The effect of the composition and maximum cycling temperature on the microstructure of Cu-Al-Mn shape memory alloys is investigated. For this purpose, two sets of alloys with constant Al content (close to 10.7 wt%) and Mn (7.0 wt%) were prepared. The influence of the composition on the microstructure of Cu-Al-Mn shape memory alloys was carried out in chill casting alloys, annealed for 5 min at 170°C. The effect of maximum cycling temperature was carried out in a Cu-10.8% wt Al-4.0% wt Mn alloy. The reverse transformation is not detected at temperatures below 400°C. Thermal cycling between 400°C and 510°C produces the precipitation of different phases, which varies the initial martensite composition and their characteristic transformation temperatures.
Fatigue crack initiation in coarse-grained Astroloy was studied by initiating cracks under bending loads. The specimen was not replicated in order to preserve any debris or extrusions associated with the cracks. Cracks were found to have initiated at pores and along slip lines. Debris and tonguelike extrusions were found coming from some cracks, but not all of them. Illustrations are given of the five different crack extrusion-related phenomena that were observed. It is concluded that the extrusion of material in the form of tonguelike or other debris may accompany the initiation of fatigue cracks, but that it is not a requisite part of the processes causing the crack.
A microstructural examination (using optical and scanning electron microscopy) has been conducted of samples of a Hastelloy B2 alloy, which have been aged from 550 to 850°C for 200, 400, 800, and 1,200 hr. The nickel-based alloy contained 27.7% Mo, 1.0% Cr, 0.8% Fe, 0.3% Mn, and 0.002% C (wt. %). In the solution-annealed condition, the structure is facecentered cubic α. The study used samples of 50 and 800 μm initial α grain sizes. At all aging temperatures, the hardness increased and approximately doubled after aging for about 400 h at 550, 650, and 750°C. At 850°C, it was still increasing after 1,200 h. In the 50 μm initial grain-sized material, aging produced increasing amounts of a Widmanstatten plate structure, with the most prolific amounts at 750°C and the least at 550°C. The formation of this structure was considerably slower in the larger-grained size material since it initiated at the α grain boundaries. The initial formation was by a plate of high Mo content (relative to the matrix) surrounded by a region of low Mo content. This structure developed into a lamellar product, growing lengthwise by the movement of an interface. It appears that the plates are the γ phase (Ni3Mo). Since considerable hardening occurs even when the Widmanstatten structure is not prominent, it is believed that the α has, at least initially, transformed to the β phase (Ni4Mo), as this is known to cause hardening. The structures are discussed in terms of the known Ni-Mo-Fe and Ni-Mo-Cr ternary phase diagrams.
The composition and morphology, and the size and distribution of the nonmetallic inclusions found within a series of shielded metal arc (SMA), C-Mn steel weld deposits, produced using AWS E7016 type electrodes, have been investigated using transmission electron microscopy (TEM) and associated x-ray microanalysis techniques. The majority of the inclusions were ≈ 0.1 μm to ≈ 1.0 μm in diameter, and microanalysis generally showed them to be rich in Mn, Si, and Ti, with lower levels of Al, Cu, and S. Most of the inclusions were heterogeneous, often containing Mn- or Cu-rich sulphide globules embedded in their surfaces.
The morphology and growth characteristics of rust phases formed on ASTM A-588 weathering steel in three different types of laboratory tests—accelerated atmospheric exposure simulation tests (AAEST), salt fog test, and continuous immersion test in plain as well as salt water—are analyzed using microstructural information obtained from representative exposed specimens studied in a scanning electron microscope (SEM). The ultimate and most dominant phase in the AAEST was α-FeOOH whereas an amorphous phase designated as amorphous bulk (AB) appeared as “cotton bolls” in the adherent, sedimentary layer formed on the steel surface during continuous immersion. Crystalline phases α-, δ-, and γ-FeOOH as well as γ-Fe2O3.H2O were found developed on top of the first-formed sedimentary amorphous layer, containing another amorphous phase designated as amorphous mix (AM). Magnetite was the dominant phase obtained in the salt fog test. It forms in layers and seems to transform to α-FeOOH through formation of whiskers and rods on its surface. Sandy grains of γ-Fe2O3.H2O were also seen in the rusts obtained in this test.
In the present study, a method for the preparation of reproducible Ni 3 Al specimens is described. It can also be applied to other brittle materials. A further advantage is that the region of the crack development can be predicted
Evolution of microstructures leads to formation of lineal microstructural features, which can be often regarded as plane convex loops. Average loop size is an important stereological parameter for characterization of such features. It is shown that the perimeter averaged caliper diameter Dp is a quantitative descriptor of the average loop size. This parameter is related to the total perimeter of the loops per unit volume, Lv, and the total planar area enclosed by the loops per unit volume, Av, as follows: Dp=4AvLv A practical example involving experimental measurement of Dp is discussed.
A method is discussed, which provides quick and unique indexing of a planar defect only from the relative change of its apparent width when the crystal is tilted in the goniometer stage of a transmission electron microscope. Two micrographs are necessary, showing different apparent widths of the defect and three diffraction patterns of arbitrary negative beam direction B, to index the foil normal and the goniometer axis II. Neither a calibration of the magnification nor the determination of the foil thickness is necessary, since the apparent widths may be supplied to the computer in arbitrary units. In addition to being of less experimental effort, this method also has accuracy that proves to be equal or better than that of other methods.
Presentation des resultats experimentaux concernant l'analyse des phases T 2 dans l'alliage traite thermiquement.Observation de la symetrie icosaedrique de la phase,et de son amincissement preferentiel lors de la preparation des echantillons.
A Co-32Ni-21Cr-8Al-0.5Y alloy coating was plasma sprayed on Hastelloy X. The microstructure of the coating layer consists of γ phase solid solution, γ′ phase, and Y-rich intermetallic phase. This coating exhibits excellent oxidation and sulfidation resistance after exposure in air and in sodium sulfate at 1,000°C for 60 h, due to the formation of α-Al2O3 oxide scale. However, the presence of chloride in the sodium sulfate leads to rupture of the aluminium oxide scale, and this results in the precipitation of chlorides and sulfides within the coating layer.
The microstructures of alloys that show peritectic formation of the compounds Mg3Hg and Mg5Hg2 were studied after continuous cooling. Identification of phases present was performed by means of x-ray diffraction, microhardness tests, and differential thermal analysis.
The excessive reactivity in galvanizing of steels containing silicon has been studied for many years, but a fundamental understanding of the phenomenon is still lacking. The present work was done to show the effect that Γ, the most iron-rich phase in the Fe-Zn system, has on the reaction kinetics. It was shown that the onset of reactive coating formation is coincident with Γ destabilization. The instability was observed in both solid state and solidliquid interactions. The metallographic results and the implications of such an instability will be presented.
The method of Roosz, Gacsi, and Baan for determining true pearlite lamellar spacings from the apparent spacing frequencies observed on a metallographic surface is examined. It is shown that the method as it stands does not give very accurate results. Modifications to the method are proposed, and these are shown to lead to large improvements in the accuracy of the method.
Weld solidification cracking in the duplex stainless steel SAF 2205 has been investigated and compared with that of alternate duplex and austenitic stainless steels. Varestraint weld-ability testing showed SAF 2205 to exhibit a lower cracking susceptibility than that of the duplex stainless steel Ferralium 255 but greater than that of a Type 304 austenitic stainless steel which solidified as ferrite and exhibited Ferrite Number 8 (FN 8) in the weld fusion zone. The high augmented strain levels required to induce cracking in these three alloys during Varestraint testing indicated a high resistance to solidification cracking at strain levels normally encountered in structural weldments. Cracking susceptibilities of the duplex and Type 304/FN-8 stainless steels were appreciably lower than that of a Type 304L stainless steel which solidified entirely to austenite and exhibited less than FN 1 in the weld fusion zone.
Alloy 718 tubes were subjected to rotary friction welding to understand to the process fundamental and grain structure evolution during welding. The distribution of grain size, low-angle grain boundaries (LAGBs), and twin boundaries throughout the joints were quantitatively analyzed. The weld power, axial load, and weld temperature were monitored. The grain structure evolution during friction welding was clarified. The grain structure in the recrystallization zone (RXZ) of the weld was a result of competition between dynamic recrystallization and grain boundary sliding (GBS), which is controlled by the local deformation condition. The axial force during welding decreased with reducing the rotation rate from 1000 rpm to 500 rpm. This anomalistic phenomenon can be ascribed that a decrease in rotation rate resulted in finer grain size in the RXZ of the weld, which required lower applied force to enable GBS.
A microstructural analysis has been conducted of 12% Cr stainless steel (essentially Type 422) bolts that were found to be embrittled after service in a fossil-fired power plant for approximately 30 years at about 565–590°C. In the as-received condition, the Charpy V-notch absorption energy at 25°C was about 4 J. Fracture in the bolts and in the impact samples was intergranular, along the prior austenite grain boundaries. However, the fracture surfaces did have a fine, rough topology. The microstructure consisted of a rather uniform dispersion of fine particles in a ferrite matrix, with particles on the prior austenite grain boundaries. X-ray diffraction analysis of extracted particles, and TEM and STEM analysis of metallographic extraction replicas and thin foils, and of extraction replicas from the fracture surface, identified most of the particles as M23C6 carbides. In addition, Laves phase and a Ni-rich (e.g., 30 wt. % Ni) phase were found.
The microstructure of weld metal of duplex Fe-30Mn-9Al-0.4C alloy was investigated using optical metallography and transmission electron microscopy. It was found that DO3 superlattices having an ordered FCC structure precipitated in the ferrite phase during autogenous gas tungsten arc welding. Two types of antiphase boundary were observed between precipitates in the Fe-Mn-Al weld metal.
In order to study the effect of ion implantation on the corrosion behavior of aluminum, it is important to insure that the implanted surface is free from surface irregularities that could shadow portions of the surface from the ion beam and thereby produce anomalous corrosion behavior. A chemical-mechanical sample preparation technique, employed prior to ion implantation, is presented. Electrochemical tests used to determine the pitting corrosion resistance of aluminum and ion-implanted aluminum indicated that shadowing is not a problem. The thickness of the oxide film after polishing was approximately 3.4 nm. Oxide thicknesses were determined using Rutherford backscattering spectroscopy. The root mean square surface roughness, determined by an optical technique that uses the total intergrated scattered light, was measured at 7.4 ± 0.5 nm.
A high maraging strength in Fe-Ni-Mn alloys can be achieved at the expense of a marked loss in ductility. Very fine precipitates are observed when peak strength is reached. At peak strength, the Fe-Ni-Mn alloys exhibit brittle failure, mainly along prior austenite grain boundaries, irrespective of the nickel content. The presence of a small amount of retained austenite prior to aging does not improve the ductility. Previous explanation of the embrittlement in Fe-Ni-Mn alloys was attributed to the segregation of Mn to prior austenite grain boundaries. However, this is not fully supported by the present studies. Auger electron spectroscopy reveals no decisive evidence of manganese segregation. Some degree of ductility in the aged martensite may be required in order to prevent brittle fracture. Dual aging recovers part of the ductility and improves the strength slightly. The effect of reversed austenite on ductility may vary, depending on its morphology. Matrix and recrystallized austenite are beneficial to both elongation and reduction of area, but lathlike austenite lowers the elongation, probably because of its lamellar morphology. The lamellar structure of the lath martensite is also detrimental to elongation.
The optical twins in grains of the high temperature superconducting material, YBa2Cu3Ox, lie along {110} planes, and the long boundaries of elongated grains are basal planes. Equations are presented utilizing these facts, which allow the apparent twin spacing to be converted to true twin spacing by measurement of the angles between twin sets and the basal plane boundary on the polish surface. The true thickness of the elongated grains in their C axis direction may also be determined from the analysis.
