Mechanical characterization of Mg-B_4C nanocomposite fabricated at different strain rates

Magnesium has wide application in industry. The main purpose of this investigation was to improve the properties of magnesium by reinforcing it using B_4C nanoparticles. The reinforced nanocomposites were fabricated using a powder compaction technique for 0, 1.5 vol%, 3 vol%,5 vol%, and 10 vol% of B_4C. Powder compaction was conducted using a split Hopkinson bar(SHB), drop hammer(DH), and Instron to reach different compaction loading rates. The compressive stress–strain curves of the samples were captured from quasi-static and dynamic tests carried out using an Instron and split Hopkinson pressure bar, respectively. Results revealed that, to achieve the highest improvement in ultimate strength, the contents of B_4C were 1.5 vol%, 3 vol%, and 3 vol% for Instron, DH, and SHB, respectively. These results also indicated that the effect of compaction type on the quasi-static strength of the samples was not as significant, although its effect on the dynamic strength of the samples was remarkable. The improvement in ultimate strength obtained from the quasi-static stress–strain curves of the samples(compared to pure Mg) varied from9.9% for DH to 24% for SHB. The dynamic strength of the samples was improved(with respect to pure Mg) by 73%, 116%, and 141% for the specimens compacted by Instron, DH, and SHB, respectively. The improvement in strength was believed to be due to strengthening mechanisms,friction, adiabatic heating, and shock waves.     

Pseudo-<Emphasis Type="Italic">in-situ</Emphasis> stir casting: a new method for production of aluminum matrix composites with bimodal-sized B<Subscript>4</Subscript>C reinforcement

A new method was applied to produce an Al-0.5wt%Ti-0.3wt%Zr/5vol%B₄C composite via stir casting with the aim of characterizing the microstructure of the resulting composite. For the production of the composite, large B₄C particles (larger than 75 μm) with no pre-heating were added to the stirred melt. Reflected-light microscopy, X-ray diffraction, scanning electron microscopy, field-emission scanning electron microscopy, laser particle size analysis, and image analysis using the Clemex software were performed on the cast samples for microstructural analysis and phase detection. The results revealed that as a consequence of thermal shock, B₄C particle breakage occurred in the melt. The mechanism proposed for this phenomenon is that the exerted thermal shock in combination with the low thermal shock resistance of B₄C and large size of the added B₄C particles were the three key parameters responsible for B₄C particle breakage. This breakage introduced small particles with sizes less than 10 μm and with no contamination on their surfaces into the melt. The mean particle distance measured via image analysis was approximately 60 μm. The coefficient of variation index, which was used as a measure of particle distribution homogeneity, showed some variations, indicating a relatively homogeneous distribution.     

反应键合B4C–SiC复合材料的组织和力学性能

Reaction-bonded B₄C–SiC composites are highly promising materials for numerous advanced technological applications. However, their microstructure evolution mechanism remains unclear. Herein, B₄C–SiC composites were fabricated through the Si-melt infiltration process. The influences of the sintering time and the B₄C content on the mechanical properties, microstructure, and phase evolution were investigated. X-ray diffraction results showed the presence of SiC, boron silicon, boron silicon carbide, and boron carbide. Scanning electron microscopy results showed that with the increase in the boron carbide content, the Si content decreased and the unreacted B₄C amount increased when the sintering temperature reached 1650°C and the sintering time reached 1 h. The unreacted B₄C diminished with increasing sintering time and temperature when B₄C content was lower than 35wt%. Further microstructure analysis showed a transition area between B₄C and Si, with the C content marginally higher than in the Si area. This indicates that after the silicon infiltration, the diffusion mechanism was the primary sintering mechanism of the composites. As the diffusion process progressed, the hardness increased. The maximum values of the Vickers hardness, flexural strength, and fracture toughness of the reaction-bonded B₄C–SiC ceramic composite with 12wt% B₄C content sintered at 1600°C for 0.5 h were about HV 2400, 330 MPa, and 5.2 MPa·m0.5, respectively.     

Structural design inspired by beetle elytra and its mechanical properties

On the basis of the microstructure of the cross-section of a beetle’s elytra,three bio-inspired lightweight structures were designed and built from acrylonitrile butadiene styrene plastic with a three-dimensional printer.The mechanical properties of three lightweight structures were analyzed and compared employing the finite element method,and quasi-static compression experiments and a three-point bending test on the structure samples were carried out using an electronic universal testing machine to verify the effectiveness of the finite element method.The results show that all three bio-structures were lightweight and had excellent mechanical properties.In particular,the bio-structure with spherical holes and hollow columns perpendicular to the top and bottom surfaces best imitated the microstructure of the cross-section of the Cybister elytra and had the greatest specific strength/stiffness in compression and bending.Finally,a preliminary optimization design was obtained for this bio-structure to further improve its specific strength and specific stiffness to 31.82 kN m/kg and 108.73 kN m 2 /kg respectively.     

纤维素纳米晶对胶凝复合材料耐酸性能的影响

Acid mine drainage presents an important threat to cementitious structures. This study is aimed at investigating the effect of cellulose nanocrystals (CNCs) on the acid resistance of cementitious composites. CNCs were added to mortar mixtures as additives at cement volume ratios of 0.2vol%, 0.4vol%, 1.0vol%, and 1.5vol%. After 28 d of standard curing, the samples were immersed in a sulfuric acid with a pH value of 2 for 75 d. The unconfined compressive strength (UCS) test, the density, water absorption, void volume test, and thermogravimetric analysis were carried out to investigate the properties of CNC mixtures before sulfuric acid immersion. It was found that the addition of CNC reduced the volume of permeable voids and increased the hydration degree and mechanical strength of the samples. Changes in mass and length were monitored during immersion to evaluate the acid resistance of mixtures. The mixture with 0.4vol% CNC showed a reduced mass change and length change indicating its improved acid resistance.     

Cu-Zn-Al2O3 nanocomposites:study of microstructure,corrosion,and wear properties

Alumina nanoparticles were added to a Cu-Zn alloy to investigate their effect on the microstructural, tribological, and corrosion properties of the prepared alloys. Alloying was performed using a mixture of copper and zinc powders with 0vol% and 5vol% of α-Al nanopowder in a satellite ball mill. The results showed that the Cu-Zn solid solution formed after 18 h of mechanical alloying. The mechanically alloyed powder was compacted followed by sintering of the obtained green compacts at 750℃ for 30 min. Alumina nanoparticles were uniformly distributed in the matrix of the Cu-Zn alloy. The tribological properties were evaluated by pin-on-disk wear tests, which revealed that, upon the addition of alumina nanoparticles, the coefficient of friction and the wear rate were reduced to 20% and 40%, respectively. The corrosion properties of the samples exposed to a 3.5wt% NaCl solution were studied using the immersion and potentiodynamic polarization methods, which revealed that the addition of alumina nanoparticles reduced the corrosion current of the nanocomposite by 90%.     

Cu-Zn-Al_2O_3 nanocomposites:study of microstructure,corrosion,and wear properties

Alumina nanoparticles were added to a Cu-Zn alloy to investigate their effect on the microstructural,tribological,and corrosion properties of the prepared alloys. Alloying was performed using a mixture of copper and zinc powders with 0vol% and 5vol% of α-Al nanopowder in a satellite ball mill. The results showed that the Cu-Zn solid solution formed after 18 h of mechanical alloying. The mechanically alloyed powder was compacted followed by sintering of the obtained green compacts at 750°C for 30 min. Alumina nanoparticles were uniformly distributed in the matrix of the Cu-Zn alloy. The tribological properties were evaluated by pin-on-disk wear tests,which revealed that,upon the addition of alumina nanoparticles,the coefficient of friction and the wear rate were reduced to 20% and 40%,respectively. The corrosion properties of the samples exposed to a 3.5wt% Na Cl solution were studied using the immersion and potentiodynamic polarization methods,which revealed that the addition of alumina nanoparticles reduced the corrosion current of the nanocomposite by 90%.     

Production and characterization of high porosity porous Fe-Cr-C alloys by the space holder leaching technique

Spherical carbamide particles were employed to produce porous Fe-Cr-C alloy with high porosity and large aperture via the space-holder leaching technique. A series of porous samples were prepared by regulating the processing parameters, which included the carbamide content and the compaction pressure. The pore characteristics and compression properties of the produced samples were investigated. The samples were characterized by scanning electron microscopy, image analysis, and compression tests. The results showed that the macro-porosity and the mean pore size were in the ranges 40.4%–82.4% and 0.6–1.5 mm, respectively. The compressive strength varied between 25.38 MPa and 127.9 MPa, and was observed to decrease with increasing total porosity.     

Preparation of aluminum borate whisker reinforced aluminum phosphate wave-transparent materials

Aluminum borate whisker/aluminum phosphates composites were prepared with normal pressure sintering, It was found that the aluminum borate whisker was effective to suppress the crystallization of aluminum phosphates in the aluminum borate whisker/aluminum phosphates composites and contributed to the improvement of the mechanical properties. After sintered at 1050℃ for 1 h, the flexural strength of the composites with 30 vol% aluminum borate whisker addition reached 215.3 MPa. The dielectric constant and loss tangent of the composites were 4.23 and 0.0024 at 10 GHz.     

利用回收磷青铜片经热挤压法制备的磷青铜/铝两相材料及其组织和力学性能的研究

Despite the existence of conventional methods for recycling chips, solid-state techniques have become popular, whereby waste metals are directly recycled into consolidated products with the desired shapes and designs. We investigated the feasibility of recycling phosphor bronze chips through a hot extrusion process using aluminum powder as a metal binder for the fabrication of a metal-fiber-reinforced aluminum matrix composite. To do so, mixtures containing 20vol%–50vol% of chips were prepared, cold-compacted, and extruded. The quality of the consolidated samples was evaluated by determining the density of the fabricated composites and studying their microstructures. In addition, we performed tensile and hardness tests to evaluate the mechanical properties of the fabricated composites. We also analyzed the fracture surfaces of the samples to study the fracture mechanism as a function of the volume fraction of phosphor bronze chips in the fabricated composite. The results indicated that the most effective consolidation occurred in the sample containing 20vol% of chips extruded at 465°C in which the chips serve as ideal fibers for improving the mechanical properties, especially the ultimate tensile strength, in comparison with those of Al matrixes that contain no chips but are produced under the same conditions.     

高应变率扭转SHB实验技术

介绍自制分离式霍布金生扭杆试验设备；详细推导出按扭转ＳＨＢ方法测量材料的动态剪应力和剪应变关系的基本方程；对软铝进行高应变率试验得到动态剪应力和剪应变关系曲线。     

水泥砂浆应变率效应研究

为分析试件长径比和应变率对水泥砂浆动态力学性能的影响规律,采用准静态实验测量了砂浆的单轴压缩强度、弹性模量以及泊松比.利用分离式SHPB压杆进行1维应力压缩实验,对水泥砂浆试件进行了回收和比较,获得了不同长径比水泥砂浆试样在不同加载速率下的动态损伤破坏过程和应力应变曲线,研究了动态强度增强因子随应变率的变化规律.结果表明:动态压缩强度随应变率的增加而显著增大;试件直径相同时,厚度小的获得的动态强度增强因子要小于厚度大的;随着应变率提高,试件损伤增加,碎块尺寸越小,表明破坏能量随应变率的增加而提高.      

等温处理对新型高强钢力学性能和绝热剪切行为的影响

为了使新型高强钢更好地在冲击领域得到应用,采用等温盐浴方法对新型高强度钢进行热处理.通过OM、SEM、TEM对材料的微观组织进行观察,采用万能试验机对材料进行准静态拉伸力学性能测试,通过分离式霍普金森压杆（SHPB）对材料进行动态性能测试并捕捉临界应变率下萌生发展的绝热剪切带形貌.研究结果表明:随着等温温度的升高,对应材料的主要组织由马氏体+下贝氏体,马氏体+下贝氏体+上贝氏体变化为马氏体+上贝氏体,材料的屈服强度和塑性逐渐降低.330℃等温处理的材料绝热剪切带萌生的临界应变率为3种等温处理材料中最低,上贝氏体组织的出现使材料对绝热剪切变形的敏感性降低.      

试件尺寸对金属材料Hopkinson拉杆实验结果的影响研究

分离式Hopkinson拉杆是当前进行材料高应变率加载条件下力学行为的主要研究手段.受加载条件的限制，往往采用小尺寸的试件开展实验.因此，试件在加工过程中的加工硬化对实验结果会产生一定的影响，从而不能完全反映真实材料的力学性能.为研究这一效应的影响，首先利用微纳米压痕法测量了小尺寸不锈钢试件沿表面厚度方向的维氏硬度变化规律，得到了加工硬化层的厚度及其对材料性能的影响.在此基础上，通过理论分析研究了加工硬化对试件拉伸试验结果的影响，并结合不同尺寸试件的准静态拉伸试验结果，给出了分离式Hopkinson拉杆动态拉伸结果的修正方法.     

泡沫铝的动态力学性能研究

粉末冶金法是制备泡沫铝的一种操作简便、实用性强的工艺。通过调整造孔剂含量、粒径、加热温度及时间等手段 ,可控制其细观结构 ,获得孔洞分布均匀的泡沫铝样品。采用 Hopkison压杆测试技术 ,对泡沫铝试样的动态压缩性能进行测试和分析。结果表明 ,泡沫铝的平台强度取决于应变速率的高低和相对密度的大小。泡沫铝材料具有很好的吸能缓冲性能。     

超高韧性水泥基复合材料与活性粉末混凝土动态性能研究

研究超高韧性水泥基复合材料（UHTCC）及活性粉末混凝土（RPC）在不同冲击荷载下的动态劈裂强度、拉压比、能量吸收量以及破坏状态.试验采用Ф80 mm的分离式霍普金森压杆以5组不同的冲击速度,打击UHTCC和RPC的平台巴西圆盘试样.研究结果表明：两组材料的动态劈裂强度随着应变率的上升而上升,具有明显的应变率敏感性,RPC的动态劈裂强度约为UHTCC的1.35倍;不同冲击荷载下两组材料的动态能量吸收量相近.     

研究混凝土材料动态力学性能的实验技术

采用波形整形技术、万向头技术和应变直接测量技术测量了混凝土材料的动态应力-应变曲线;通过损伤"冻结"试验及相应的MTS静态试验研究了混凝土材料的损伤演化规律;利用Hopkinson压杆技术直接测得层裂信号,进而可确定混凝土材料的层裂强度.     

动态劈裂实验的数值模拟及讨论

利用分离式霍普金森压杆(SHPB)可以进行混凝土类材料高应变率下的劈裂实验,目前公认的方法是通过透射波信号得到劈裂强度和相应的平均应变率.文章通过数值模拟对这种方法的有效性进行了讨论,发现在高应变率下,混凝土劈裂不满足中心开裂条件.因此上述基于弹性静力学平面问题的传统处理方法会产生一定的误差.文章还分析了不同试样尺寸对动态劈裂实验精度的影响.     

退火及化学钢化硅酸盐玻璃的动态弯曲力学行为研究

利用电子万能试验机和改进的分离式Hopkinson压杆装置分别对退火硅酸盐玻璃及化学钢化硅酸盐玻璃试样进行了加载速率为1.67×10-6 m/s和2.5 m/s的三点弯曲实验,动态加载实验中验证了杆端的加载载荷平衡,确保实验结果的有效性,同时实验过程中利用高速摄像机记录了玻璃试样的动态破坏过程.实验结果表明:由于表面钢化层预压应力的存在,化学钢化玻璃弯曲强度较退火玻璃有较大提高,且强度分散性降低.随着加载速率的提高,玻璃试样的弯曲强度有明显提高,强度分散性有所下降,动态加载条件下试样表现出多裂纹成核扩展的破坏模式.      

高强混凝土SHPB试验研究

采用变截面大尺寸的Ｈｏｐｋｉｎｓｏｎ压杆,对直径分别为７２ｍｍ,６２ｍｍ的高强混凝土试件进行了冲击压缩（ＳＨＰＢ）试验,得到了不同应变率下的混凝土动态压缩强度及应力－应变全过程曲线,证实了混凝土材料的应变速率敏感性,结果对于计算高强混凝土在高速冲击及爆炸条件下的响应很有意义。