Flow softening and dynamic recrystallization behavior of a Mg-Gd-Y-Nd-Zr alloy under elevated temperature compressions

Flow softening behavior of a homogenized Mg-7Gd-4Y-1Nd-0.5Zr alloy under compression to a final strain of∼1.8 at elevated temperatures of 450∼550℃ and a constant strain rate of 2s^(−1) has been investigated by optical microscopy,scanning electron microscopy,electron back-scattered diffraction and transmission electron microscopy.The results show that true stress first rises to the peak point and then drops to the bottom value and increases again with further increasing strain at each temperature.Twinning dynamic recrystallization(DRX)and continuous DRX contribute to the formation of new fine grains at temperatures 450∼475℃ when the restoration is caused by both DRX and texture change due to extension twinning,resulting in the larger softening degrees compared with the softening effects owing to continuous DRX and discontinuous DRX at 500∼550℃ when twinning activation is suppressed.500℃ is the transition temperature denoting a significant decline in the contribution of twinning and TDRX to the strain with increasing temperature.The cuboid-shape phase exists in both homogenized and compressed samples,while the compositions are varied.

High temperature deformation and recrystallization behavior of magnesium bicrystals with 90°<1010>and 90°<1120>tilt grain boundaries

The deformation mechanisms and dynamic recrystallization(DRX)behavior of specifically grown bicrystals with a symmetric 90°<1010>and 90°<1120>tilt grain boundary,respectively,were investigated under deformation in plane strain compression at 200℃and 400℃.The microstructures were analyzed by panoramic optical microscopy and large-area electron backscatter diffraction(EBSD)orientation mapping.The analysis employed a meticulous approach utilizing hundreds of individual,small EBSD maps with a small step size that were stitched together to provide comprehensive access to orientation and misorientation data on a macroscopic scale.Basal slip primarily governed the early stages of deformation at the two temperatures,and the resulting shear induced lattice rotation around the transverse direction(TD)of the sample.The existence of the grain boundary gave rise to dislocation pile-up in its vicinity,leading to much larger TD-lattice rotations within the boundary region compared to the bulk.With increasing temperature,the deformation was generally more uniform towards the bulk due to enhanced dislocation mobility and more uniform stress distribution.Dynamic recrystallization at 200℃was initiated in{1011}-compression twins at strains of 40%and higher.At 400℃,DRX consumed the entire grain boundary region and gradually replaced the deformed microstructure with progressing deformation.The recrystallized grains displayed characteristic orientations,such that their c-axes were perpendicular to the TD and additionally scattered between 0°and 60°from the loading axis.These recrystallized grains displayed mutual rotations of up to 30°around the c-axes of the initial grains,forming a discernible basal fiber texture component,prominently visible in the{1120}pole figure.It is noteworthy that the deformation and DRX behaviors of the two analyzed bicrystals exhibited marginal variations in response to strain and deformation temperature.

Dynamic recrystallization of AZ91 magnesium alloy during compression deformation at elevated temperature

High temperature compressive tests of AZ91 Mg alloy were carried out at 573723 K and strain rates of 0.0011 s-1. The microstructures of as-compressed samples were observed by optical microscopy and transmission electron microscopy （TEM）, and the microhardness was also tested. It is shown that with the increase of temperature or the decrease of strain rate, the flow stress decreases, at the same time the dynamic recrystallization （DRX） of the alloy is more noticeable. The microstructures reveal that continuous dynamic recrystallization, which develops through conversion of low-angle grain boundaries into high-angle boundaries, occurs preferentially at the grain boundary.

Characteristic Analysis of DS18B20 Temperature Sensor in the High-voltage Transmission Lines’ Dynamic Capacity Increase

Dynamic capacity increase in high voltage electric power transmission line is currently the most economical method for solving electric power transmission bottleneck nowadays. DS18B20 temperature sensor is applied to the dynamic capacity increase of high voltage transmission lines to measure the conductor temperature and ambient temperature. The paper is focused on the experiment of DS18B20 both in the laboratory and outside. From the result of the lab temperature measurement data analysis, using 4 DS18B20’s is the most suitable plan, considering both accuracy and economical efficiency. In the experiment outside, we get four groups of conductor (uncharged) temperature and four groups of ambient temperature. The data proved that DS18B20 has good stability, and small measurement error. It is suitable for measuring the temperature of conductor and ambient in dynamic capacity increase, and helpful to improve the accuracy of the calculation of capacity increasing.

Dynamic property tests of frozen red sandstone using a split hopkinson pressure bar

In this study, frozen red sandstone specimens were impacted by a Split Hopkinson bar (SHPB), with a velocity of 4.558 ~ 6.823 ms-1. The temperature of the specimens was maintained at -15℃ during the experiment. For comparison purposes, static uniaxial compression tests were conducted in advance using a freezing triaxial test machine. Four stress-strain curves were obtained in diff erent average strain rates. The test results suggested that when the average strain rate is low, the specimen strength changes gradually;but when it is high, its strength changes rapidly. When the average strain rate is 120.73 s-1, the peak value of stress is as high as 82.96 MPa, which is about two times that of the static compressive strength of 44.1 MPa. A constitutive model was established that was composed of the damaged, viscoelastic and spring bodies, and revealed the variations of compressive strength and strain for the frozen red sandstone under diff erent high strain rates. The test results also showed that the failure form was correlated to the average strain rate of the frozen red sandstone. When the average strain rate is low, the damage was only distributed on the specimen’s edges. However, as the average strain rate increases, the damage range extended to the central parts of the specimen. When the average strain rate reached 107.34s-1, the specimen was smashed.

Dynamic compressive property and failure behavior of extruded Mg-Gd-Y alloy under high temperatures and high strain rates

For the purpose of investigating the dynamic deformational behavior and failure mechanisms of magnesium under high strain rates,the Split Hopkinson Pressure Bar(SHPB)was used for investigating dynamic mechanical properties of extruded Mg-Gd-Y Magnesium alloy at ambient temperature(300 K),200℃(473 K)and 300℃(573 K)temperature.The samples after compression were analyzed by scanning electron microscope(SEM)and metallographic microscope.Dynamic mechanical properties,crack performance and plastic deformation mechanism of extruded Mg-Gd-Y Magnesium alloy along the extrusion direction(ED)were discussed.The results show that,extruded Mg-Gd-Y Magnesium alloy has the largest dynamic compressive strength which is 535 MPa at ambient temperature(300 K)and strain rate of 2826 s^(−1).When temperature increases,dynamic compressive strength decreases,while ductility increases.The dynamic compression fracture mechanism of extruded Mg-Gd-Y Magnesium alloy is multi-crack propagation and intergranular quasi-cleavage fracture at both ambient temperature and high temperature.The dynamic compressive deformation mechanism of extruded Mg-Gd-Y Magnesium alloy is a combination of twinning,slipping and dynamic recrystallization at both ambient temperature and high temperature.

不同温度下再生骨料沥青混凝土动态抗压性能研究

为进一步研究再生骨料沥青混凝土在水工建筑物中的适用性,本文在不同温度环境中对再生骨料水工沥青混凝土的动态力学特性进行试验研究。对再生骨料水工沥青混凝土在温度为-10℃~10℃和应变速率为10^(-5)~10^(-2)s^(-1)的条件下进行动态抗压性能试验研究。试验结果表明:再生骨料水工沥青混凝土的抗压强度和弹性模量与温度成反比关系,与加载速率成正比关系;峰值应变与温度成正比关系,与加载速率成反比关系。通过温度影响因子和相关性系数分析,发现抗压强度和弹性模量随着温度升高逐渐减小并趋于某一定值。

纳米多晶Mg-Y合金压缩力学行为的分子动力学研究

目的研究纳米多晶Mg-Y合金在不同温度和应变速率下的力学响应以及不同变形条件下的晶粒变形行为。方法通过ATOMSK软件构建了晶粒取向随机的纳米多晶Mg-Y二元合金模型,利用LAMMPS软件,在400~520 K温度及1×10^(10)s^(−1)、1×10^(9)s^(−1)应变速率下完成了纳米多晶Mg-Y合金的压缩模拟,借助后处理OVITO软件对模拟结果进行了分析。结果随着温度的升高,纳米多晶Mg-Y合金的力学性能降低,且在温度上升到一定程度后具有一定的梯度规律。在微观层面,温度的上升使总体位错密度下降,但减少了晶粒内部的长程位错和位错扭结,并且<c+a>类位错与类位错的比例上升。纳米多晶Mg-Y合金在高应变速率下的屈服强度更高,在原子排布层面上,六方最密堆积的比例降低。结论温度升高带来的更多能量使低能态的Y原子更容易参与到位错产生、消失过程中,还加剧了<c+a>类型位错向类位错的转变。高应变速率通过大幅度转化基体Mg的六方最密堆积形式,增加了晶界的破碎和形核,影响了合金的力学性能。

低温下TiZrHf系高熵合金的动态压缩力学特性与点火行为

针对高熵合金在低温条件下力学特性以及点火行为研究不足的现状,以含微量元素Cu、Al或Ni的TiZrHf系多主元高熵合金(HEA)为研究对象,结合SHPB试验系统与高速摄像系统,开展不同温度下高熵合金的动态压缩实验及点火特性实验,获得了弹性模量、应力峰值、硬化模量以及应变率与温度、元素种类的关联规律,并明确了元素种类对火焰面积、点火延迟时间、点火对应时刻以及剧烈反应持续时间的影响规律。结果表明,低温条件下3种HEA的应力-应变曲线均经历弹性、强化以及卸载阶段;在相近载荷条件下(~20 m/s,-40℃),含Cu和Ni的2类试件均发生了点火现象,而含Al试件只有细微点火,试件的点火延迟时间均为25μs,含Cu试件的剧烈反应持续时间与点火持续时间均大于含Ni试件。

基于微观机理的高温后大理岩动态力学特性分析

为研究温度对大理岩动态力学特性的影响,对常温(25℃)及高温(200℃、400℃、600℃和800℃)后的大理岩试样开展SHPB试验、XRD和SEM微观试验,探究了高温后大理岩动态力学特性与温度的关系及微观损伤机理。结果表明:高温后大理岩的动态压缩应力-应变曲线分为弹性、屈服、破坏3个阶段;大理岩的峰值应力及破坏特征均存在明显的温度效应,即随温度的升高,大理岩的峰值应力逐渐下降,破碎程度加剧;大理岩的动态力学特性与其矿物组分及晶体结构密切相关,200℃~600℃时,微缺陷增多,且白云石发生了热分解转化,在800℃时,矿物分解生成CO_(2)气体和氢氧化钙,且晶体结构发生熔融破坏,内部损伤严重,塑性增强,从而导致宏观力学特性的大幅下降。

Nb-10Zr合金的热变形行为、组织特征及热加工图

Nb-10Zr合金可作为特种薄膜功能材料应用于太阳能行业。深入理解Nb-10Zr合金的热变形行为是实现该应用的前提,然而国内目前围绕该合金热加工过程的材料加工性能相关研究十分匮乏。建立热材料加工图可实现描述指定条件下的材料可加工性,明确合金的变形窗口,指导材料加工工艺的制定和优化。选用均匀化处理后的电铸熔炼铸锭Nb-10Zr合金,采用热模拟试验机开展了热模拟压缩试验,并基于动态材料模型,通过对应变速率敏感系数m、功率耗散系数η和失稳系数ξ的数据分析,建立了材料不同温度和应变速率条件下的流变稳态区和非稳态区的热加工图。同时,通过微观组织观察,分析和验证了加工图的准确性。研究结果表明,Nb-10Zr合金铸锭在1300℃下经24 h均匀化处理后,未出现Zr元素偏聚所形成的缺陷,也未见裂纹、气孔、疏松和夹渣等其他类型的缺陷。铸态组织中存在粗大晶粒和细小晶粒,晶粒尺寸分别为500—800μm和20—30μm。在应变为0.4和0.6条件下,Nb-10Zr合金存在2个合理的热加工窗口,即变形温度1060—1100℃和应变速率0.01—0.04 s^(-1),以及变形温度1080—1100℃和应变速率0.3—1 s^(-1)。在不同变形条件下,变形后的Nb-10Zr合金均获得了细小的动态再结晶组织。在温度1100℃和应变速率0.01 s^(-1)下,合金晶粒尺寸为80—100μm;而在温度1100℃及应变速率1 s^(-1)下,合金晶粒尺寸为40—60μm。此外,通过不同工艺制备参数下合金组织形貌的观察,证明了所确定加工窗口的合理性。本研究为Nb-10Zr生产过程中的工艺选择和工艺参数的优化提供了理论指导。

Experimental Study on Dynamic Mechanical Properties of Sprayed Concrete-Surrounding Rock Combined Body

To investigate the dynamic response problem of the double medium formed by the adherence of sprayed concrete and surrounding rock in the tunnel,a split Hopkinson pressure bar of 75 mm in diameter was adopted at the ages of 3,7 and 10 d.Experimental results showed that dynamic compressive strength and dynamic increase factors(DIF)of the combined bodies increase with the strain rate.With the growth of strain rate,the critical strain of the combined bodies first increases,then deceases.Furthermore,the combined bodies of 3 d reveal the plastic property and brittle property for 7 d and 10 d when the strain rate is over 80/s.The failure characteristic of the sprayed concrete changes from tearing strain damage to crushing damage as the growth of strain rate,and the failure characteristic of rock presents the tensile failure mode as demonstrated by the scanning electron microscope(SEM).

Investigation on the plastic work-heat conversion coefficient of 7075-T651 aluminum alloy during an impact process based on infrared temperature measurement technology

The plastic work-heat conversion coefficient is one key parameter for studying the work-heat conversion under dynamic deformation of materials. To explore this coefficient of 7075-T651 aluminum alloy under dynamic compression, dynamic compression experiments using the Hopkinson bar under four groups of strain rates were conducted, and the temperature signals were measured by constructing a transient infrared temperature measurement system. According to stress versus strain data as well as the corresponding temperature data obtained through the experiments, the influences of the strain and the strain rate on the coefficient of plastic work converted to heat were analyzed.The experimental results show that the coefficient of plastic work converted to heat of 7075-T651 aluminum alloy is not a constant at the range of 0.85–1 and is closely related to the strain and the strain rate. The change of internal structure of material under high strain rate reduces its energy storage capacity, and makes almost all plastic work convert into heat.

纳米多晶铝压缩塑性力学行为分子动力学研究

目的研究纳米多晶铝在不同温度与应变速率下的力学响应与塑性变形行为以及不同变形条件下的塑性力学行为。方法通过ATOMSK软件构建了晶粒取向随机的纳米多晶铝模型,利用LAMMPS软件在300~700 K温度以及1×10^(9)、5×10^(9)、1×10^(10)、1×10^(11)s^(-1)应变速率下完成了纳米多晶铝的压缩模拟,借助后处理OVITO软件对模拟结果进行了分析。结果随温度的升高,晶界原子所占比例增大,纳米多晶铝的弹性模量逐渐下降,在压缩过程中总位错密度随温度的升高而增大。随着应变速率的增大,材料硬化速率增加,纳米多晶铝表现出更高的屈服强度。当应变速率较低时,位错大量存在于小晶粒之中,且中央大晶粒相较于初始位置旋转了20°。当应变速率达到1×10^(11)s^(-1)时,材料的硬化速率极大提高,且在晶粒内部出现了孪晶。在塑性变形过程中,1/6<112>(不全位错)的数量最多,在位错运动中占主导地位。结论温度升高导致材料弹性模量降低,这主要是由于高温提供了更多能量,晶界原子占比增加。应变速率会影响纳米多晶铝的塑性变形方式,应变速率的增大使其由晶粒旋转变形转变为孪生变形与位错湮灭机制,导致纳米多晶铝硬化速率与屈服强度提高。

AZ31连铸板的高温压缩变形行为及组织演变

合金在高温下的流变应力与组织演变特征是构建合金热加工参数的重要基础。基于此,利用Gleeble 1500D对AZ31连铸板在温度为450和500℃、应变速率为0.15 s^(-1)的压缩行为进行了研究,并将其与400℃及以下温度的压缩变形行为进行了对比。结果发现:随着温度的升高,合金的稳态流变应力值下降,在500℃进行35%变形时仅约28.2 MPa;在较低温度时,合金流变应力取向差异(0°试样<5°试样<10°试样)随温度升高而消失;不同于稳态流变应力的变化,再结晶比例则随变形温度的升高而增大,变形量与再结晶比例之间保持正相关,而取样角度使得再结晶比例的取向性较强,500℃时再结晶比例在不同取样方向上遵循0°试样<5°试样<10°试样的变化趋势;随着变形量的进行,动态再结晶越来越充分,其组织形貌逐渐由晶界处的零散分布再结晶晶粒向“项链状”再结晶分布转变,并且逐渐呈包围状态向晶体内部扩展,最后完全取代原始连铸板组织。此外,通过示意图的方式再现了合金再结晶的形核与长大过程。

车用6061铝合金冶金管准静态压缩特性分析

6061铝合金冶金管材广泛应用于车轻量化领域。为了提高对其力学性能的认识,分析其在高温下动态压缩力学性能,主要从流动应力、材料应力-应变关系曲线等不同条件下变化规律展开分析。研究结果表明:当受热温度不断升高时,应力应变曲线开始呈现下移趋势,并且随着温度升高越快而越显著。当温度升至200℃时,应力应变曲线下降趋势相对较小,当温度继续升温至300℃和400℃时,应力-应变曲线下降趋势明显。该研究为后续的合金性能综合分析奠定一定的力学参数基础。

不同煤矸石粗骨料替代率下混凝土的低温力学性能研究

为研究煤矸石粗骨料对C40混凝土低温力学性能的影响,制作4种替代率下煤矸石粗骨料试件,以全替代率为基础,对比设计强度为C30,C50煤矸石粗骨料混凝土,在低温下恒温4 h,分别测试在20,-10,-20,-30℃下的抗压强度、劈裂抗拉强度、抗折强度和相对动弹性模量.结果表明,在低温条件下,煤矸石骨料混凝土的强度和动弹性模量均提高,且温度越低,其抗压强度、劈裂抗拉强度、抗折强度以及动弹性模量提升幅度越大.

高温后钢纤维橡胶自密实混凝土动态冲击性能

为探究高温后钢纤维橡胶自密实混凝土(steel fiber rubber self-compacting concrete,SFRSCC)的动态力学性能,采用直径为80 mm的分离式-霍普金森压杆装置对其进行动态冲击试验,在确定钢纤维最优掺量基础上重点探讨了橡胶掺量对高温后SFRSCC破碎形态、动态抗压强度、冲击韧性以及动态应力增长因子等的影响规律。试验结果表明:与常温下相同,随着橡胶掺量的增加,高温冷却后SFRSCC的动态抗压强度呈逐渐降低的趋势;与常温下相反,高温冷却后SFRSCC抵抗冲击破碎能力、冲击韧性、动态应力增长因子均随着橡胶掺量的增加呈逐渐降低的趋势。对比试验得到的动态应力增长因子与CEB-FIP建议公式计算值的差异,提出了高温后SFRSCC的动态增长因子的修正公式,修正公式的计算值与试验结果和其他文献的试验结果吻合较好,可为进一步分析探讨高温后SFRSCC动力特性提供一定参考。

压缩二氧化碳和甲基吡咯烷酮剥离石墨烯的分子动力学模拟

通过分子动力学模拟,研究在压缩二氧化碳(CO_(2))和有机溶剂混合溶剂体系中液相剥离石墨烯的重要环境参数,包括CO_(2)和甲基吡咯烷酮的分子个数比、体系的温度以及环境压强对石墨烯剥离的影响。通过对设置了不同参数的体系中溶剂分子的插层速度以及解吸附能的表征和比较,结果表明,CO_(2)和甲基吡咯烷酮个数比为3.5∶1~5.5∶1,温度在300 K以上,压强为10~19 MPa时,更有利于获得高的剥离产率。该研究结果有利于压缩CO_(2)与有机溶剂体系剥离石墨烯选择合适的环境变量以获得更高的产率和单层率及少层率。

基于导线温度的架空线路弧垂新算法

为保证输电线路增容运行的安全性,研究导线温度与弧垂的关系有很现实的意义。常用的状态方程法和应力积累法只适用于较低的导线温度范围,一旦导线温度过高,铝股发生径向膨胀,弧垂的计算结果将会出现很大误差。输电线路导线温度——弧垂新模型的提出解决了上述问题,当导线温度逐渐升高,铝股受力由拉力变为压力时,将利用纯钢特性继续求解弧垂。实验结果表明,该算法使得计算的弧垂误差均小于其他两种方法,新模型的应用将为输电线路增容安全运行提供保障。