U-Ti合金的动态力学性能

采用分段式霍普金森杆（SHPB）及材料试验机测试了3种时效状态的U-Ti合金在室温下的静动态力学曲线,采用扫描电镜分析了U-Ti合金动态压缩后的剪切断裂特征,采用XRD分析了动态压缩前后的结构变化。结果表明,3种状态的U-Ti合金都产生了应变速率强化效应;当应变速率约为2500s^-1时,U-Ti合金发生了剪切断裂;断口出现取向性的韧窝且在局部区域的韧窝非常细小,以及烧蚀变形。动态压缩使3种状态的U-Ti合金的晶粒取向改变。     

高应变率下U-5.7Nb合金的绝热剪切变形

利用分离式Hopkinson压杆实验和显微分析技术,研究了U-5.7Nb合金在应变率范围(102/s?104/s)内的变形特征和微观组织.结果表明:U-5.7Nb合金的动态屈服强度明显高于其静态屈服强度,表现出明显的应变率敏感效应;当U-5.7Nb合金应变率为4300/s,试件在应变达到64%时发生剪切破坏.材料内部形成了明显的绝热剪切带,剪切带在与压缩方向垂直的平面内呈环形,这与动态压缩过程中应力分布不均匀有关.     

CLAM钢的热变形行为及热加工图

利用Gleeble-3500热模拟试验机对中国低活化马氏体（CLAM）钢进行热模拟压缩试验,研究变形温度为950～1150 ℃、应变速率为0.01～5 s^-1和真应变为1.2条件下的热变形行为。建立了CLAM钢的本构方程,绘制了热加工图,并采用Leica-DMI5000M金相显微镜观察了CLAM钢在热加工稳定区和失稳区的变形条件下的微观组织。结果表明：在较高变形温度和较低应变速率条件下的主要变形机制是动态再结晶;在较低温度和较高应变速率下主要发生动态回复;应避免的失稳区存在于较低变形温度和较高应变速率区域。对比分析热加工图和变形后的组织得到最佳热加工工艺参数为变形温度955～1035 ℃、应变速率0.03～0.32 s^-1,在此条件下,功率耗散率大于36%。     

Zr-Sn-Nb-Fe-Cr合金热变形行为

应用Gleeble-1500热模拟机对Zr-Sn-Nb-Fe-Cr合金进行热压缩试验,研究该合金在600～980℃和应变速率0.001～50 s-1条件下的热变形行为.根据材料动态模型,计算并分析该合金的加工图.利用加工图确定了热变形的流变失稳区,并获得了试验参数范围内两个区域热变形过程的如下最佳工艺参数:热加工温度600～630℃,应变速率0.001 s-1;热加工温度高于980℃,应变速率1～10 s-1.     

稀土元素Gd对2308双相不锈钢热变形行为的影响

为解决含Gd双相不锈钢热加工不足问题,本文以含2%Gd的双相不锈钢为研究对象,在不同温度下开展热模拟压缩实验,研究含Gd双相不锈钢热变形行为及组织演变。利用Gleeble-1500D热模拟试验机对含Gd双相不锈钢进行变形量为50%的单道次热变形试验。根据真应力-真应变曲线计算了该合金的热变形激活能Q_d,建立本构方程。同时对热变形后的组织进行了分析,探究稀土元素Gd对含Gd双相不锈钢热变形行为的影响,结果表明,在热变形过程中,合金的动态软化机制主要为动态再结晶。合金包含两种含Gd析出相,即条带状的脆性析出相M₃Gd相和M₁₇Gd₂相（M=Fe、Cr、Ni）,均为六方结构。当变形温度为1 050 ℃时,脆性M3Gd相破坏了基体的连续性,无法与基体协同变形,降低了合金的热塑性,导致合金在热变形过程中出现沿晶开裂。含Gd双相不锈钢适宜的热加工工艺区间的应变速率为0.01~0.1 s^-1,变形温度为950~1 000 ℃。     

Zr-Sn-Nb-Fe合金两相区流变行为与本构模型研究

为准确描述材料的高温流变行为，本文通过利用单向压缩试验对Zr-Sn-Nb-Fe合金在温度为650～800℃、应变速率为0.01～1 s^-1时的热变形行为进行研究，建立了Zr-Sn-Nb-Fe合金应变补偿型Arrhenius本构模型，并评估了该模型的预测能力。结果表明，Zr-Sn-Nb-Fe合金是一种对温度和应变速率较敏感的材料，其流变曲线呈现为动态再结晶型，流动应力随变形温度的升高和应变速率的降低而减小。采用本文所建本构模型对Zr-Sn-Nb-Fe合金的流动应力进行预测，结果显示，各工艺参数下流动应力预测值与实验值均吻合良好，模型预测值与实验值的平均相对误差绝对值为4.77%,相关系数为0.988 3,表明模型的预测精度较高。     

7AXX铝合金在热压缩状态下的流变行为

研究7AXX铝合金在热变形过程中的流变行为对于提高专用设备锻件综合性能、避免材料热开裂有重要意义。本文利用Gleeble-3500热模拟试验机在变形温度360～460 ℃、应变速率0.001～1 s^-1的条件下,对7AXX铝合金铸锭进行高温压缩实验,研究了热压缩状态下温度和应变速率对该合金流变行为的影响程度,拟合得到了含Zener-Hollomon参数的7AXX铝合金流变应力表达式,建立了基于双曲正弦关系的合金热变形本构方程及热加工图。结果显示,变形温度及应变速率对7AXX铝合金的流变行为有显著影响。合金的流变应力随温度的升高而降低,随应变速率的增加而增加。合金在热压缩过程中的应力指数为3.21,热激活能为137.31 kJ/mol。变形温度385～450 ℃、应变速率0.01～0.1 s^-1是适用于7AXX铝合金的热加工条件范围。     

HMD法制备U-10wt%Mo合金粉末的粒度控制

对氢化-破碎-脱氢(HMD)法制备U-10wt%Mo合金粉末的粒度控制工艺进行了研究.U-Mo合金在950℃分别退火4、10、24h,然后经过γ→α相转变热处理、HMD法和α→γ相转变热处理等过程成为粉末.分析结果表明:950℃下U-Mo晶粒长大较慢,可通过调节保温时间使晶粒尺寸控制在40～100μm;γ相分解程度强烈依赖于晶粒尺寸,晶粒尺寸越大,γ相向α相的转变程度越低;粉末粒度大小与原始合金γ相的晶粒度密切相关,原始合金γ相晶粒度越大,粉末粗粉比重越大.     

低活化马氏体钢热变形行为及机理型本构建模研究

利用Gleeble-3500热模拟试验机研究了低活化马氏体钢在变形温度为850～950 ℃、应变速率为0.001～1 s^-1条件下的热变形行为。建立了流变应力本构方程,并评估了该方程的预测能力。绘制了低活化马氏体钢在不同应变下的热加工图。结果表明：在较高的应变速率条件下,该材料主要发生动态回复,在较高变形温度和较低应变速率下具有明显的动态再结晶特征;本构方程的预测结果与实验结果符合良好;变形温度870～930 ℃、应变速率0.001～0.01 s^-1和变形温度920～950 ℃、应变速率0.3～1 s^-1分别是真应变为0.4和0.6下最优的热加工区域。     

304不锈钢在单轴应变棘轮变形下的随动硬化实验研究

对304不锈钢在室温下进行了单轴应变控制下的应变棘轮变形与失效以及低周疲劳试验研究,对材料在循环过程中材料的硬化行为进行了系统的揭示。在对称应变循环下,研究了不同应变幅值下弹性区尺寸和背应力演化规律;在给定工程应变幅值和循环棘轮应变增量组合的应变棘轮变形下进行了弹性区尺寸和背应力演化研究。观察到了各向同性硬化和随动硬化演化对加载历史的依赖性。     

Measurement and analysis of fission rates in depleted uranium and depleted uranium–polyethylene shells with neutrons from D-T reaction

To validate the concept design of a novel fusion–fission hybrid energy reactor, a depleted uranium assembly and a combined assembly of uranium and polyethylene were designed and assembled based on a depleted uranium spherical shell and a polyethylene spherical shell. The distribution of the fission rates for the depleted uranium and enriched uranium in the two assemblies, as a function of the distance of the detection position to the centre, was measured using a plate fission chamber bombarded by D-T neutrons. The addition of a polyethylene shell significantly changed the neutron spectrum; in particular, the neutron fluxes with energies of 1 MeV and lower were changed. Using MCNP5 and the attached libraries, the fission rate experiments were simulated, and the experimental configuration, including the wall of the experimental hall, was described in detail in the model. The fission rate distributions for depleted uranium and enriched uranium in the two assemblies were reproducible. The difference between the calculated results with different libraries and different tallies is as small as 1.0%. By considering the neutron flux, the fission rate and the C/E values, it is concluded that the fission rates of depleted uranium and enriched uranium induced by the fast neutrons were overestimated, and it is proposed that the fission parameters of uranium for fast neutrons should be re-evaluated, or the margin of the concept design should be enlarged, to make the concept effective.     

The effect of deformation temperature on the microstructure evolution of Inconel 625 superalloy

Hot compression tests of Inconel 625 superalloy were conducted using a Gleeble-1500 simulator between 900 °C and 1200 °C with different true strains and a strain rate of 0.1 s⁻¹. Scanning electron microscope (SEM) and electron backscatter diffraction technique (EBSD) were employed to investigate the effect of deformation temperature on the microstructure evolution and nucleation mechanisms of dynamic recrystallization (DRX). It is found that the relationship between the DRX grain size and the peak stress can be expressed by a power law function. Significant influence of deformation temperatures on the nucleation mechanisms of DRX are observed at different deformation stages. At lower deformation temperatures, continuous dynamic recrystallization (CDRX) characterized by progressive subgrain rotation is considered as the main mechanism of DRX at the early deformation stage. However, discontinuous dynamic recrystallization (DDRX) with bulging of the original grain boundaries becomes the operating mechanism of DRX at the later deformation stage. At higher deformation temperatures, DDRX is the primary mechanism of DRX, while CDRX can only be considered as an assistant mechanism at the early deformation stage. Nucleation of DRX can also be activated by the twinning formation. With increasing the deformation temperature, the effect of DDRX accompanied with twinning formation grows stronger, while the effect of CDRX grows weaker. Meanwhile, the position of subgrain formation shifts gradually from the interior of original grains to the vicinity of the original boundaries.     

Strain rate dependence of impact properties of sintered 316L stainless steel

This paper uses a material testing system (MTS) and a compressive split-Hopkinson bar to investigate the impact behaviour of sintered 316L stainless steel at strain rates ranging from 10⁻³ s⁻¹ to 7.5 × 10³ s⁻¹. It is found that the true stress, the rate of work hardening and the strain rate sensitivity vary significantly as the strain rate increases. The flow behaviour of the sintered 316L stainless steel can be accurately predicted using a constitutive law based on Gurson’s yield criterion and the flow rule proposed by Khan, Huang and Liang (KHL). Microstructural observations reveal that the degree of localized grain deformation increases, but the pore density and the grain size decrease, with increasing strain rate. Adiabatic shear bands associated with cracking are developed at strain rates higher than 5.6 × 10³ s⁻¹. The fracture surfaces exhibit ductile dimples. The depth and density of these dimples decrease with increasing strain rate.     

铀尾矿粒度对氡析出影响的蒙特卡罗模拟

基于核反冲理论建立了铀尾矿氡析出的数学模型和蒙特卡罗模拟方法,模拟了不同含水饱和度条件下铀尾矿颗粒粒度对氡析出的影响。结果表明,镭分布于颗粒表面时,4种含水饱和度（5%、20%、50%、75%）条件下的K均随颗粒尺寸的增大先增大后趋于稳定。镭分布在距颗粒表面34 nm和68 nm时,含水饱和度为5%或20%的条件下,K先增加后趋于稳定,而含水饱和度为50%或75%的条件下,K随颗粒尺寸的增大先稍有下降然后逐渐稳定。运用蒙特卡罗方法计算了各铀尾矿样品的氡析出率,计算的氡析出率与实测铀尾矿氡析出率符合较好,相对误差为3%～9%。     

回收铀燃料元件UO2芯块辐照后显微组织研究

为评价回收铀燃料元件中UO₂芯块的辐照稳定性,采用热室金相显微镜对辐照后高放射性UO₂芯块沿轴向及径向的辐照肿胀、裂纹分布、晶粒尺寸及分布和晶粒长大行为进行了观察和分析。结果表明:燃料元件芯块中均存在大量的裂纹,回收铀燃料元件UO₂芯块裂纹呈明显的环形分布特征,天然铀燃料元件UO₂芯块呈放射性发散分布特征。两者的燃料芯体晶粒呈等轴状,均出现从边缘区域向芯块中心区域晶粒逐渐长大现象,辐照后晶界变粗化。两者晶粒尺寸、形貌及分布特征并无明显差别。此外,在相同的堆内运行工况条件下,回收铀燃料元件UO₂芯块辐照肿胀不明显,芯块破碎程度及晶粒长大过程与天然铀并无明显差别。      

Effect of specimen sizes, specimen shapes, and placement directions on compressive strength of concrete

The compressive strength of concrete is used as the most basic and important material property when reinforced concrete structures are designed. It has become a problem to use this value, however, because the control specimen sizes and shapes may be different from country to country.In this study, the effect of specimen sizes, specimen shapes, and placement directions on compressive strength of concrete specimens was experimentally investigated based on fracture mechanics. Experiments for the Mode I failure were carried out by using cylinder, cube, and prism specimens. The test results are curve-fitted using least square method (LSM) to obtain the new parameters for the modified size effect law (MSEL). The analysis results show that the effect of specimen sizes, specimen shapes, and placement directions on ultimate strength is present. In addition, correlations between compressive strengths with size, shape, and placement direction of the specimen are investigated.     

Deformation of polycrystalline Li2O

Constant-crosshead-speed compression of Li₂O polycrystals has been performed at temperatures of 700–950°C with strain rates ranging from about 10⁻⁶-10⁻⁴s⁻¹. For temperatures greater than 850°C, data suggest deformation rates are controlled by recovery via dislocation climb. For lower temperatures, impurity effects may alter the deformation mechanism. At 950°C, viscous creep occurs at low strain rates. For porosities ranging from about 7 to 21%, deformation is strongly dependent on porosity, but is virtually independent of grain size.     

铀表面脉冲偏压MSIP铝镀层的电化学腐蚀行为

采用脉冲偏压磁控溅射离子镀（MSIP）技术在贫铀表面制备铝镀层,利用电化学测试技术、扫描电镜（SEM）及X射线能谱（EDS）对铝镀层在50μg/gCl－水溶液中的电化学腐蚀行为进行研究。结果表明：铝镀层的腐蚀电位－534.8mV高于贫铀的腐蚀电位－641.2mV,它对贫铀是一种阴极性镀层;镀铝贫铀样品的极化电阻和电化学阻抗幅值远大于贫铀,腐蚀电流远小于贫铀,铝镀层对贫铀基体具有良好的防腐蚀性能;镀铝贫铀样品的腐蚀特征为局部腐蚀,并出现镀层破裂、剥落,抗腐蚀性能变差;铝/铀界面伪扩散层具有一定的抗腐蚀能力。     

草酸铈和草酸铀的热分解机理及煅烧遗传性北大核心CSCD

在核燃料后处理过程钚尾端处理中,通常利用草酸制备草酸钚沉淀然后进一步煅烧获得氧化钚颗粒、用于MOX燃料的制造,因此掌握草酸钚热分解机制、控制氧化钚产品颗粒的形貌和粒度具有实际意义。针对草酸盐的热分解机理和沉淀在煅烧后颗粒形貌遗传性问题,本工作选用草酸铈和草酸铀作为研究对象,系统研究了草酸铈、草酸铀的热分解反应,结合同步热分析仪(TG/DSC)与X射线衍射仪(XRD)的表征,获得了草酸铈和草酸铀的热分解数据;制备了不同粒度的草酸铈和草酸铀,用激光粒度仪考察了煅烧前后颗粒的粒度,并用扫描电镜(SEM)观察颗粒聚集状态、粒度和形貌,结果表明煅烧分解会导致粒径有规律地下降,但形貌得以保留。