纳米镁合金块体的制备及其力学性能

通过氢化-歧化-脱氢-重组,结合放电等离子体烧结以及热挤压技术,获得了平均晶粒尺寸约15 nm的纳米晶镁合金块体。纳米晶AZ91镁合金棒材的力学性能表征显示其室温拉伸强度高达267 MPa,同时也表明当晶粒尺寸小于20 nm时纳米晶镁合金具有特殊的力学行为,证明了一种反Hall-Petch关系的存在。     

纳米晶锰的制备与力学性能研究

采用电化学沉积法，在Cu基片上制备出了平均晶粒尺寸为3nm的纳米晶α-Mn通过对其在不同温度下真空退火，得到具有不同平均晶粒尺寸的纳米晶α-Mn系列样品。同时分析了电沉积、热处理条件对结构的影响，研究了结构与力学性能的关系。结果表明，随晶粒尺寸的减小，纳米晶α-Mn的力学性能表现出正Hall—Petch关系和反Hall—Petch关系；当晶粒小于临界晶粒尺寸42nm时，正Hall—Petch关系转变为反Hall—Petch关系。     

多晶体材料的晶界强化模型研究

近几十年来,材料的细晶强化研究大量开展。在一般晶粒尺寸范围内,材料的强度随晶粒尺寸的变化是符合Hall-Petch关系的,但在纳米晶体材料中出现了偏离甚至反Hall-Petch关系的现象,因此Hall-Petch关系的使用具有一定的局限性。文章从塑性变形理论出发,建立了晶界强化数学模型,通过该模型,研究了晶粒大小、晶界厚度及晶界体积百分数、晶界能、弹性模量与多晶体材料强度的关系,发现除了晶粒大小及晶界厚度影响材料强度之外,弹性模量、晶界能越大,多晶体材料的强度越高。     

偏压对钛薄膜显微组织及力学性能的影响(英文)

为了研究纳米结构金属Ti的纳米力学性能,在偏压为0~140 V的范围内,采用磁控溅射方法制备纯钛薄膜。并采用X射线衍射仪(XRD)、扫描电子显微镜(SEM)和高分辨率透射电子显微镜(HRTEM)表征钛薄膜的显微组织。结果表明:钛薄膜呈现非晶与纳米晶的混合结构,且晶化程度随着偏压的升高而增大。纳米压痕测试结果表明:钛薄膜的硬度与晶粒尺寸在6~15 nm的范围内符合Hall-Petch关系。但其Hall-Petch关系的斜率与采用其他强烈塑性变形法制备的超细晶纯钛相比,明显偏小,且呈现软化趋势。此外,讨论偏压对钛薄膜生长取向的影响。     

球磨法制备钛纳米晶

用球磨法制备出钛纳米晶粉末.结果表明,经过24h球磨后,晶粒平均尺寸达到12—13nm粉末的氧、氮含量极低、粉末的硬度与晶粒尺寸的关系为正常的Hall-Petch关系。     

退火时间对块体纳米晶Fe3Al材料组织性能的影响

在800℃下对铝热反应法制备的含10%Ni的纳米晶Fe3Al材料进行了不同时间的等温热处理,保温时间分别为4、8、12、16、20、24和48 h。利用XRD和TEM分析了不同保温时间下材料的平均晶粒尺寸,并对硬度进行了测试,研究了晶粒尺寸变化趋势以及硬度变化规律,探讨了两者之间的变化关系。结果表明:材料的平均晶粒尺寸在不同时间的退火处理后,呈现出两次减小,两次增大,最后趋于平稳的过程。晶粒尺寸在4 h等温处理后达到最小值16 nm,在24 h等温处理后达到最大值35 nm。存在一个临界值dc=20 nm,当晶粒尺寸小于dc时,维式硬度和晶粒尺寸之间满足反Hall-Petch关系,当晶粒尺寸大于dc时,两者之间呈现正的Hall-Petch关系。16 h退火处理后维氏硬度最大为490 HV,24 h退火后维氏硬度最小为410 HV。     

晶粒尺寸对γ-TiAl力学性能影响的纳米压痕研究

为研究纳米压痕过程中晶粒尺寸对γ-TiAl合金力学性能及变形行为的影响,利用Voronoi方法建立多晶γ-TiAl模型,采用分子动力学方法模拟压头压入不同晶粒尺寸模型的压痕过程,得到相应尺寸下的载荷-深度曲线,并计算了7种晶粒尺寸下γ-TiAl的硬度。结果表明：当晶粒尺寸小于9.9nm时,晶粒尺寸与硬度表现出反Hall-Petch关系,位错和晶界活动共同促使材料发生塑性变形,晶界活动起主导作用。当晶粒尺寸大于9.9nm时,晶粒尺寸与硬度符合Hall-Petch关系,晶界对材料变形影响较小,位错主导基体发生塑性变形。另外,分析了γ-TiAl在压痕过程中的应力传递和形变恢复过程,发现致密晶界网格结构能够有效抑制压痕缺陷及内应力向材料内部传递;晶粒尺寸越小,压头下方的内应力分布越均匀,沿压痕方向的弹性恢复比越小。     

晶粒尺寸对γ-TiAl合金力学性能影响的纳米压痕研究

为研究纳米压痕过程中晶粒尺寸对γ-Ti Al合金力学性能及变形行为的影响,利用Voronoi方法建立多晶γ-Ti Al模型,采用分子动力学方法模拟压头压入不同晶粒尺寸模型的压痕过程,得到相应尺寸下的载荷-深度曲线,并计算了7种晶粒尺寸下γ-Ti Al的硬度。结果表明:当晶粒尺寸小于9.9 nm时,晶粒尺寸与硬度表现出反Hall-Petch关系,位错和晶界活动共同促使材料发生塑性变形,晶界活动起主导作用。当晶粒尺寸大于9.9 nm时,晶粒尺寸与硬度符合Hall-Petch关系,晶界对材料变形影响较小,位错主导基体发生塑性变形。另外,分析了γ-Ti Al在压痕过程中的应力传递和形变恢复过程,发现致密晶界网格结构能够有效抑制压痕缺陷及内应力向材料内部传递;晶粒尺寸越小,压头下方的内应力分布越均匀,沿压痕方向的弹性恢复比越小。     

纳米晶体材料屈服应力与晶粒尺寸的依赖关系

本文把纳米晶体材料等效成由晶粒基体和晶间界面夹杂组成的复合材料，对纳米材料屈服应力特征作了详细讨论。依此得到了纳米晶体材料屈服应力偏离Ｈａｌｌ－Ｐｅｔｃｈ关系的尺寸范围，这一范围强烈地依赖于界面的性质。依据所得到的结果，解释了屈服应力随晶粒尺寸减小而降低的反常实验现象。文中还指出屈服应力对晶粒尺寸的依赖曲线可划分为线性区，非线性区，反常偏离区和不确定区四个区域，屈服应力的尺寸效应不但需要指定晶粒尺     

钛上阳极氧化生成TiO2光催化薄膜的结构与性能

研究了电化学阳极氧化在钛上制备的纳米晶多孔TiO2光催化薄膜的结构与光催化特性。将工业纯钛片或钛箔暴露于电介质溶液并加一定电压，钛表面将氧化生长多孔TiO2薄膜。适当控制氧化电压、溶液温度，得到非晶氧化膜，再进行晶化处理，得到锐钛矿相纳米晶TiO2薄膜。其晶粒尺寸约在10nm～30nm。用SEM，TEM，XRD表征TiO2薄膜的形貌与相结构。用光谱仪测定了薄膜对入射光的吸收特性，表明电化学氧化制备的纳米晶二氧化钛薄膜对近紫外入射光产生强烈的吸收，显示纳米结构的量子效应。测定了薄膜对酸性红溶液的光催化降解效率，结果表明反应30min后薄膜对酸性红的光催化降解率可达95％以上。     

The inverse Hall-Petch effect in nanocrystalline ZrN coatings

For the purpose of studying the inverse Hall-Petch effect in nanocrystalline hard coatings, nanocrystalline ZrN coatings have been fabricated using magnetron sputtering with grain sizes ranging from 45 nm to 10 nm by varying negative biases from 0 V to 150 V. The transition from the classical Hall-Petch effect to an inverse Hall-Petch effect in nanocrystalline ZrN coatings is observed at a grain size between 19.0 nm and 14.2 nm. The reality of the inverse Hall-Petch effect in the present study is validated by exclusion of other possible effects on hardness of nanocrystalline ZrN coatings, such as porosity, multiphase, chemical composition, texture, and residual stress. Furthermore, a concise model based on lattice dislocations piling up mechanism is proposed to illustrate the breakdown of the Hall-Petch effect and calculate the critical grain size. The predictions of the model fit well with experimental data in some nitride and carbide nanocrystalline coatings. Both experimental and theoretical results indicate that the inverse Hall-Petch effect is an essential property of nanocrystalline hard coatings as similar to nanocrystalline metals and alloys.     

The effect of solid solution W additions on the mechanical properties of nanocrystalline Ni

Although pure metals with grain sizes below about 10 nm are very difficult to prepare, alloying enables the realization of finer grain sizes, often down to the amorphous limit. In this work, the role of solid solution additions of ~13 at% W are considered with respect to the structure and mechanical properties of electrodeposited Ni alloys with grain sizes below 10 nm. Structure of the nanocrystalline alloys is analyzed by high-resolution transmission electron microscopy, and related to the mechanical properties assessed by instrumented nanoindentation and nano-scratch experiments. The Ni-W alloys exhibit higher hardness and scratch resistance as compared to the finest pure nanocrystalline Ni alloys, although the contribution of solid solution strengthening from W is expected to be essentially negligible. The improved properties are therefore most likely due to the finer length scale available in multicomponent nanocrystalline alloys, and suggest that alloying may suppress the breakdown of Hall-Petch strengthening to finer grain sizes. Finally, the present data are shown to smoothly bridge the hardness-grain size trend between nanocrystalline Ni (grain size>10 nm) and amorphous Ni-based alloys.     

Hardness of porous nanocrystalline Co-Ni electrodeposits

The Hall-Petch relationship can fail when the grain size is below a critical value of tens of nanometres. This occurs particularly for coatings having porous surfaces. In this study, electrodeposited nanostructured Co-Ni coatings from four different nickel electroplating baths having grain sizes in the range of 11–23 nm have been investigated. The finest grain size, approximately 11 nm, was obtained from a coating developed from the nickel sulphate bath. The Co-Ni coatings have a mixed face centred cubic and hexagonal close-packed structures with varying surface morphologies and different porosities. A cluster-pore mixture model has been proposed by considering no contribution from pores to the hardness. As the porosity effect was taken into consideration, the calculated pore-free hardness is in agreement with the ordinary Hall-Petch relationship even when the grain size is reduced to 11 nm for the Co-Ni coatings with 77±2 at% cobalt. The present model was applied to other porous nanocrystalline coatings, and the Hall-Petch relationship was maintained.     

A New Model for Inverse Hall-Petch Relation of Nanocrystalline Materials

In the present article, a new model for inverse Hall-Petch relation in nanocrystalline materials has been proposed. It is assumed that lattice distortion along grain boundaries can cause internal stresses and high internal stresses along grain boundaries can promote the grain boundary yielding. The designed model was then verified using the nanocrystalline-copper data. The minimum grain size for inverse Hall-Petch relation is determined to be about 11 nm for Cu.     

Microstructure evolution and mechanical properties of nanocrystalline FeAl obtained by mechanical alloying and cold consolidation

In the present study high energy mechanical milling followed by cold temperature pressing consolidation has been used to obtain bulk nanocrystalline FeAl alloy. Fully dense disks with homogenous microstructure were obtained and bulk material show grain size of 40 nm. Thermal stability of the bulk material is studied by XRD and DSC techniques. Subsequent annealing at a temperature up to 480 °C for 2 h of the consolidated samples enabled supersaturated Fe(Al) solid solution to precipitate out fine metastable Al₅Fe₂, Al₁₃Fe₄ and Fe₃Al intermetallic phases. Low temperature annealing is responsible for the relaxation of the disordered structure by removing defects initially introduced by severe plastic deformation. Microhardness shows an increase with grain size reduction, as expected from Hall-Petch relationship at least down to a grain size of 74 nm, then a decrease at smallest grain sizes. This could be an indication of some softening for finest nanocrystallites. The peak hardening for the bulk nanocrystalline FeAl is detected after isochronal ageing at 480 °C.     

Effect of bias voltage on microstructure and nanomechanical properties of Ti films

In order to investigate nanomechanical properties of nanostructured Ti metallic material, pure Ti films were prepared by magnetron sputtering at the bias voltage of 0-140 V. The microstructure of Ti films was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). It is interesting to find that the microstructure of pure Ti films was characterized by the composite structure of amorphous-like matrix embodied with nanocrystallines, and the crystallization was improved with the increase of bias voltage. The hardness of Ti films measured by nanoindentation tests shows a linear relationship with grain sizes in the scale of 6-15 nm. However, the pure Ti films exhibit a soft tendency characterized by a smaller slope of Hall-Petch relationship. In addition, the effect of bias voltage on the growth orientation of Ti films was discussed.     

Room temperature mechanical behavior of silicon-doped TiAl alloys with grain sizes in the nano- and submicron-range

Nano- and submicron-grained intermetallic compounds consisting of γ-TiAl and ξ-Ti₅(Si,Al)₃ were produced by high energy milling and hot isostatic pressing. Owing to the pure and controlled processing conditions, the mechanical properties may be indubitably related to the microstructure. Both yield strength and hardness show a Hall–Petch-type dependence on grain size, resulting in extremely high flow and fracture stresses under compression of up to 3 GPa. With a reduction of grain size, the coefficient of strain hardening as well as the compressive fracture strain decrease and drop to zero for alloys with grain sizes of about 150 nm. Deformation at room temperature is accomplished by dislocation glide and mechanical twinning, with twinning attaining more and more importance as the grain size is further reduced. Diffusion-controlled deformation mechanisms can be ruled out even for intermetallics with crystallite sizes as small as 50 nm. A room temperature ductilization of intermetallic compounds by switching to a nanocrystalline microstructure seems to be rather unlikely.     

碳化帆颗粒尺寸对纳米晶硬质合金组织和性能的影响

利用原位还原碳化反应制备纳米尺度的WC-Co复合粉体,应用放电等离子烧结(SPS)技术制备出纳米晶WC-Co硬质合金块体材料。分析了晶粒长大抑制剂碳化钒(VC)颗粒尺寸对纳米晶硬质合金的显微组织、晶粒尺寸及分布和力学性能的影响。结果表明:当VC的粒径减小到100 nm以下时,利用快速烧结技术可制备得到平均晶粒尺寸约为70 nm的致密WC-Co硬质合金块体材料,其物相纯净,晶粒尺寸分布均匀,维氏硬度为19.84 GPa,断裂韧性达到12.10 MPa·m1/2。     

Anomalies in Hall-Petch strengthening for nanocrystalline Au-Cu alloys below 10 nm grain size

The mechanical behavior of an electrodeposited nanocrystalline alloy is assessed with regards to the experimentally measured strain-rate sensitivity. Foils are characterized with grain sizes as small as 3 nm, a nano-scale regime that has previously gone without detailed experimental examination. It is found from micro-scratch measurements that hardness, hence strength, approaches ideal values as the grain size decreases to 7 nm. Below 7 nm, softening in strength and departure from Hall-Petch behavior is related to an increase in the activation volume for deformation as grain size decreases further.