纤维状WC晶粒硬质合金的制备

以高能球磨方法处理制备的纳米晶复合粉末为原料，通过真空烧结制备硬质合金块体，研究该纳米晶复合粉末的烧结致密化行为和显微结构特征。结果表明：该纳米晶粉末的烧结致密化可依烧结温度从低至高分为、3个阶段，而在高于1250℃的液相烧结阶段，将温度提高至1375℃烧结30min，可获得密度为14．46g／cm^3、烧结收缩率为27．2％的致密硬质合金。此时，WC晶粒呈纤维状，随机分布在烧结体中，其长度约为1．2μm，径向尺寸约为100nm；采用高能球磨处理工艺可以获得原位生成的纤维状WC晶粒增强的硬质合金。     

纳米晶Al-Cu-Mg合金的微观组织与力学性能研究

以工程实际中应用较广的Al-Cu-Mg铝合金作为研究对象, 用Al-Cu-Mg铝合金气体雾化粉末作为原材料, 通过低温液氮球磨获得纳米晶后, 再经真空热压和热挤压制备了致密的大块体纳米材料. 通过力学性能测试, 挤压态的纳米晶Al-Cu-Mg块体材料抗拉强度达470 MPa, 经过T4处理后, 抗拉强度达到590 MPa, 远远超过常规方法制备的Al-Cu-Mg铝合金抗拉强度. 对纳米晶Al-Cu-Mg块体材料进行微观组织观察, 分析了材料强度提高的原因.     

机械合金化纳米晶Cu-5% Cr粉末的热静液挤压研究

采用机械合金化制备了纳米晶Cu-5%(质量分数,下同)Cr粉末,然后对其进行了热压制坯和热静液挤压致密化研究。结果表明,经10h高能球磨,Cu-5%Cr粉末中Cu的晶粒尺寸细化到约50nm,成为纳米晶粉末。采用热压制坯和热静液挤压工艺可以使纳米晶Cu-5%Cr粉末接近完全致密化。球磨10h的Cu-5%Cr合金粉末经400℃热压制坯和600℃、挤压比为4的热静液挤压后相对密度达到99.3%。热静液挤压致密化后的Cu-5%Cr合金的晶粒有所长大,Cu基体的平均晶粒尺寸达到了500nm左右,变成了亚微米晶材料。该亚微米晶Cu-5%Cr合金具有较好的性能。     

纳米稀土硬质合金原料粉末的制备及研究

采用高能球磨法制备出了用于生产纳米晶稀土硬质合金的原料粉末。通过XRD、SEM和DTA等分析检测手段，研究了该纳米WC—Co—RE粉末的结构、形貌和相的变化。结果表明：高能球磨45h，可获得晶粒尺寸约为8．45mm的WC—Co—RE粉末；微量稀土的加入，有利于粉末晶粒的细化；在25～45h范围内，随着高能球磨时间的延长，粉末晶粒尺寸的减小趋势符合直线变化规律，且掺稀土粉末的晶粒尺寸比未掺稀土粉末的晶粒尺寸减小一半；高能球磨25h，粉末中Co相的X射线衍射峰消失。高能球磨ⅥE—Co—RE粉末的DTA曲线在597℃出现了一个尖锐的放热峰。高能球磨WC—Co—RE粉末固结之后，所制得合金的晶粒细小且机械性能较好。     

热机械合金化法制备纳米晶W-Cu复合粉末

采用热机械合金化制备纳米晶W-Cu复合粉末。通过XRD、SEM、激光粒度测试等方法对球磨后的粉末进行表征。结果表明:随球磨时间延长,W的晶粒尺寸不断减小,球磨30 h后W的平均晶粒尺寸为41 nm左右;球磨初期,粉末迅速细化;随球磨时间延长,粉末粒度有所增加;进一步增加球磨时间,粉末粒度减小。球磨粉末还原后有较高的烧结活性,1 200℃烧结后相对密度可达97%以上。烧结材料的组织非常均匀,且晶粒细小。     

机械球磨法制备纳米晶复相Nd2Fe14B/α-Fe永磁粉末

通过将成分为Nd2Fe14B（原子比）的铸态合金与羰基铁粉的混合粉末进行搅拌式机械球磨，并对球磨后的合金粉末进行真空晶化处理，制备了纳米复相Nd2Fe14B/α-Fe永磁合金。通过X射线衍射（XRD）、差示扫描量热法（DSC）、透射电子显微镜（TEM）等分析方法研究了球磨时间及晶化处理温度对合金微观组织的影响。结果表明，随球磨时间的延长，Nd2Fe14相及α-Fe的晶粒尺寸迅速减小，球磨5h后粉末由Nd2Fe14B非晶相和晶粒尺寸约为10nm左右的α-Fe组成，在随后的晶化热处理过程中转变成Nd2Fe14B／α-Fe纳米复相组织。     

高能球磨合成纳米WC-Co复合粉末的特性

采用变转速多次循环高能球磨工艺在32min制备出了平均晶粒尺寸约为25nm的纳米WC-10CO-0．8VC-0．2Cr3C2(重量分数)复合粉末，并用化学元素分析、XRD，TEM，DTA对纳米WC—Co复合粉末的特性进行了表征和分析。结果表明，变转速多次循环高能球磨工艺制备的纳米WC—CO复合粉末，化学成分合格，杂质含量低，球磨效率高；球磨过程是一个晶粒逐渐细化的过程，同时也是一个晶格畸变逐渐增加、粉末体系能量逐渐增大的过程；球磨得到的WC-Co纳米复合粉末颗粒形貌基本为球形，粒径分布较宽，颗粒中存在着一些团聚体，平均颗粒尺寸约为50nm；纳米WC-10Co-0．8VC-0．2Cr3C2(wt％)复合粉末的共晶点约为1280℃。纳米复合粉末中W，Co，V，Cr元素分布均匀弥散。     

高强AZ61Mg?18%Ti复合材料的制备及力学性能

通过机械球磨、真空热压和热挤压制备了AZ61Mg?18%Ti（质量分数）复合材料,研究了复合材料的微观组织、室温力学性能和强化机制。结果表明,采用真空热压+热挤压制备的复合材料镁基体平均晶粒尺寸为180 nm,Ti颗粒及纳米级Ti₃Al相弥散分布于镁基体中,Ti颗粒和Ti₃Al相平均尺寸分别为265 nm和10 nm。超细晶AZ61Mg?18%Ti复合材料具有优异的室温力学性能,其屈服强度、抗压强度和断裂应变分别达到606 MPa、698 MPa和12%。     

高能球磨制备纳米晶镁合金粉末的研究

利用氩气保护下的高能球磨,制备了纳米晶AZ31镁合金粉末。采用X射线衍射(XRD)、扫描电镜(SEM)和透射电镜(TEM)等方法,研究了高能球磨过程中粉末微观组织与形貌演变规律。结果表明:随着球磨时间的延长,镁合金粉末的晶粒尺寸逐渐减小,微观应变和晶格常数逐渐增大;粉末颗粒首先被碾压成扁平状并相互焊合使颗粒尺寸粗化,然后随球磨的继续进行发生断裂,使颗粒尺寸逐渐减小;球磨80h后,粉末组织与形貌均趋于稳定,获得了平均颗粒尺寸为15～20μm、晶粒尺寸为85nm左右的纳米晶AZ31镁合金粉末。     

超细晶钛铝基合金的高温压缩性能研究

采用高能球磨及真空热压烧结的方法制备超细晶/纳米晶双相γ-TiAl基合金,将名义成分为Ti-45Al-7Nb(%,原子分数)的混合粉末经40 h高能球磨后,粉末达到纳米级。球磨后的混合粉末经真空热压烧结(烧结温度1200℃,压力30 MPa,保温保压1 h)。研究该合金在温度为1000,1050和1100℃,应变速率为1×10^-4,1×10^-3和1×10^-2s^-1 3个变形速率条件下的高温压缩组织、流变行为和本构关系。研究结果表明：经过高能球磨及真空热压烧结原位合成的组织为超细晶α₂-Ti₃Al及γ-TiAl双相等轴状合金组织,晶粒尺寸小于5μm。合金为热敏感型和应变速率敏感型合金,合金压缩流变应力随应变速率的降低和温度的升高而下降。高温热压缩时,合金组织由规整等轴状被压变形为长条形,形变主要发生在γ-TiAl相中,晶界和γ相晶内可见位错及孪晶,孪晶及位错为主要的形变机制。在1000,1050和1100℃,1×10^-4,1×10     

Bulk nanocrystalline aluminum 5083 alloy fabricated by a novel technique: Cryomilling and spark plasma sintering

Dense, bulk nanocrystalline aluminum 5083 alloy was fabricatedvia a combined technique: cryomilling (mechanical milling at cryogenic temperature) to achieve the nanocrystalline Al 5083 powder and spark plasma sintering (SPS) to consolidate the cryomilled powder. The results of X-ray diffraction analysis indicate that the average grain size in the SPS consolidated material is 51 nm, one of the smallest grain sizes ever reported in bulk Al alloys produced by powder metallurgy derived methods. In contrast, transmission electron microscopy (TEM) analysis revealed a bimodal grain size distribution, with an average grain size of 47 nm in the fine-grained regions and approximately 300 nm in the coarse-grained regions. Nanoindentation was used to evaluate the mechanical properties and the uniformity of the consolidated nanocrystalline Al 5083. The hardness of the material is greatly improved over that of the conventional equivalent, due to the fine grain size. The mechanisms for spark plasma sintering and the microstructural evolution are discussed on the basis of the experimental findings.     

Mechanical behavior and microstructure of a thermally stable bulk nanostructured Al alloy

A commercial aluminum alloy, 5083, was processed using a cryomilling synthesis approach to produce powders with a nanostructured grain size. The powders were subsequently degassed, hot isostatically pressed, and extruded. The grain size at each processing step was measured utilizing both X-ray diffraction and transmission electron microscopy (TEM). The mechanical properties of the n-5083 extruded material were determined utilizing ASTM E8-93, Standard Test Methods for Tension Testing of Metallic Materials. This processing technique was found to produce a thermally stable nanostructured aluminum alloy which maintained an average grain size of 30 to 35 nm through several processing steps up to 0.61 T _mp . The thermal stability was attributed to Zener pinning of the grain boundaries by AIN and Al₂O₃ particles and solute drag of numerous atomic species. The nanostructured 5083 was found to have a 30 pct increase in yield strength and ultimate strength over the strongest commercially available form of 5083, with no corresponding decrease in elongation. The enhanced ductility is attributed to the presence of a few large, single-crystal aluminum grains acting as crack-blunting objects.     

Thermal stability and recrystallization of nanocrystalline Ti produced by cryogenic milling

The grain growth, thermal stability, and recrystallization behavior of a cryomilled Ti alloy with a grain size of about 21.2 nm were examined using differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. Isochronal heat treatments at different temperatures were applied to study the thermal stability and recrystallization behavior of this alloy system. The average grain size increased from 20 to 80 nm in the temperature range of 200 °C to 350 °C, and then significantly decreased to 15 nm during annealing at 400 °C to 450 °C. This phenomenon was rationalized on the basis of a recrystallization mechanism. When the annealing temperature increased from 450 °C to 720 °C, the grain size increased slightly from 15.2 to 27.5 nm. In addition, the isothermal grain growth behavior in this alloy was investigated in the temperature range of 150 °C to 720 °C, and the resulting grain growth activation energy was analyzed to rationalize the underlying grain growth mechanisms. An interesting scientific question that arises from the present work is whether a decrease in grain size can be obtained in nanocrystalline (nc) materialsvia a recrystallization mechanism. The present results show that indeed a smaller grain size is obtained after annealing at elevated temperatures (500 °C to 720 °C) in cryomilled nc Ti, and the experimental results are explained on the basis of a recrystallization mechanism.     

机械合金化Mg-LaNi5合金的研究

采用X射线衍射(XRD)、扫描电子显微镜(SEM)、差热分析(DTA)等测试方法研究了新型材料Mg-69％LaNi5(质量分数)的组织形貌及热稳定性能等。结果表明：该合金在转速为280r／min的条件下球磨250h后形成了短程有序或无序的镧、镁、镍等非晶及MgNi2纳米晶(3nm)组织；所得样品的颗粒形状主要为规则的球形或近球形，还有少量多角形等不规则形状。球磨样品在763K温度下保温35d，得到热稳定性较好的具有纳米尺度(2．5nm)的Mg2NiLa，Mg2Ni，MgNi2三相合金。     

Microstructural evolution and deformation of cryomilled nanocrystalline Al-Ti-Cu Alloy

The microstructural evolution during processing and tensile deformation of a nanocrystalline Al-Ti-Cu alloy was investigated using transmission and scanning electron microscopy. Grain refinement was achieved by cryomilling of elemental powders, and powders were consolidated by hot isostatic pressing (“HIPing”) followed by extrusion to produce bulk nanocrystalline Al-Ti-Cu alloys. In an effort to enhance ductility and toughness of nanocrystalline metals, multiscale structures were produced that consisted of nanocrystalline grains and elongated coarse-grain (CG) bands of pure aluminum. Examination of bulk tensile fracture samples revealed unusual failure mechanisms and interactions between the CG bands and nanocrystalline regions. The ductile CG bands underwent extensive plastic deformation prior to fracture, while nanocrystalline regions exhibited nucleation and growth of voids and microcracks. Cracks tended to propagate from nanocrystalline regions to the CG bands, where they were effectively arrested by a combination of crack blunting and crack bridging. These processes were instrumental in enhancing the toughness and ductility of the nanocrystalline alloy.     

Synthesis of nanocrystalline Zn-22 Pct Al using cryomilling

In the present investigation, the synthesis of nanocrystalline Zn-22 pct Al by ball milling was studied. The microstructural evolution during cryomilling and subsequent annealing was characterized using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray diffraction (XRD). Observations made during the cryomilling of the alloy reveal three findings. First, minimum average grain sizes of about 33 nm for the Al phase and 41 nm for the Zn phase are reached as cryomilling time increases to 16 hours. Second, the morphology of the powders changes from spherical (as-sprayed) to flaky (milled 8 hours) and then back to spherical again (milled 16 hours). Third, the microstructure transforms from two-phase coarse structure (0.8 μm, as-sprayed) to bimodal structure (milled 8 hours) and then to a uniform fine-grained structure (milled 16 hours). The minimum grain size characterized by the peak broadening of the XRD patterns is much larger than that reported in previous work on Al and Zn but agrees well with those predicted from the approximate linear relationship between the minimum grain size and the critical equilibrium distance between two edge dislocations in a pileup. The mechanism of grain size refinement is discussed at three different levels: macroscopic level (individual powders), mesoscopic level (individual small fragments), and microscopic level (individual grains). The excellent thermal stability of the milled powders during subsequent annealing has been attributed to three factors: the nature of the eutectoid structure, grain-boundary pining by dispersions, and microporosity in the particles.     

Grain growth behavior of cryomilled INCONEL 625 powder during isothermal heat treatment

Nanocrystalline INCONEL 625 powders were fabricated via cryomilling (mechanical alloying under a liquid nitrogen environment), and their grain growth behavior during isothermal heat treatment was investigated in detail. The grain size after milling for 8 hours was approximately 22 nm, measured by transmission electron microscopy (TEM) observations and X-ray diffraction (XRD). Along with this refined structure, the NiO and Cr₂O₃ oxide particles were distributed in the cryomilled material with average size of 3 nm. Following heat treatment at 800 °C, correspond to T/T _m = 0.65, for 4 hours, the grain size was approximately 240 nm, which represents an improved grain stability compared to that of conventional INCONEL 625 and cryomilled pure Ni. The improved grain stability of cryomilled INCONEL 625 is originated from a particle pinning effect by the oxide particles in addition to solute drag. The grain stability of the cryomilled powders at 900 °C was better than that at lower temperatures. This behavior was attributed to the formation of two types of secondary particles that precipitated at this temperature, which were identified as spherical NbC carbides and cylindrical-shaped Ni₃Nb intermetallic precipitates. These precipitates promote grain growth resistance at this particular temperature via a grain-boundary pinning effect. Contribution of 30 pct Nb solute atoms in alloy on the forming precipitates on grain boundary, the grain growth will be restricted to approximately 200 nm, on the basis of a Zener mechanism. This calculation is in qualitative agreement with the experimental results. The observation that precipitation kinetics were accelerated over those of conventional INCONEL 625 was rationalized on the basis of the shortened diffusion paths and more nucleation sites available in the nanocrystalline materials.     

Thermal expansion behaviors in nanocrystalline materials with a wide grain size range

Thermal expansion behaviors of nanocrystalline (NC) Ni-P alloy samples with grain sizes ranging from a few to 127 nm were studied experimentally using thermal mechanical analysis. Porosity-free NC Ni-P samples with a wide grain size range were synthesized by completely crystallizing amorphous Ni-P alloy at different annealing temperatures. Measurements showed that the linear thermal expansion coefficient (α_L) increases markedly with a reduction of the average grain size in the as-crystallized NC Ni-P samples. The thermal expansion coefficient was also found to decrease during grain growth in the as-crystallized NC sample upon annealing. From the grain size dependence of α_L in these NC samples, we deduced that the difference in thermal expansion coefficients between the interfaces and the nm-sized crystallites diminishes when the grain size becomes smaller. This tendency agrees well with other experimental results on the structural characteristics of the interfaces and the nm-sized crystallites in NC materials.     

初始晶粒尺寸对2024铝合金热变形行为和组织的影响

利用永磁搅拌近液相线铸造和普通铸造方法制备不同晶粒尺寸的2024铝合金铸锭,利用Gleeble-1500热模拟试验机研究初始晶粒尺寸对不同压缩变形条件下2024铝合金的热变形行为和变形后显微组织的影响。研究表明:2024铝合金的热变形行为依赖于变形条件和初始组织。初始晶粒尺寸对流变应力的影响是:当应变速率小于0.1 s~(-1)时,流变应力随晶粒尺寸减小而减少;当应变速率为10 s~(-1)时,流变应力随晶粒尺寸减小而增大。降低变形温度会弱化晶粒尺寸对流变应力的影响。热压缩流变应力随应变速率增大而增大,随变形温度升高而减小。应变速率为10 s~(-1)时,热压缩应力应变曲线呈现周期性波动;只在粗晶2024铝合金中发现变形剪切带。     

Ti-Y2O3 Composites with Nanocrystalline and Microcrystalline Matrix

Mechanical milling of a Ti-2 pct Y₂O₃ powders mixture led to the synthesizing of a composite powder with a nanocrystalline Ti matrix having a mean crystallite size of 19 nm. Both the nanocomposite powder prepared through milling and the initial mixture of powders were consolidated by hot pressing under the pressure of 7.7 GPa at the temperature of 1273 K (1000 °C). The transmission electron microscopy (TEM) investigations of the bulk sample produced from milled powder revealed that Y₂O₃ equiaxial particles of less than 30 nm in size are distributed uniformly in the Ti matrix with a grain size in the wide range from 50 nm to 200 nm. The microhardness of the produced nanocrystalline material is 655 HV0.2, and it significantly exceeds the hardness of the microcrystalline material (the consolidated initial mixture of powders), which is equal to 273 HV0.2. This finding confirms that reducing the grain size to the nanometric level can have a beneficial influence on the hardness of titanium alloys. Dispersion hardening also contributes to the hardness increase.