Stabilization of Cubic Zirconia with Manganese Oxide

A directionally solidified eutectic (DSE) of MnO-ZrO₂ has been investigated using a variety of electron optical techniques. It is found that considerable MnO goes into ZrO₂ to form a substitutional solid solution. About 14 wt% of MnO is soluble in ZrO₂ close to the eutectic temperature. The solubility of ZrO₂ in MnO, however, is quite low, less than 0.50 wt%. Electron diffraction experiments indicate that ZrO₂ (MnO) has the cubic fluorite structure. Diffuse scattering, similar to other cubic zirconias (e.g., CaO, MgO stabilized zirconia), is also observed in manganese-stabilized zirconia. Diffuse scattering indicates the presence of oxygen vacancies and thus confirms the defect nature of the fluorite structure. Electron energy loss spectrometry (EELS) fine structure analysis of the Mn L₂₃ edge provided clear evidence that Mn is present as Mn²⁺ in Mn-stabilized cubic ZrO₂.     

周志宏对魏氏组织及马氏体的形成和形貌的早期金相研究

关于铁合金中相变的详尽显微组织方面的早期研究之一是由艾伯特·苏佛教授的学生周志宏（哈佛大学博士，１９２８年）完成的。该项研究描述了铁碳合金中魏氏组织和马氏体的形成，所用合金具有宽的碳含量范围，热处理时采用的冷却速率范围也是宽的。还描述了一种独特的汞浴淬火方法。此研究工作从未完全发表过。本文援引该博士论文中的一些原始显微组织照片和部分原文，并试图用我们现今对于铁合金相变的理解来阐明和重新诠释。文中还包括中国引进现代工业和科学的历史背景，以及周志宏对苏佛教授的个人回忆。     

Low-energy interfaces in NiO-ZrO2(CaO) eutectic

Interphase interfaces in the directionally solidified eutectic (DSE) NiO-ZrO₂(CaO) have been investigated using transmission electron microscopy (TEM) techniques. Arguments are presented, based on extensive experimental results, to show that the observed interface plane, (111) NiO//(100) ZrO₂, corresponds to a minimum in interface energy. The possible relaxation events associated with this interface are identified with the aid of imaging and diffraction analyses. A recently introduced technique of convergent beam electron diffraction for a plan-view bicrystal is attempted in order to identify rigid body translation associated with this interface. Some of the difficulties associated with this technique are discussed. This paper is based on a presentation made in the symposium “Interface Science and Engineering” presented during the 1988 World Materials Congress and the TMS Fall Meeting, Chicago, IL, September 26–29, 1988, under the auspices of the ASM-MSD Surfaces and Interfaces Committee and the TMS Electronic Device Materials Committee.     

Ordering transformation and spinodal decomposition in Au−Ni alloys

A transmission electron microscope (TEM) was employed to study the ordering and phase separation processes in Au-40 at. pct Ni and Au-50 at. pct Ni alloys in order to test for the possible existence of a transient long-range-order (LRO) phenomenon within a disordered miscibility gap. An L1₀ LRO phase was found in the Au-50 at. pct Ni alloy when spinodally decomposed specimens were reannealed at ∼490°C on a TEM hot stage. This observation, together with the literature results, indicates that a transient LRO did exist in the Au−Ni system, although it appeared during a reversion process. Attempts to find the L1₀ and L1₂ LRO phases during decomposition processes of a single-phase homogeneous solid solution were unsuccessful, except in very thin sections of the TEM foils. Elastic strain energy relaxation was employed to explain the experimental observations. Time-temperature—transformation (TTT) diagrams for the two Au−Ni alloys were constructed based on the TEM characterization. In bulk-annealed samples, only spinodal decomposition and discontinuous precipitation structures were observed. JI-CHENG ZHAO, formerly Graduate Student, Department of Materials Science and Engineering, Lehigh University     

Phase equilibria and thermal analysis near the Au-Pb-Sn ternary eutectic

In an attempt to better determine the composition of the ternary eutectic in the Au-Pb-Sn system, a series of alloys were fabricated near the proposed eutectic composition. These alloys were observed using standard metallographic techniques in addition to thermal analysis to determine which, if any, were closer to the true ternary eutectic. Results from this work have shown that Prince’s currently accepted eutectic is near but not at the true ternary eutectic. In addition, this work also shows that thermal analysis coupled with metallographic techniques is an improved method for determination of phase equilibria in this system.     

Interface stability in the Ni-Cr-AI system: Part I. morphological stability of β-γ diffusion couple interfaces at 1150°C

Aluminide coatings on Ni-base superalloys offer resistance to oxidation and hot corrosion at elevated temperatures. Complex depositional and subsequent diffusional interactions of the coating with the substrate result in a multiphase product consisting primarily of β-NiAl and γ′-Ni₃Al intermediate phases. An understanding of interfacial stability between the coating and the substrate is therefore necessary in order to explain the formation of such phases. The Ni-Cr-AI system serves to simplify the complex chemistry of most Ni-base superalloys. In this study, reaction diffusion and interfacial stability were investigated in solid-solid diffusion couples, consisting of a common β-Ni50Al end-member and a series of γ-pure Ni, binary Ni-Cr, and ternary Ni-Cr-Al alloys, isothermally annealed at 1150 °C for 49 hours. The morphological development of the interface was examined using optical metallography and quantitative information obtained using electron-probe microanalysis. A transition from a stable or planar to an unstable or nonplanar interface in the β-γ diffusion couples was observed with the systematic variation in Cr content of the γ end-member. Interface breakdown in the β-γ couples was explained by means of microstructural information gathered about interfaces, measured diffusion paths, and a knowledge of phase constitution relationships.     

Inversion Domain Boundaries in Aluminum Nitride

Inversion domain boundaries (IDBs) have been identified in undoped, hot-pressed AIN. The microstructure and micro-chemistry of the IDBs have been studied using conventional transmission electron microscopy, convergent-beam electron diffraction, and analytical electron microscopy. Two distinct IDB morphologies are present: a planar variant which lies on the basal plane (0001), and a curved variant which does not possess a particular habit plane, but portions of the boundary are often seen lying on one of the {1011} planes. The boundaries exhibit α-like fringe contrast, indicating that a translation exists across the boundary. The displacement vectors R_F for the planar and curved boundaries have been investigated using both two-beam and multiple-beam techniques. Microchemical analysis has revealed oxygen segregation to the planar IDB; when present on the curved IDB, oxygen is at a lower concentration than in the planar case. Lattice fringe imaging and long-exposure selected area electron diffraction patterns have indicated the presence of thin, platelike precipitates at the planar IDBs. Sintering and annealing studies indicate that oxygen is necessary for the formation of the planar IDBs and that oxygen is not uniformly distributed along the curved IDBs.     

Phase transformation kinetics and the assessment of equilibrium and metastable states

The general characteristics of phase transformation kinetics during cooling and heating are introduced. It is demonstrated that the cooling process always depresses the phase boundaries away from equilibrium and towards lower temperatures; similarly, the heating process always shifts the phase boundaries to higher temperatures. Moreover, the larger the cooling or heating rate, the larger the discrepancy. According to this observation, the reliability of cooling and heating data (irrespective of the methods used to determine them: electrical resistance, dilatometry, etc.) for phase diagram assessments is discussed. The principle for correct assessment of metastable phase information is also briefly introduced. It is pointed out that each kind of metastable phase has its own transformation-start temperature, T_s (M_s, etc.), during cooling. More specifically, each kind of martensite has its own Ms. Examples are shown for Fe-Ni, Fe-Mn, and Ti-Cr alloys.     

The reaction between solid iron and liquid Al-Zn baths

The reaction which occurred between iron panels and Al-Zn baths during hot dipping was investigated. Three baths were studied: 45Al-55Zn, 55Al-45Zn, and 75Al-25Zn (in wt pct) in the temperature range of 570 to 655 °C. The reaction between the iron panel and the Al-Zn bath was very severe and in all cases the iron panel was totally consumed by the bath in less than two minutes. The rapid attack of the iron panels by the Al-Zn baths was attributed to two separate causes depending on growth conditions. First, in some panels the intermediate layer which formed between the iron panel and the molten bath was nonadherent. This resulted in the direct contact of the molten bath with the iron panel at a nonequilibrium interface, which presented a large driving force and little inhibition for the reaction. Second, in panels containing an adherent alloy layer, the layer had channels of liquid Zn which extended from the molten bath to the iron panel. These channels allowed rapid transport of Zn and Al to the iron panel which resulted in a very high reaction rate. The controlling step in the reaction between the iron panel and molten Al-Zn bath was the diffusion rate of Al in the molten bath to the surface of the iron panel. The diffusion coefficient of Al in the molten bath was found to be in the range of 1 × 10^-5 to 5 × 10^-5 cm²/s. Microstructural, electron microprobe, and X-ray diffraction data are presented to support the above-mentioned mechanisms and conclusions.