态再结晶显著改变铸态组织，从而提高材料的最终性能。Among the magnesium alloys containing rare earth elements,the Mg alloyed with Gd and Y is the most common system 921but the MgGdZn alloys are relatively new 22,23,and their hot deformation behaviors have not been investigated in detail. Hence,for future applications of MgGdZn alloys with high strength, high creep and corrosion resistance, a thorough knowledge of the structureproperty correlation is essential.In the current work, the hot deformation behavior of Mg3Gd1Zn (GZ31)magnesium alloy as a famous wrought alloy in this system is studied during hot compression test.含稀土元素的镁合金中，钆镁合金和钇镁合金是最常见的，但是MgGdZn合金却相对较新，其热变形行为并没有被详细研究。因此，为了高强度，高抗蠕变和高耐腐蚀性的MgGdZn合金的未来应用，对其结构与性能相关性进行深入了解是必不可少的。在当前的工作中，研究了此合金系统中著名的锻造合金Mg3Gd1Zn (GZ31)镁合金在热压缩实验过程中的热变形行为。2.Experimental and procedures2.1.Material and processing The Mg3Gd1Zn alloy was prepared from high purity Mg, Zn, and an Mg30Gd master alloy,which were melted in an electric furnace under a covering flux.The details of melting,alloying,and casting processes have been described elsewhere for a similar alloy 24 and will not be explained here.The homogenization treatment was performed at 500 C for 10h.The homogenized slab was then hot rolled with a light reduction of 10% at 480 C.The rolled slab was annealed at 400 C for 1 h and quenched in water.Hot compression test specimens with height of 8 mm and diameter of 5 mm with compression axis parallel to the transverse direction of rolling were prepared by machining.2. 实验步骤2.1.材料与加工 Mg3Gd1Zn合金是通过高纯度镁和在电炉内覆盖层下熔融的Mg30Gd中间合金制备。融化，合金化和铸造工艺的细节已经在另一种类似合金那里描述过了，这里不再解释。均质化处理是在500度下进行10小时。然后将均质板在480度下进行10%小压下量的热轧。卷板在400度下退火一小时并且在水中淬火。最后用机械加工的方法制备用于热压缩实验的高8毫米，直径5毫米的试样，压缩轴方向与轧制方向的横向平行。2.2.Hot compression testCompression tests were carried out at temperatures of 300500 C under strain rates of 0.00010.1 s-1 using an MTS universal testing machine.Mica sheets were used as the lubricant at the interface between the anvil and the sample. Specimens were heated at a rate of 1.5 C/s to the desired deformation temperature and held for 15 min to eliminate thermal gradients.The specimens were then water-quenched immediately after compression to preserve the deformed microstructure.The microstructural observations were carried out on the longitudinal sections after etching in a solution containing 100 ml ethanol,2.5 g picric acid, 25 ml acetic acid and 25 ml water.2.2.热压缩实验压缩实验使用MTS万能试验机在300-500度和0.0001-0.1每秒的变形速率下进行。云母片被用做铁毡和样品之间界面的润滑剂。样品以1.5度每秒的速度加热到要求的变形温度，并且保持15分钟以消除热梯度。在压缩后立即进行水淬以保存变形组织。在100毫升乙醇，2.5克苦味酸，25毫升乙酸和25毫升水的混合溶液中侵蚀以后，对纵截面进行微观组织观察。3.Results and discussion3.1.Hot flow behavior The true stressstrain curves obtained at different deformation temperatures and different strain rates are shown in Fig.1a.In many cases such as 400 C/0.01 s-1,the material exhibited typical broad single peak dynamic recrystallization behavior,followed by a gradual fall toward the steady-state stress 8.However,for some deformation conditions,the imposed strain is insufficient for the completion of DRX and the steady-state condition is not attained.Fig.1 b and c reveal that the lower temperatures and higher strain rates will increase the flow stress of the GZ31 magnesium alloy,which is consistent with the general behavior of materials in hot working.The drop in flow stress with deformation temperature may be attributed to the increase in the rate of restoration processes and decrease in the strain hardening rate.In the same way,the increase in the flow stress with strain rate can be ascribed to the decrease in the rate of restoration processes and increase in strain hardening rate 25.However,in some samples such as 350 C/0.01 s-1,an unexpected steep loss in flow stress occurred due to the intense deformation twinning and shearing as can be seen in Fig.2b.As a result of this strange behavior,the flow stress of this sample falls under that of 400 C/0.01 s-1 by continued straining.3. 结果与讨论3.1.热流动性不同变形温度和变形速率下获得的实际应力应变曲线如Fig.1a图所示。在很多情况下，例如温度400度，变形速率0.01每秒，材料表现出典型的宽单峰的动态再结晶行为，其次是对稳态应力的逐渐下降。不过，在某些变形条件下，施加压力不足以完成动态再结晶，且达不到稳态条件。图Fig.1 b和c显示较低的温度和高应变速率将增加GZ31镁合金的流动应力，这与材料热加工的通常表现一致。流动应力随变形温度的下降而下降可以归因于恢复过程速率的上升和应变硬化速率的下降。同样的，流动应力随变形速率的上升可以归因于恢复过程速率的下降和应变硬化速率的上升。不过在某些样品如300度，0.01每秒下，由于强烈的孪生和剪切变形，流动应力发生了意想不到的大幅度降低，正如图Fig.2b中所示。由于这种特殊的现象，这种试样的流动应力在持续变形下跌落到了400度，0.01每秒的试样之下。3.2.Microstructure evolution Some representative micrographs of the GZ31 alloy after casting are shown in Fig.3. The as-cast microstructure of Fig.3a shows the typical dendritic microstructure and Fig.3b reveals that the as-cast microstructure includes coarse grains with eutectic -Mg5Gd phase between dendrite arms 26.Most of the phase exists as network structure,while some particles are distributed inside grains.The presence of the phase was subsequently verified by XRD patterns as shown in Fig.4.After homogenization at 500 C for 10 h,the dendritic structure disappears as shown in Fig.5 and the average linear intercept length was determined as 300 m.The homogenized plate was hot rolled and then annealed followed by water quenching. The resulting microstructure is shown in Fig.6,showing equiaxed grains with considerable amount of mechanical twins.This is the initial microstructure form which the samples for hot compression were machined.3.2.组织演变GZ31合金铸造后一些代表性的显微照片如图Fig.3所示。Fig.3a中的铸态组织显示了典型的树枝状结构，Fig.3b显示，铸态组织包含了枝间是-Mg5Gd共晶相的粗晶粒。大部分的相以网状结构存在，而一些颗粒散布在晶粒内。如图Fig.4所示，相的存在随后通过XRD图谱被验证。在500度下均质化处理10小时后，树枝状结构消失，如表Fig.5所示，平均线性截距长度被确定为300微米。均质钢板热轧之后进行退火然后水淬，由此产生的微观结构展示在图Fig.6，显示了有相当数量机械孪晶的等轴晶粒。这是热压缩变形试样经机械加工后的初始组织形式。 Some representative micrographs of the GZ31 alloy after hot deformation are shown in Fig.7.It is apparent that the necklace DRX (Figs.7 and 2a)is responsible for significant grain refinement(Fig.7b).This grain refinement ability can be ascribed to the fact that in single peak DRX,nucleation occurs essentially along existing grain boundaries and the growth of each grain is stopped by the concurrent deformation as a result of increasing the dislocation density of the new grains and reducing the driving force for their further growth 27.The DRX process continues until the completion of the first layer of necklace to cover the entire grain boundary.Afterwards,the subsequent layers form at the recrystallization front between the recrystallized and unrecrystallized portions to continue the recrystallization process.GZ31合金经过热变形后的一些有代表性的显微照片显示在图Fig.7。很明显，这些再结晶结构（图Figs.7和2a）是负责显著细化晶粒的（图Fig.7b）。这种细化晶粒的能力可以归因于以下事实：单峰的动态再结晶中，成核的发生基本沿着现有的晶界，并且并行变形会阻止每个晶粒的生长，因为（这）增大了新晶粒的位错密度并且减小了（晶粒）进一步生长的驱动力。动态再结晶过程继续下去，直到（ ）再结晶第一层覆盖整个晶界。随后，后续层在再结晶区和非结晶区之间的再结晶区前沿形成，继续进行再结晶过程。3.3.Grain refinement Since DRX involves repeated nucleation but limited growth of new grains,the mean DRX grain size varies slightly as recrystallization proceeds.However,in a partially recrystallized structure,deformed grains contribute to the measurement of grain size.As a result,the average grain size ( D) continuously decreases until the probable completion of DRX 25.But in this investigation,there were partially recrystallized samples that deformed less than true strain of 0.6 at 400 C with strain rate of 0.01 s-1 and they do not show their final DRX microstructure.Therefore,for grain size analysis, only those cases in which steady state were reached before quenching were used.Hence,the average grain size( D ) was equal to fully dynamically recrystallized grain size ( Ds)25.Fig.8 shows the variation of the average grain size ( D ) versus Z.As can be seen in this figure,the DRX grain size significantly decreases as Z increases.The increase in grain size with rising temperature and declining strain rate could be attributed to decline in dislocation density and increase in the mobility of grain boundaries and hence the growth rate 25.The smallest grain size obtained in the present study is about 10m when the strain rate is 0.001 s-1 and the deformation temperature is 400 C.The data in Fig.8 can be fitted to the following power relationship：where Ds and rp are expressed in m and MPa,respectively.3.3.晶粒细化因为动态再结晶包含重复的形核过程并且新晶粒的生长受到抑制，所以再结晶过程中再结晶晶粒的平均尺寸略有不同。然而，在部分再结晶结构，变形的晶粒有助于晶粒尺寸的测量。因此，晶粒平均尺寸不断地减小直到动态再结晶完成。但在本次研究中，有部分再结晶的样品在400度，应变速率0.01每秒下的变形比真应变小0.6，并且不显示最后的动态再结晶组织。所以，粒度分析只在材料淬火前就达到稳定状态的情况下使用。因此平均晶粒尺寸（D）等于完全动态再结晶的晶粒尺寸（DS）。图Fig.8显示了平均晶粒尺寸对Z的变化。如在此图中可以看到随着Z的增大，再结晶晶粒尺寸明显减小。晶粒尺寸随着温度的升高和应变速率的下降而增加，可以归因于位错密度的下降，上升的晶界迁移率和增长速率。本研究中最小的晶粒尺寸出现在应变速率为0.001每秒和变形温度为400度的情况下，大约为10微米。Fig.8的数据适用于以下的动力关系Ds和Rp分别以微米和兆帕表示。3.4.Constitutive analyses Sellars and Tegart,using hyperbolic function proposed by Garofalo,showed that hot deformation can be considered as athermally-activated process which can be described by strain rate equations similar to those employed in creep studies.The ZenerHollomon parameter ( Z ),which is the temperature compensated strain rate,can be associated with the flow stress in different ways,as shown in Eq.(2).These are the power law at relatively low stresses,exponential law at high stresses,and hyperbolic sine law for a wide range of deformation conditions 28,29where Q is the deformation activation energy,_e is the strain rate, T is the absolute temperature,R is the universal gas constant,and A0,A00,A,n0,n,b and a(b/n0) are material constants.The stress multiplier is an adjustable constant which brings ar into the correct range that gives linear and parallel lines in ln_e versus ln sinh（） plots 5.Based on Eq.(2),the expressions of n0=【ln_e=lnrp】T,b=【ln_e=rp】T,and n=【ln_e=lnfsinhep 】T can be derived and the values of n0,b,and n can be calculated30.The required plots are shown in Fig.9.Subsequently,the value of a=b/n0=0.024 can be calculated from these results. The following expression can also be derived from the hyperbolic sine law of Eq.(2) 5,30:3.4.本构分析塞拉斯和特加特利用加罗法洛提出的双曲函数，表明热变形可以当做一个热激活过程，用应变率方程来研究，这一方程类似于在蠕变研究中所用的方程。齐纳变形常数（Z），即温度补偿应变速率，可以用不同方式与流动应力联系起来，如公式 Eq.(2)。这些是幂定律在较低的应力，指数定律在高应力和双曲正弦定律在广泛的变形条件下。其中Q是变形激活能，_e是应变率，T是绝对温度，R是通用气体常数，和A0，A00，一，N0，N，B和（BN0）是材料常数。应力乘数 是一个可变常数，可以使rou到正确的范围，以在以下公式中给出直线和平行线。基于方程Eq.2，以下几个公式可以被推导，n，和n可以被计算出来。所需图如 Fig.9所示。随后=/N=0.024的值可以从这些结果中计算出来。下面的表达式也可由图Eq.（2）的双曲正弦定律推导出来。 It follows from these expression that the slope of the plots of ln_e and 1/T against lnsinhea rp） can be used for obtaining the value of Q .The required plot is shown in Fig.10 and the value of Q was determined as 173.2 kJ/mol which is higher than the lattice self-diffusion activation energy of magnesium (135 kJ/mol)5 or the grain boundary diffusion activation energy (92 kJ/mol)9.This can be ascribed to the presence of gadolinium，showing the importance of rare earth elements on increasing the deformation resistance at high temperatures 31,32.For instance,as shown in Fig.1b,the level of flow stress of the GZ31 alloy at the temperature 500 C and strain rate of 0.01s-1 is significantly higher than that of the AZ31 alloy,which is one of the most commercially important Mg alloys 5,33. The value of 173.2 kJ/mol was used to calculate theZparameter.According to Eq.(2), plots of LnZversus lnfsinhe arpTHg,rp and lnrp may be used to find the relationship between Z and rp25.The corresponding curves are shown in Fig.11.The resultant equations with new regression constants are shown in Eq.(4):从这些A式对B式斜率的表达式，可以获得Q的值。如Fig.10图所示，Q的值被确定为173.2 kJ/mol，高于镁的晶格自扩散激活能（135 kJ/mol）或晶界扩散激活能（92 kJ/mol）。这可以归因于钆元素的存在，显示了稀土元素在高温下提高变形阻力中的重要性。例如，在图Fig.1b，GZ31合金在500度，0.01每秒的形变速率下的流动应力水平显著高于AZ31合金，而后者是商业上最重要的一种镁合金。173.2 kJ/mol这个值被用来计算Z的值。如式Eq.2，式A对式B和式C可以用来找到Z与rp之间的关系。相应的曲线如图Fig.11所示。新的回归常数所得的方程如（4）式所示。In Fig.11a,the excluded point corresponds to the deformation temperature of 300 C under strain rate of 0.01 s-1.The point does not follow the trend due to inability of the power law at high stresses.The excluded points in Fig.11b correspond to deformation temperature of 500 C under strain rates of 0.01,0.001,and 0.0001s-1.Again,these points do not follow the general trend due to inability of the exponential law at low stresses.However,the hyperbolic sine law 5,as shown in Fig.11c,can give the appropriate constitutive equation for a wide range of deformation conditions.A comparison between the calculated and measured values of flow stress by the hyperbolic sine relation of Eq.(4) is shown in Fig.12.It can be seen that the calculated and the measured values are in a good agreement,which shows that the predictions of the proposed constitutive equation are satisfactory.在表Fig.11a中，排除的点对应变形温度300度，应变速率0.01每秒。并不遵循在高应力下幂定律无力的趋势。Fig.11b中的排除点对应变形温度500度，应变速率0,01，0.001和0.0001每秒。再次的，这些点不遵循低应力下指数定律无力的趋势。不过，双曲正弦定律可以给出大范围变形条件下的适当的本构方程，如Fig.11c所示。Eq.(4)中双曲正弦关系下流动应力的计算值和实测值的比较如Fig.12图所示。可以看出，计算值与实测值吻合较好，表明所提出的本构方程预测是符合要求的。4.Conclusions (1)The flow curves of GZ31 alloy showed typical broad single peak dynamic recrystallization behavior followed by a gradual fall towards the steady state stress. (2)The deformation activation energy was determined as 173.2 kJ/mol,which is higher than the lattice self-diffusion activation energy of 135 kJ/mol for magnesium.The latter can be ascribed to the presence of gadolinium and shows the importance of this element in increasing the deformation resistance at high temperatures.This was proved by comparing the flow stress of GZ31 alloy with AZ31 alloy. (3)The power and the exponential laws were found to be unsuitable for description of the flow stress of GZ31 alloy at high and low stresses,respectively.However,the hyperbolic sine law was able to represent the constitutive behavior in a wide range of Z parameter. (4)The stress multiplier and the hyperbolic sine exponent were calculated as 0.024 MPa-1 and 3.42,respectively.Therefore,the following constitutive equation,which can be used to express the hot flow behavior of this material, was proposed and verified: (5)Significant grain refinement occurred as a result of necklace DRX mechanism.The average dynamically recrystallized grain size decreased with increasing strain rate and decreasing deformation temperature.It was related to ZenerHollomon parameter and by power equations with exponents of 0.28 4. 结论（1） GZ31合金的流动曲线呈典型宽单峰动态再结晶表现，随后对稳态应力逐渐下降。（2） 变形激活能被确定为173.2 kJ/mol，高于镁的晶格自扩散激活能135 kJ / mol。后者可以归因于钆的存在，并且显示了这种元素在高温下增加变形抗力的重要性。这一点可以通过比较GZ31合金和AZ31合金的流动应力来证实。（3） 动力和指数定律被发现不适合用来描述GZ31合金分别在高低压力下的流动应力。然而，双曲正弦规律能在大范围的Z值内表示该合金的本构行为。（4） 压力乘数和双曲正弦指数分别算出为0.024 MPa-1和3.42,。因此，下面的本构方程可以用来表示该材料的高温流变行为，且已被提出并验证。（5） 显著的晶粒细化是（ ）动态再结晶机制的结果。应变速率升高，变形温度下降时平均动态再结晶晶粒尺寸减小。这与齐纳变形指数和指数为0.28的功率方程有关。References（工具书类）（6） 1H.E.Friedrich, B.L.Mordike,Magnesium TechnologyMetallurgy, Design Data Applications, Springer-Verlag, Berlin, Heidelberg, Germany,2006.（7） 2Z.Yang, J.P.Li, J.X. Zhang, G.W.Lorimer, J.Robson, Acta Metall. Sin.21 (2008)313328.（8） 3Y.Kawamura, M.Yamasaki, Mater. Trans.48 (11) (2007) 29862992.（9） 4N.Stanford, D.Atwell, M.R. Barnett, Acta Mater. 58 (2010) 67736783.（10） 5H.Mirzadeh, Mech.Mater. 77 (2014) 8085.（11） 6M.Karami, R.Mahmudi, Mater.Lett.81 (2012) 235238.（12） 7H.Mirzadeh, M.H.Parsa, J.Alloys Comp.614 (2014) 5659.（13） 8H.Mirzadeh, A.Najafizadeh, Mater.Des. 31 (2010) 11741179.（14） 9M.A.Shaojie, L.Ying, D.Xuehua, Z.Xinping, J.Wuhan Univ.