The oxidation of nanocrystalline Ni3Al fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline Ni₃Al was fabricated through mechanical alloying of elemental powders and spark plasma sintering. The nanocrystalline Ni₃Al has a nearly full density after being sintered at 1223 K for 10 min under a pressure of 65 MPa. Isothermal and cyclic oxidations of nanocrystalline Ni₃Al were tested at 1173–1373 K with intervals of 100 K. The results indicate that nanocrystalline Ni₃Al exhibits excellent isothermal and cyclical oxidation resistance. The oxide scales consist primarily of dense and continuous -Al₂O₃. The grain refinement is beneficial for improving the oxidation resistance of Ni₃Al by providing more nucleation centers for the Al₂O₃ formation, promoting the selective formation of Al₂O₃ and improving the adhesion of oxide scales to the matrix.     

Ni/Al Intermetallics Plasma Transferred Arc Processing

TAILORING COMPONENT SUFACES for elevatedtemperature operations has been a goal of researchersduring the last century.Process and material selectionare the key factors for this development.In particularlythe latter can be described to have started with thedevelopment of stainless steels.Ni and Co based alloys,the superalloys,attended the increasing technologicaldemands for higher service temperatures(1).Morerecently the higher operating temperatures ofequipments,such as gas turbines,r…     

L12型Al3Sc基金属间化合物有序化行为和力学性能的第一性原理计算

本文采用亚晶格模型,辅助以第一性原理总能计算,研究了L1₂型Al₃Sc基金属间化合物中元素的占位有序化行为和力学性能。结果表明：Al₃Sc合金呈现完全有序化,其中Al占据3c亚晶格位置,Sc占据1a亚晶格位置；L1₂-Al₃(Sc_0.75M_0.25)金属间化合物(M=Y、Ti、Zr和Hf)也呈现完全有序化,第三组元M均只占据1a亚晶格位置,这些元素的占位行为均不受温度的影响。L1₂-Al₃(Sc_0.75M_0.25)金属间化合物均满足力学稳定性条件。M为Y时,L1₂-Al₃(Sc_0.75M_0.25)金属间化合物的剪切模量、体弹模量和杨氏模量和硬度下降；M为Ti、Zr或Hf时,随着原子半径增大,剪切模量、体弹模量、杨氏模量和硬度逐渐降低,其中Ti的加入可使L1₂-Al₃(Sc_0.75M_0.25)金属间化合物的塑性和韧性达到最好。     

Thermodynamic analysis and assessment of the Ce–Ni system

An optimised set of thermodynamic parameters for the Ce–Ni system has been obtained using the CALPHAD approach. A thorough thermodynamic analysis of the system has been carried out using different calorimetric techniques and the data have been used in the assessment. The free energy of the liquid phase has been described as a function of temperature and composition using a Redlich–Kister polynomial. Solid compounds have been considered as stoichiometric with the exception of the Laves phases. The phase diagram and thermodynamic quantities calculated from assessed parameters agree well with experimental data.     

Mg,Ca,Y,La和Ce在Ni3Al中的合金化行为

利用Ｘ射线衍射法确定了合金元素Ｍｇ，Ｃａ，Ｙ，Ｌａ及Ｃｅ在Ｎｉ３Ａｌ中的合金化行为。结果表明，Ｍｇ，Ｃａ，Ｙ，Ｌａ及Ｃｅ原子占据Ｎｉ３Ａｌ中的Ａｌ位，并按以下顺序增加Ｎｉ３Ａｌ的有序度：Ｍｇ，Ｃａ，Ｙ和Ｌａ－Ｃｅ。随着合金元素含量的增加，Ｎｉ３Ａｌ的点阵常数亦按Ｍｇ，Ｃａ，Ｙ，Ｌａ－Ｃｅ的顺序呈线性增大。     

Reactive Structural Materials: Preparation and Characterization

Reactive structural materials, which can serve both as structural elements as well as a source of chemical energy released upon initiation have emerged as an important class of metal‐based composites for use in various energetic systems. Such materials rely on a variety of exothermic reactions, from oxidation to formation of metal‐metalloid and intermetallic phases. The rates of these reactions are as important as the energy that may be released, in order for them to occur at the time scales compatible with the requirements of applications. Therefore, chemical composition, scale at which reactive components are mixed, and the structure and morphology of materials are important and can be controlled by the method of preparation and compaction of the composite materials. Methods of preparation of the composite structures are briefly reviewed as well as methods of characterization of their mechanical and energetic properties. In addition to common thermo‐analytical and static mechanical property measurements, dynamic tests of mechanical properties as well as ignition and combustion experiments are necessary to understand the fragmentation, initiation, and heat release expected for these materials when they are stimulated by an impact, shock, or rapid heating. Reaction mechanisms are studied presently for the thin layers and small samples of reactive materials initiated in carefully designed experiments. In other experiments, impact and explosive initiation are characterized for larger material compacts in the conditions imitating practical scenarios. Examples of results describing thermal, impact, and explosive initiation of some of the reactive materials are presented.

     

Using Selective Electron Beam Melting to Enhance the High-Temperature Strength and Creep Resistance of NiAl–28Cr–6Mo In Situ Composites

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF-EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate-like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF-EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material.     

The isothermal section of the La–Si–Mg system at 500 C

The isothermal section of the La–Si–Mg system at 500 ^°C was constructed in the whole concentration range by means of the SEM-EDXS and XRPD characterization of about forty alloys prepared by induction melting and then annealed. Phase equilibria are characterized by the following ternary phases: τ₁-La_2+xSi₂Mg_1−x (0≤x≤0.35,tP10-Mo₂FeB₂), τ₂-LaSi₂Mg₂ (tP5-CeSi₂Mg₂), τ₃-LaSi₂Mg (structure still unknown) and τ₄-La₆SiMg₂₃ (cF120-Zr₆SiZn₂₃). The high temperature binary phase LaMg₂ (cF24-MgCu₂) has been found to be stabilized at 500 ^°C probably by a small amount of Si. Phases in binary subsystems do not generally form extended ternary solid solutions except for (La_1−xMg_x)₃Si₂ (0≤x≤0.167,tP10-U₃Si₂). Crystal structures of phases τ₁-La_2+xSi₂Mg_1−x and (La_1−xMg_x)₃Si₂ are correlated, the former being a substitution derivative of the latter.     

多相NiAl-Cr合金的微观组织和韧脆转变行为的研究

研究了NiAl-25Cr合金微观组织和韧脆转变行为.NiAl-25Cr合金的铸态组织为β-Ni(Al,Cr),γ'-Ni3(Al,Cr)和α-Cr三相组成,挤压后沿着挤压方向共晶区被显著拉长、变直,共晶区之间并列平行排列.经挤压变形后材料为完全再结晶的细小等轴晶,晶粒大小约3～5μm.韧脆转变温度明显依赖于应变速率,应变速率提高2个数量级,韧脆转变温度(BDTT)相应提高80K.在BDTT以下,断口形貌以β/γ'解理为主,而在BDTT以上,断口形貌出现大量的裂纹,表明合金的韧脆转变后的塑性变形由相界强度所控制.