FeMnNi合金的冲击相变和层裂特性的实验研究

对FeMnNi合金进行了轻气炮平板撞击实验研究,实验中材料发生了相变和层裂。得到的FeMnNi合金的相变阈值压力为6.3~6.9 GPa,远低于纯铁的相变阈值压力13 GPa,说明Mn、Ni合金元素的加入会极大的降低相变阈值。回收试样观测表明,当应力高于FeMnNi合金的相变阈值时,样品中可能产生二次层裂现象和浅表层裂新现象。实验结果还表明该合金相变后层裂强度没有明显的提高。     

冷热循环处理对Fe基块体非晶合金摩擦磨损性能的影响

本文中采用电弧熔炼和感应熔炼后喷铸的方法制备了厚度为4 mm的Fe基块体非晶合金. 通过对Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂块体非晶合金进行30次和60次冷热循环处理,并在往复式摩擦磨损条件下,研究冷热循环工艺对其摩擦磨损性能的影响. 结果表明:冷热循环处理没有显著改变铁基非晶合金的非晶态结构. 30次冷热循环处理后,Fe基块体非晶合金发生了明显的软化,平均硬度由铸态的16.06 GPa降为14.06 GPa,平均弹性模量由241 GPa降为216 GPa. 随着冷热循环次数和载荷的增加,非晶合金的平均摩擦系数和磨损率先减小后增大. 冷热循环处理有利于降低非晶合金的平均摩擦系数和磨损量. 当冷热循环次数为30次、载荷为30 N时,铁基非晶的摩擦系数由0.77降至0.72,表现出最小的摩擦系数,同时磨损率降低13.3%,表现出最小的磨损率[1.04×10?6 mm3/(m·N)]. 铸态Fe基块体非晶合金的磨损机理以疲劳断裂为主,伴随着轻微的磨粒磨损. 随着冷热循环次数的增加,疲劳导致的脆性断裂程度降低,磨损机制向磨粒磨损和疲劳断裂的共同作用转变. 所以,冷热循环处理有望成为调控非晶态金属材料摩擦学性能的有效手段. 进一步深化对冷热循环处理的理解,必将有利于推动非晶态材料在摩擦学领域的应用.      

无定形水化硅酸钙纳米压痕模拟及卸载段处理

基于预测水泥及其水化物体积性能的简化物理模型Wittmann模型，结合分子动力学研究方法，构建水 化硅酸钙(C-S-H)凝胶模型用来模拟纳米压痕实验．在弛豫阶段使用高温淬火方法将模型转变为无定形态，并 测定模型高温淬火前后的径向分布函数，确定了无定形态下的水化硅酸钙(C-S-H)压痕模型，计算得出荷载深 度曲线 （P-h 曲线）．考虑到高温淬火对模型内粒子热力学运动的影响以及带来的位错效应，分别采用 OliverPharr方法与 Cheng-Cheng 方法对荷载深度曲线的卸载段进行分析处理．其中 Oliver-Pharr方法计算得到硬度 H 为 0.92 GPa，折合模量 Er为 29.56 GPa；Cheng-Cheng方法计算得到折合模量 Er为(34.92±7.57) GPa．与文献结 果及标准试块实验结果进行对照，探究两种计算方法的合理性及适用范围，并提出研究展望     

纯铁材料的冲击相变与"反常"层裂

采用VISAR、X射线衍射和金相处理联合测试分析技术,开展等厚对称碰撞实验,研究了纯铁材料的冲击相变与层裂行为特征。结果发现,冲击加载压力大于纯铁材料相变阈值(13 GPa)时,等厚对称碰撞样品发生反常2次层裂。结合相关文献的实验结果,从应力波相互作用的角度分析了反常层裂形成的原因,指出纯铁材料的冲击相变和卸载逆转变及引发的稀疏冲击波是导致反常2次层裂的物理机制。     

Hf基非晶合金夹芯结构长杆弹的侵彻行为

为研究Hf基非晶合金的变形行为及高速侵彻性能,分别开展了Hf基非晶合金材料静动态力学性能和Hf基非晶合金夹芯结构长杆弹高速侵彻45钢靶体试验研究,并与45钢夹芯长杆弹侵彻结果进行对比。研究发现:Hf基非晶合金具有较高的断裂强度,断裂时伴随有能量释放现象;Hf基非晶合金夹芯长杆弹侵彻钢靶过程可分为3个阶段:开坑、夹芯结构侵彻和剩余弹体侵彻。Hf非晶合金在侵彻过程中发生了明显的释能反应,显著地增强了弹体毁伤效应,扩大了侵彻弹孔直径,增加了弹体侵彻深度和弹孔体积。在高速冲击下,Hf基非晶合金夹芯长杆弹表现出优异的侵彻性能,可以为非晶合金材料在高效毁伤领域的应用提供新思路。     

磁控溅射Al靶功率对类金刚石薄膜的结构和摩擦学性能影响

本文采用中频磁控溅射金属Al靶,以CH4为反应气体,通过调整Al靶溅射功率,在p(100)单晶硅片和不锈钢基底上成功制备出不同Al含量的Al/a-C:H纳米复合薄膜。并利用HR-TEM、XPS、纳米压痕仪和摩擦磨损试验机等手段分析和研究了Al/a-C:H薄膜的结构、机械及摩擦学性能。结果表明：金属Al以纳米晶颗粒形式镶嵌在非晶碳网络中,使得所制备Al/a-C:H薄膜呈现出典型的纳米晶/非晶复合结构;同时,Al掺杂促进薄膜中sp2杂化碳形成,且有效地释放残余内应力。Al靶溅射功率为800 W时所制备的Al/a-C:H薄膜具有结构致密、内应力低、硬度高的特性;在大气环境中,该薄膜与Si3N4陶瓷球干摩擦时显示出优越的摩擦学性能,其摩擦系数约为0.055,磨损率约为2.9×10-16 m3/(N?m)。     

Detonation waves in trinitrotoluene

Fabry-Perot, ORVIS, and VISAR laser interferometry are used to obtain nanosecond time resolved particle velocity histories of the free surfaces of copper and tantalum discs accelerated by detonating trinitrotoluene (TNT) charges and of the interfaces between TNT detonation products and lithium fluoride crystals. TNT detonation reaction zone profiles are measured for self-sustaining detonation and piston supported overdriven (supracompressed) waves. The experimental records are compared to particle velocity histories calculated using very finely zoned meshes of the exact dimensions with the DYNA2D hydrodynamic code. The Ignition and Growth reactive flow model, which is based on the Zeldovich-von Neumann-D?ring (ZND) theory of detonation, yields excellent agreement with the experimental records for TNT using an unreacted von Neumann spike pressure of 25 GPa, a reaction rate law which releases 90% of the chemical energy within 80 ns and the remaining 10% over an additional 200 ns, and a reaction product equation of state fit to cylinder test data assuming a Chapman-Jouguet pressure of 19 GPa. The late time energy release is attributed to diffusion controlled solid carbon particle formation. Received 26 July 1997 / Accepted 29 December 1997     

斜波压缩下锡的相变和层裂行为

获取不同热力学路径下锡的动态响应实验数据,是深入研究其相变和损伤物理过程的基础.利用小型磁驱装置CQ-4完成了金属锡的斜波加载实验,获取了锡含有相变和层裂损伤物理信息的实验数据.实验结果显示,在加载段锡依次经历了弹塑性转变和β-γ相变两种物理过程,屈服强度约0.194 GPa,相变压力随着锡厚度的增加从7.54 GPa减小到7.14 GPa.在卸载段出现了明显的层裂损伤,层裂强度约1.1 GPa,与相同加载压力下冲击实验结果有巨大差异,层裂片厚度约0.38 mm.结合由锡的多相Helmholtz自由能计算的多相状态方程、Hayes相变动力学方程和损伤度理论,对斜波压缩实验过程进行一维流体动力学数值模拟,计算结果可以很好描述锡的弹塑性转变、相变和层裂三个物理过程.     

Temperature measurements of shock heated materials using multispectral pyrometry: Application to bismuth

A new device for temperature measurements of shock heated materials has been developed in order to refine their equations of state. We present temperature measurements for bismuth samples in contact with a lithium fluoride window. The shock pressure of these experiments ranges from 18 to 97 GPa. Received 2 June 1997 / Accepted 26 January 1998     

Mesoscale modeling of dynamic compression of boron carbide polycrystals

An anisotropic nonlinear elastic model is advanced for crystals belonging to either of two polytypes of boron carbide ceramic. Crystals undergo transformation to an isotropic, amorphous phase upon attainment of a local state-based criterion associated with a loss of intrinsic stability. The model is implemented using the dynamic finite element method, and is demonstrated on a representative volume consisting of fifty polyhedral grains subjected to uniaxial strain at a uniform high strain rate and shock compression at axial pressures ranging from 10 to 50 GPa. Predicted stress–strain behavior is in close agreement with experimental data. For polycrystals consisting of both polytypes, amorphization initiates at stress levels slightly below the experimental Hugoniot elastic limit, and occurs more readily than observed in experiment. For polycrystals consisting only of the CBC (polar) polytype, amorphization initiates at impact pressures similar to those suggested by experiment. In either case, transformation is promoted by dynamic stress interactions and elastic coefficient mismatch among anisotropic crystals. Results support a previous conjecture that amorphization is related to shear instability and cross-linking of the CBC chain in the polar polytype.     

涂铁基非晶涂层的磨粒磨损性能研究

用电弧喷涂技术在Q235钢基体上制备一种高非晶含量的Fe基合金涂层.用MLS-225型湿砂橡胶轮磨损试验机评价铁基复合涂层的磨粒磨损性能,利用光学显微镜、扫描电子显微镜和X射线衍射仪对涂层的显微组织结构、磨损表面进行分析.结果表明,涂层中含有一定量的非晶相,通过Verdon方法对XRD图谱进行Pseudo-Voigt函数拟合,计算得出涂层的非晶相含量约为49%.涂层呈典型的层状组织结构,结构致密,孔隙率低,具有很高的硬度和耐磨性,显微硬度在HV900~1 050范围内,属于硬质涂层.涂层耐磨粒磨损性能为Q235钢的12.2倍.其磨损机制主要为脆性剥落.     

A thermodynamic approach to model the caloric properties of semicrystalline polymers

It is well known that the crystallisation and melting behaviour of semicrystalline polymers depends in a pronounced manner on the temperature history. If the polymer is in the liquid state above the melting point, and the temperature is reduced to a level below the glass transition, the final degree of crystallinity, the amount of the rigid amorphous phase and the configurational state of the mobile amorphous phase strongly depend on the cooling rate. If the temperature is increased afterwards, the extents of cold crystallisation and melting are functions of the heating rate. Since crystalline and amorphous phases exhibit different densities, the specific volume depends also on the temperature history. In this article, a thermodynamically based phenomenological approach is developed which allows for the constitutive representation of these phenomena in the time domain. The degree of crystallinity and the configuration of the amorphous phase are represented by two internal state variables whose evolution equations are formulated under consideration of the second law of thermodynamics. The model for the specific Gibbs free energy takes the chemical potentials of the different phases and the mixture entropy into account. For simplification, it is assumed that the amount of the rigid amorphous phase is proportional to the degree of crystallinity. An essential outcome of the model is an equation in closed form for the equilibrium degree of crystallinity in dependence on pressure and temperature. Numerical simulations demonstrate that the process dependences of crystallisation and melting under consideration of the glass transition are represented.     

四面体无定型无氢非晶碳膜的制备及其摩擦学性能研究

采用磁过滤阴极真空弧系统分别在硅片[Si(100)]、W18Cr4V高速钢和Cr18Ni9不锈钢基体上沉积了一系列sp3键含量较高的四面体无定型无氢非晶碳膜(ta-C),研究了所合成薄膜的结构、硬度、附着强度和摩擦磨损性能,考察了基体和薄膜厚度对薄膜摩擦系数的影响,简要分析了相应ta-C膜的失效机理.结果表明,在高速钢基体上沉积的ta-C膜的显微硬度为76 GPa,结合力Lc值达42 N,具有优良的摩擦学性能,其摩擦系数为0.12,且摩擦系数可以在16 000 r范围内保持稳定.     

贝氏体钢在多次冲击接触载荷下的反常磨损行为研究

采用JD-125型冲击试验机对贝氏体钢表面进行40 h的往复冲击磨损试验,观察表层组织演变和磨损率的变化过程.结果表明:在6.0 J冲击下磨损率始终稳定不变,呈现出通常的冲击磨损规律;而在8.2 J冲击下,冲击10 h后磨损率急剧下降,甚至低于6.0 J冲击下的磨损率,材料显示出反常的磨损规律;8.2 J冲击下材料磨损表面出现大量非晶相,且出现少量纳米晶嵌于非晶基底中,使得普通的冲击磨损过程被强烈抑制,因此磨损率下降,其磨损表面形貌也较低能冲击下的表面更为平整和均匀.同时还证明只有冲击能量超过某个阈值,材料表面才能够形成非晶层.     

油润滑条件下FeBSiNb非晶涂层磨损性能研究

基于机器人自动化高速电弧喷涂技术在45^#钢基体上制备了FeBSiNb非晶涂层.研究了非晶涂层在油润滑条件下磨损时间、线速度和载荷对涂层的磨损行为.采用扫描电镜(SEM)、能谱分析仪(EDAX)和X射线衍射仪(XRD)等仪器对涂层的组织结构进行了表征,利用纳米压痕仪对涂层的微观力学性能进行了分析.结果表明:涂层具有非晶态结构,组织均匀,层状结构紧凑,具有较低的孔隙率(1.4%)和较高的显微硬度(16.42 GPa).随着磨损速度升高,载荷增加,涂层的磨损率也随之增加.FeBSiNb非晶涂层具有良好的耐磨性,同一磨损条件下,其相对耐磨性为3Cr13涂层的6倍.FeBSiNb非晶涂层的磨损机制主要为脆性疲劳剥落.     

Multi-scale constitutive modeling of the small deformations of semi-crystalline polymers

A multi-scale constitutive model for the small deformations of semi-crystalline polymers such as high density Polyethylene is presented. Each macroscopic material point is supposed to be the center of a representative volume element which is an aggregate of randomly oriented composite inclusions. Each inclusion consists of a stack of parallel crystalline lamellae with their adjacent amorphous layers.Micro-mechanically based constitutive equations are developed for each phase. A viscoplastic model is used for the crystalline lamellae. A new nonlinear viscoelastic model for the amorphous phase behavior is proposed. The model takes into account the fact that the presence of crystallites confines the amorphous phase in extremely thin layers where the concentration of chain entanglements is very high. This gives rise to a stress contribution due to elastic distortion of the chains. It is shown that the introduction of chains’ elastic distortion can explain the viscoelastic behavior of crystalline polymers. The stress contribution from elastic stretching of the tie molecules linking the neighboring lamellae is also taken into account.Next, a constitutive model for a single inclusion considered as a laminated composite is proposed. The macroscopic stress-strain behavior for the whole RVE is found via a Sachs homogenization scheme (uniform stress throughout the material is assumed).Computational algorithms are developed based on fully implicit time-discretization schemes.     

Metallization of fluid hydrogen at 140 GPa (1.4 Mbar) by shock compression

Shock compression was used to produce the first observation of a metallic state of condensed hydrogen. The conditions of metallization are a pressure of 140 GPa (1.4 Mbar), 0.6 g/cm (ninefold compression of initial liquid-H density), and 3000 K. The relatively modest temperature generated by a reverberating shock wave produced the metallic state in a warm fluid at a lower pressure than expected previously for the crystallographically ordered solid at low temperatures. The relatively large sample diameter of 25 mm permitted measurement of electrical conductivity. The experimental technique and data analysis are described. Received 12 November 1997 / Accepted 10 November 1998     

Phase transition and dynamics of iron under ramp wave compression

The ramp wave compression experiments of iron with different thicknesses were performed on the magnetically driven ramp loading device CQ-4. Numerical simulations of this process were done with Hayes multi-phase equation of state (H-MEOS) and dynamic equations of phase transition. The calculated results of H-MEOS are in good agreement with those of shock phase transition, but are different from those under ramp wave compression. The reason for this is that the bulk modulus of the material in the Hayes model and the wave velocity are considered constant. Shock compression is a jump from the initial state to the final state, and the sound speed is related to the slope of the Rayleigh line. However, ramp compression is a continuous process, and the bulk modulus is no longer a constant but a function of pressure and temperature. Based on Murnaghan equation of state, the first-order correction of the bulk modulus on pressure in the Hayes model was carried out. The numerical results of the corrected H-MEOS agree well with those of pure iron in both ramp and shock compression phase transition experiments. The calculated results show that the relaxation time of iron is about 30 ns and the phase transition pressure is about 13 GPa. There are obvious differences between the isentropic and adiabatic process in terms of pressure–specific volume and temperature–pressure. The fluctuation of the sound speed after 13 GPa is caused by the phase transition.     

A model for the effect of physical ageing on the stress relaxation behavior of high density polyethylene

The stress relaxation behavior of high density polyethylene (HDPE) can be affected by ageing processes; e.g., with increasing storage time at a low temperature following a quench from a high temperature (close to the melting point) the relaxation curves change shape. More specifically, the stress level approached after very long loading times in a stress relaxation experiment increases with the ageing time. Here this stress level is denoted the internal stress _i. Struik has pointed out that physical ageing may also occur in semicrystalline polymers like HDPE. The physical ageing should then be associated with that part of the amorphous phase which is closest to the surfaces of the crystallites. This part of the amorphous phase of HDPE can be assumed to have a restricted mobility at room temperature and may have a partially glassy character. In this paper a model for explaining the increase in _i for HDPE with increasing ageing time is proposed and discussed. It is based on the separation of the amorphous phase into two parts as suggested by Struik. The glassy part of the amorphous phase ages in a way similar to that of an entirely amorphous polymer quenched to a temperature below its glass transition, while the more rubbery phase is assumed not to undergo any physical ageing.     

溅射沉积WSx/Ni/a-C/Ni多层膜的微结构及大气环境摩擦学性能

采用磁控溅射技术在硅基体上交替沉积WS₂、Ni及非晶碳(a-C)层制备出不同Ni层厚度的WS_x/Ni/a-C/Ni多层膜. 利用扫描电子显微镜、X射线衍射仪、拉曼光谱仪、X射线光电子能谱仪等研究了多层膜的成分及微观结构；通过纳米压痕仪、划痕仪和球盘式摩擦磨损试验机等评价了薄膜的力学及大气环境中的摩擦学性能. 结果表明:随着Ni层厚度的增加，WS_x/Ni/a-C/Ni多层膜的致密度下降，n_s/n_w值(S与W原子百分数之比)由0.84降至0.73，WS_x层以微晶或非晶的形式存在. 多层膜的硬度和磨损率均随Ni层厚度的增加先降低后升高，但摩擦系数由0.22升至0.38，结合力先增大后减小. Ni层厚度为6 nm的多层膜的硬度可达13.4 GPa，抗氧化性能和耐磨性最优，磨损率仅为9.47×10–14 m3/(Nm).