A transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg alloy

The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at _1/9 law, whereas growth at longer times, or higher temperatures, followed at _1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed. Formerly Research Assistant, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA. 02139     

A transmission electron microscopy investigation of the early stages of precipitation in an Al-Zn-Mg alloy

The growth of spherical precipitates in an Al-5 wt pct Zn-2 wt pct Mg alloy was monitored by hardness measurements and transmission electron microscopy. The growth of precipitates from 15 to 80Å in diameter was followed for various aging times at temperatures of 30, 70, 100, and 150°C. For short times at low aging temperatures precipitate growth followed at _1/9 law, whereas growth at longer times, or higher temperatures, followed at _1’3 law. Various contrast experiments lead to the conclusion that the precipitates produced at all but the shortest aging times at the lowest aging temperatures have a hexagonal structure and are spherical precursors of the η′ phase. The advantages and disadvantages associated with the various imaging techniques for small precipitates utilized in this study are discussed.     

铝锂合金显微组织高分辨电子显微研究

以新型高强铝锂合金（Al-Cu-Li）为研究对象,合金经过520 ℃固溶淬火后分别在165 ℃时效6 h和165 ℃时效18 h,然后采用高分辨电子显微镜观察Al-Cu-Li铝锂合金显微组织中δ相和T1相的形貌和晶格像,研究了主要强化相T1相的形核机制。结果表明：单胞T1相为五层密排六方堆垛结构,T1相的化学堆垛次序为CBABC。观察发现T1相既可以单独在基体形核生长,也可以依附在原有的T1相上形核以台阶方式生长。细小的球形析出相δ相为有序结构相,并且与基体共格。     

An in situ nanoindentation specimen holder for a high voltage transmission electron microscope

BACKGROUND: Allergen-induced late nasal responses are associated with recruitment of T lymphocytes and eosinophils, and preferential messenger RNA (mRNA) expression of 'TH2-type' cytokines. We previously showed that topical steroid inhibited the late response and associated tissue eosinophilia. In this study we tested the hypothesis that granulocyte/macrophage-colony stimulating factor (GM-CSF) may contribute to late-responses and tissue eosinophilia and is inhibitable by topical corticosteroid. METHODS: Nasal biopsies were taken before and 24 h after nasal allergen provocation following 6 weeks of treatment with either a nasal corticosteroid spray (fluticasone propionate) or a matched placebo nasal spray twice daily. Cryostat sections were processed by immunohistochemistry and in situ hybridization to assess cytokine mRNA expression for GM-CSF. RESULTS: Increases in T lymphocytes and eosinophils were seen in the nasal mucosa after allergen challenge (p = 0.01) which were accompanied by a 5-fold increase in cells expressing mRNA for GM-CSF (p = 0.01). Double immunohistochemistry/in situ hybridization demonstrated that the majority of GM-CSF mRNA+ cells were co-localized to CD68+ (40%), or T cells (40%) with a lesser contribution from eosinophils (<20%). Topical steroid treatment was accompanied by a decrease in both the CD3+ and major basic protein (MBP+) cells expressing GM-CSF mRNA (p = 0.01) with a corresponding proportionate increase in the % of macrophages expressing GM-CSF. CONCLUSIONS: The results indicate that after allergen provocation, eosinophils are recruited to the nasal mucosa and that, at least in part, this may be due to GM-CSF. Topical nasal corticosteroid inhibits late responses and the associated eosinophilia, possibly indirectly by decreasing GM-CSF from T lymphocytes or reducing autocrine production of GM-CSF from eosinophils.     

A high-resolution transmission electron microscopy study of the precipitation process in a dilute Ti-N alloy

The precipitation processes in dilute nitrogen alloys of titanium have been examined in detail by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The alloy Ti-2 at. pct N on quenching from its high-temperatureβ phase field has been found to undergo early stages of decomposition. The supersaturated solid solution (α″-hcp) on decomposition gives rise to an intimately mixed, irresolvable product microstructure. The associated strong tweed contrast presents difficulties in understanding the characteristic features of the process. Therefore, HREM has been carried out with a view to getting a clear picture of the decomposition process. Studies on the quenched samples of the alloy suggest the formation of solute-rich zones of a few atom layers thick, randomly distributed throughout the matrix. On aging, these zones grow to a size beyond which the precipitate/matrix interfaces appear to become incoherent and theα′ (tetragonal) product phase is seen distinctly. The structural details, the crystallography of the precipitation process, and the sequence of precipitation reaction in the system are illustrated.     

High-temperature transmission electron microscopyin situ study of lower bainite carbide precipitation

In situ observation of the bainite carbide precipitation processes in 40CrMnSiMoV steel by means of high-temperature transmission electron microscopy (TEM) is conducted. It is evident that carbides can precipitate either in bainitic ferrite or from austenite when carbide-free bainite (meta-bainite) obtained by isothermal transformation is tempered at higher temperatures. In view of the quantity of carbides precipitated from ferrite in combination with the result of an X-ray diffraction analysis of the bainitic ferrite carbon content, it can be concluded that bainitic ferrite growth involves supersaturation of carbon content to some degree. Formerly with Northwestern Polytechnical University Formerly with Northwestern Polytechnical University This paper is based on a presentation made in the symposium “International Conference on Bainite” presented at the 1988 World Materials Congress in Chicago, IL, on September 26 and 27, 1988, under the auspices of the ASM INTERNATIONAL Phase Transformations Committee and the TMS Ferrous Metallurgy Committee.     

A transmission electron microscope study of 1 Pct Mn ductile iron with different austempering treatments

A transmission electron microscope (TEM) equipped with an energy dispersive spectroscopy (EDS) system was used to study the bainitic reaction in a conventional and a successive austempering process for 1 wt pct Mn ductile iron. In the case of conventional austempering, the specimens were full austenitized at 900 °C and then austempered at 375 °C (high austempering temperature) and 315 °C (low austempering temperature) for different periods. In the case of the successive austempering process, following austempering at 375 °C for different periods, specimens were austempered at 315 °C, and subsequently quenched in ice water. The TEM-EDS study showed that carbide precipitation in the ferritic and retained austenitic component of bainite is a function of the local concentrations of the alloying elements, austempering time, and temperature. After a short time at high austempering temperature, carbide-free bainite forms near graphite nodules. Longer austempering time or lower austempering temperature encourages carbide precipitation in the bainitic ferrite. A long austempering time at high temperature leads to decomposition of retained austenite to ferrite and carbide. A rough inspection shows that the precipitated carbides in the ferritic component of specimens austempered at low temperature lie at an angle of about 40 to 50 deg to the sheaf axis.     

A transmission electron microscopy study of copper precipitation in the cementite phase of hypereutectoid alloy steels

The precipitation of copper has been detected and studied in three of the main decomposition products of austenite: allotriomorphic grain-boundary cementite, pearlitic cementite, and Widmanstätten cementite plates. The investigation has been carried out on two high-alloy hypereutectoid steels containing copper contents of 1.0 and 2.5 wt pct. The main advantage of these high-alloy steels is that the parent austenite phase remains stable upon cooling to room temperature, thus preserving the parent phase and the parent/product interfaces in the microstructure for subsequent examination. Transmission electron microscopy (TEM) revealed that the copper precipitation occurs in proeutectoid allotriomorphic grain-boundary cementite in association with the transformation interface. The copper particles were dispersed in the form of rows (or sheets) within the allotriomorphs of cementite. Evidence for copper precipitate particles nucleated at structural features imaged at the growth interface was also obtained. Copper precipitation was found to occur in both the ferrite and cementite lamellae of pearlite, and again, examination of partially decomposed structures revealed copper particles nucleated at the austenite/pearlite transformation interface. In addition, copper particles were also observed at the ferrite/cementite interface of pearlite. Copper precipitation observed in Widmanstätten cementite plates revealed a precipitate-free midrib region in the plates and a higher concentration of copper particles toward the broad faces of the plate. Copper particles were also found located at coarse linear interface defects at the broad faces of the plate.     

AnIn Situ transmission electron microscope deformation study of the slip transfer mechanisms in metals

The slip transfer mechanisms across grain boundaries in 310 stainless steel, high-purity aluminum, and a Ni-S alloy have been studied by using thein situ transmission electron microscope (TEM) deformation technique. Several interactions between mobile lattice dislocations and grain boundaries have been observed, including the transfer and generation of dislocations at grain boundaries and the nucleation and propagation of a grain boundary crack. Quantitative conditions have been established to correctly predict the slip transfer mechanism. 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.     

Photochemically induced focal cochlear lesions in the guinea pig: II. A transmission electron microscope study

We report the characterization of a de novo unbalanced chromosome rearrangement by comparative genomic hybridization (CGH) in a 15-day-old child with hypotonia and dysmorphia. We describe the combined use of CGH and fluorescence in situ hybridization (FISH) to identify the origin of the additional chromosomal material on the short arm of chromosome 6. Investigation with FISH revealed that the excess material was not derived from chromosome 6. Identification of unknown unbalanced aberrations that could not be identified by traditional cytogenetics procedures is possible by CGH analysis. Visual analysis of digital images from CGH-metaphase spreads revealed a predominantly green signal on the telomeric region of chromosome 10p. After quantitative digital ratio imaging of 10 CGH-metaphase spreads, a region of gain was found in the chromosome band 10p14-pter. The CGH finding was confirmed by FISH analysis, using a whole chromosome 10 paint probe. These results show the usefulness of CGH for a rapid characterization of de novo unbalanced translocation, unidentifiable by karyotype alone.     

High resolution microanalytical study of precipitation in a powder metallurgical high speed steel

The microstructure of a powder metallurgical high speed steel (ASP 60) has been investigated by combined analytical electron microscopy and atom-probe field-ion microscopy. The compositions of MC and M₆C primary precipitates were determined. The tempering reaction at 560°C was studied in detail and it was shown that extremely fine M₂C precipitates dominated at optimum hardness (3 h). Some fine MC precipitates were also present. The compositions of the secondary carbide precipitates were determined by atom-probe analysis. Overageing at 700°C resulted in the transformation of the fine secondary carbide dispersion to coarse M₆C and M₂₃C₆ and a change in the matrix morphology from martensitic plates to equiaxed recovered ferrite subgrains.     

A study of stress-corrosion in Al-Zn-Mg alloys

This study of Al-Zn-Mg alloys was undertaken to establish the specific combination of mechanical and thermal conditions which is necessary for stress-corrosion susceptibility to occur in these alloys. The work included a study by electron transmission microscopy of the microstructure of this alloy system. A new mechanism is proposed to account for the observations made. It suggests that the high corrosion potential at grain boundaries in stress-corrosion susceptibile Al-Zn-Mg alloys is due to segregation of zinc and magnesium. By taking precautions to reduce this segregation, the stress-corrosion life can be increased.     

In-situ straining of Ni3Al in a transmission electron microscope

TEM in-situ straining experiments have been performed on thin foils of Ni₃Al and Ni₃Al containing 750 ppm (0.35 at.%) boron. During the straining, gliding APB-coupled dislocations were observed to leave superlattice intrinsic stacking faults (S-ISF's) as debris in their wake confirming a mechanism for S-ISF formation first suggested by Pak et al. [Scripta metall.19, 1081 (1976)]. The APB-coupled dislocations (i) pile-up at grain boundaries, (ii) become extrinsic grain boundary dislocations and (iii) initiate slip in adjacent grains. The characteristics of the crack propagation process indicated that plastic flow, albeit localised, precedes fracture.