Metallurgical Microstructure Research Articles

Ductile failure as a result of a void-sheet instability: experiment and computational modeling

The ductile fracture of circumferentially notched HY-100 steel specimens tested at high stress triaxiality is characterized by a fracture surface with a zig-zag profile created by large elongated voids separated by inclined sheets of microvoids. The failure path suggests that a local deformation instability, triggered by the growth of MnS inclusion-nucleated voids, may be responsible for this form of fracture. Measurements show that the failure strains are small and decrease slowly with increasing stress triaxiality as compared to a more global void coalescence. Micro-mechanical modeling by finite element analysis of a geometry based on the observed metallurgical microstructure shows a distinct localization of plastic flow. The localization of plastic flow leads to a void-sheet failure prediction and the experimental results are correctly characterized by the computational model.

V<SUB>3</SUB>Si極細多芯超伝導線の微視的組織と電磁特性

The V3Si multifilamentary superconductor, which can be fabricated by controlling the diffusion reaction between V and Cu-Si solid-solution alloy, is promising as a new A15-type multifilamentary conductor for ac use, since it is possible to leave appropriately a normal-conducting V5Si3 layer as a high-resistivity barrier around V3Si filaments. In the present study, the flux pinning mechanism and the ac loss characteristics are investigated in correlation with metallurgical microstructure. Angular dependence of flux pinning force with respect to the field direction, for a flattened conductor, was well fitted to the proposed model in which two kinds of ellipsoidal-microstructure are mutually perpendicular and act as pining centers, respectively. We could thereby confirm that both the grain boundary and the superconducting(S)/normal conducting(N) interface act as the pinning centers. Elemental pinning force of the SN interface is 2∼3 times larger than that of grain boundaries. It was revealed that the V5Si3 layer is effective in suppressing the proximity coupling between filaments and hence to reduce the hysteresis loss.

Shape Memory Effect and Magnetostriction in Rapidly Solidified Fe-29.6 at%Pd Alloy

Rapidly solidified ferromagnetic shape memory Fe-29.6 at% Pd alloy was fabricated to develop a new type of actuator material responded to magnetic field. The thin ribbon samples which were made by the originally designed electromagnetically controlled nozzleless melt-spinning method showed stronger crystal anisotropy and larger shape memory effect. Magnetostriction increased with increasing magnetic field and its value changed with increasing temperature up to about 1800 microstrain at an applied magnetic field, H=8.0 × 10 5 A/m (=10 kOe), at just below the inverse phase transformation temperature (A s ). This phenomenon may be caused by the re-arrangements of the activated martensitic twin variants due to applying magnetic field. The dependencies of shape memory effect and giant magnetostriction on metallurgical microstructures are discussed from the viewpoint of the crystal anisotropy and the grain boundary character distribution.

Fabrication and performance of A1N insulator coatings for application in fusion reactor blankets

The liquid-metal blanket concept for fusion reactors requires an electrically insulating coating on the first-wall structural material to minimize the magnetohydrodynamic pressure drop that occurs during the flow of liquid metal in a magnetic field. Based on the thermodynamics of interactions between the coating and the liquid lithium on one side and the structural V-base alloy on the other side, an A1N coating was selected as a candidate. Detailed investigations were conducted on the fabrication, metallurgical microstructure, compatibility in liquid Li, and electrical characteristics of A1N material obtained from several sources. Lithium compatibility was studied in static systems by exposing A1N-coated specimens to liquid Li for several time periods. Electrical resistance was measured at room temperature on the specimens before and after exposure to liquid Li. The results obtained in this study indicate that A1N is a viable coating from the standpoint of chemical compatibility in Li, electrical insulation, and ease of fabrication; for these reasons, the coating should be examined further for fusion reactor applications.

Metallurgical micro-structure of liquid metal-emitter interface of field emission liquid metal ion source

The stability of the liquid metal field-emission ion source for submicrometre lithography and ion implantation with a focused ion beam (FIB) system is of prime importance, Liquid metal ion sources (LMIS) in FIB systems have emerged as a new tool for a variety of microelectronic fabrication processes. Some micro-fabrications, such as etch rate enhancement, resist exposure, and direct microbeam ion implantation is possible because of the small virtual emitting source size, of the order of 5-60 nm [1, 2]. The basic configuration of LMIS in the FIB system consists of a wetted polycrystalline needle emitter with tip diameter in the order of 1-10 #m, and an alloy source reservoir (Fig. 1). Because of the nature of a polycrystalline emitter with grain boundaries, it is weil established that the mean jump frequency of an atom in these regions is much higher than that of an atom in the lattice [3]. Several possibilities can occur in these regions, such as liquid metal diffusion into the grain boundary (Fig. 2a) and formation of solid precipitates along the shank of the emitter tip (Fig. 2b), thus leading to source emission instability by impeding the flow of liquid metal from the reservoir to the emitter tip, causing variation in dopant concentration. Analysis of the high diffusivity path in this region, namely between two adjacent grains, is important for two reasons. Firstly, there is the question of how much these paths contribute to the calculated value of the diffusion coefficient, and, secondly by analysing the grain boundary diffusion, it is possible to approximate the diffusion coefficient in each of these regions. A more accurate solution to determine the grain boundary diffusion coefficient is to use a tracer concentration gradient as a driving force and radioactive tracer to measure the total amount of material transported. However, for the purpose of approximating the grain boundary diffusion of Pd73B27 alloy into polycrystalline Re, it is assumed that Pd is the main diffusant (because of higher Pd concentration than B) with its known self-diffusion coefficient in order to limit the com-

Thermal stabilities and microstructures of tungsten compound contacts on n-GaAs fabricated by low-energy ion-beam-assisted deposition

A low-energy ion-beam-assisted deposition (IBAD) technique has been developed to fabricate W-Si-N Schottky contacts of enhanced thermal stabilities on n-GaAs. By implementing remote sputtering and glancing angle low-energy N+ ion irradiation, the thermal stabilities of the W-Si-N/n-GaAs Schottky contacts were enhanced to be stable up to 850 °C, while keeping the Schottky barrier heights to the best values obtained with conventional sputtering. Thermal stabilities of the IBAD refractory metals/n-GaAs interfaces were investigated also for various W-Si-N compositions by examining the microstructure and Schottky diode characteristics. WSi0.3N0.4 showed the most stable Schottky contacts on n-GaAs of which the barrier height changed only 20 meV after thermal annealing between 700 and 850 °C, and that is proposed to be due to the stable metallurgical microstructure compared to the element tungsten and tungsten nitride.

THE MAGNETIC PROPERTIES OF HIGH STRENGTH STEELS

Studies of the interrelation of metallurgical and magnetic properties of high strength pearlitic steels are described. High voltage Lorentz electron microscopy has been used to identify directly the pinning sites for domain boundaries. The results lead via a physical model to a formula which permits the prediction of coercivity from the metallurgical microstructure. The empirical relationship suggested by Kneppo is found to describe well the virgin magnetization curve for a range of steels including those heat treated. It is shown that weld renormalization leads to magnetic parameters almost indistinguishable from the parent plate. Heat input variations do not significantly affect the magnetic properties.

Analytical ultrasonics for characterization of metallurgical microstructures and transformations: Rosen, M. Analytical ultrasonics in materials research and testing. Proceedings of a conference, NASA Lewis Research Center, Ohio (United States), 13–14 Nov. 1984. pp. 83–102. NASA conference publication 2383 (1986)

The application of contact (piezoelectric) and noncontact (laser generation and detection) ultrasonic techniques for dynamic investigation of precipitation hardening processes in aluminum alloys, as well as crystallization and phase transformation in rapidly solidified amorphous and microcrystalline alloys is discussed. From the variations of the sound velocity and attenuation the precipitation mechanism and kinetics were determined. In addition, a correlation was established between the observed changes in the velocity and attenuation and the mechanical properties of age-hardenable aluminum alloys. The behavior of the elastic moduli, determined ultrasonically, were found to be sensitive to relaxation, crystallization and phase decomposition phenomena in rapidly solidified metallic glasses. Analytical ultrasonics enables determination of the activation energies and growth parameters of the reactions. Therefrom theoretical models can be constructed to explain the changes in mechanical and physical properties upon heat treatment of glassy alloys. The composition dependence of the elastic moduli in amorphous Cu-Zr alloys was found to be related to the glass transition temperature, and consequently to the glass forming ability of these alloys. Dynamic ultrasonic analysis was found to be feasible for on-line, real-time, monitoring of metallurgical processes.

The interaction between low cycle fatigue and lamellar tearing in AISI 316L welded joints

Experimental joints were manufactured by arc welding relatively thick plates of AISI 316L austenitic stainless steel. These joints simulated genuine joints employed in high-temperature nuclear plants. Mechanical testing of small specimens machined from the experimental joints and from base materials revealed the loss of ductility which is commonly encountered in thick welded plates when they are subjected to loading in the short transverse (through thickness) direction. Low cycle fatigue experiments, both at ambient temperature and at 550°C (1020°F) in air, were performed on base material and on welded specimens, after any surface roughness and geometrical notch effects had been removed by machining. In the case of specimens machined from a T-joint, it was observed that the strain was concentrated in the fusion zone, because of its lower strain hardening properties, but that the ruptures nucleated and grew in the short transverse direction. The fatigue life of the welded joint specimens was in all cases shorter than that of the base materials, but the life reduction was more marked at ambient temperature than at high temperature. Both phenomena can be explained with reference to the metallurgical microstructures. Observation made both before and after the tests revealed that inclusions of precipitated phases ( ie δ-ferrite and σ-phase) were responsible for a mechanism of interaction between lamellar tearing and low cycle fatigue, which is governed by the number, size and orientation of the inclusions.

Developments in Quantification and Use of Environmental Cracking Mechanisms for Design and Lifetime Prediction

ABSTRACTFollowing a long period of qualitative assessment of environmental cracking phenomenology, there has been recent emphasis on developing quantitative, mechanistically-based predictive models. This focus is driven by scientific and engineering concerns. For example, neither pressure vessel design nor evaluation codes incorporate the contribution of environmental cracking understaticloading, nor do they differentiate between the large possible variations in both environment and material chemistry. Similarly, while numerous scientific hypotheses have been proposed to explain the crack advance process and related, contributing phenomena, ultimately their validity must withstand the scrutiny of quantification and comparison to measured cracking kinetics and observed component lifetimes.The benefits and approaches involved in predicting environmental cracking based on mechanistic modeling of crack advance and its underlying phenomena are discussed. The benefits and approaches involved in predicting environmental cracking based on mechanistic modeling of crack advance and the underlying phenomena are discussed. The formulation of a quantitative model from fundamental mechanisms has progressed most rapidly for the film rupture / slip dissolution mechanism of environmental crack advance. Specifically, for iron and nickel base alloys in high temperature water, identification, quantification and integration of the important underlying parameters has led to algorithms which predict crack growth rates over a wide continuum in static and dynamic stressing, metallurgical microstructure and solution chemistry applicable to light water reactors. For widely studied systems, e.g., stainless and low alloy steels in 288°C water, the predictions are within a factor of 2–3 for 90% of the data which span “initiation”, the possible roles of contributing, “secondary” crack advance processes in oxidation-based mechanisms, such as environmentally enhanced cleavage, spontaneous oxidation, etc.Similar efforts to identify and quantify the critical parameters and underlying processes in crack advance mechanisms involving hydrogen embrittlement, environmentally assisted cleavage, etc. are summarized. Conceptual approaches to and progress toward integrating these models with in-situ chemical and crack monitoring devices, with the goal of providing on-line evaluation, control and lifetime prediction of environmental cracking, are also discussed.

A modeling of crack initiation of grain-boundary type under multiaxial low-cycle fatigue.

Cracking behavior under multiaxial fatigue has been observed to be affected by not only stress multiaxiality but also metallurgical microstructure. In copper which was used in this study, cracks were observed to be nucleated and to grow along grain boundaries. It was also found that the crack growth was assisted by coalescence among initiated cracks. These observations suggest that the direction of the main crack might be determined by the distribution of grain-boundary cracks. A modeling of grain-boundary cracking is required to establish the way to predict the direction of the main crack, depending on stress multiaxiality. In this analysis, two components of shear deformations on the grain-boundary plane were taken into account to describe incompatibility along the grain boundary. New parameters based on two shear deformations were proposed to represent the orientational distribution of grain-boundary crack. Calculated results were composed with experimental crack distribution during biaxial fatigue tests with constant and variable strain-biaxialities.

Correlating Microstructure with Backscattered Ultrasonic Energy

AbstractHigh frequency ultrasonic energy interacts with metallurgical microstructure to produce scattered energy fields that are useful in characterizing the material. By using a focused transducer whose focal spot size and ultrasonic wavelength are both comparable to the scale of grain structure in high purity copper specimens, the authors have shown a strong correlation between microstructure and the pattern of backscattered ultrasonic signals. A systematic series of experiments logged the ultrasonic returns from cold-rolled copper with the direction of the ultrasonic beam both parallel to and transverse to the rolling direction. Fourteen samples were annealed to various degrees, capturing several stages of recovery and recrystallization. Use of backscattered ultrasonic energy rather than specularly reflected energy allows isolation of grain boundary scattering in an otherwise quiet acoustic environment. Clear differences between returns of backscattered shear waves from parallel and transverse beam orientations relative to the rolling direction gradually disappear as the annealing process re-establishes a field of equiaxial crystals. Backscattered surface waves also show variations that appear to correspond with the relaxation of residual stresses before the onset of recrystallization, and preferred orientation after recrystallization.

Metallic glass as a temperature sensor during ion plating

An investigation was conducted to examine the temperature of the interface and/or of a surface layer of a substrate during ion plating. A metallic glass of the composition Fe 67Co 18B 14Si 1 was used as a substrate and as the temperature sensor. Transmission electron microscopy and diffraction studies were also conducted to determine the microstructure of the ion-plated gold film and the substrate. The results of this investigation indicate that crystallization occurred not only in the film but also in the substrate. The grain size of crystals formed during ion plating was 6–60 nm in the gold film and 8–100 nm in the substrate at a depth of 10–15 μm from the ion-plated interface. The temperature rise of the substrate during ion plating was approximately 500°C. Discontinuous changes in the metallurgical microstructure and the physical, chemical and mechanical properties during the amorphous-to-crystalline transition in metallic glasses make these materials extremely useful for temperature sensor applications in coating processes such as ion plating.

Mechanical-contact-induced transformation from the amorphous to the partially crystalline state in metallic glass

Sliding friction and wear experiments and electron microscopy and diffraction studies were conducted to examine the metallurgical microstructure of a metallic glass surface strained in sliding contact. Friction and wear experiments were conducted with aluminium oxide spheres 3.2 and 6.4 mm in diameter sliding, in reciprocating motion, on a metallic foil with a composition of Fe 67Co 18B 14Si 1 at a sliding velocity of 1.5 mm s -1 (frictional heating is negligible) with a load of 2.5 N at room temperature and in a laboratory air atmosphere. The results of the investigation indicate that the amorphous alloy (metallic glass) can be crystallized during mechanical contact. Crystallites with a size range of 10–150 nm are produced on the wear surface of the amorphous alloy. A diffused honeycomb-shaped structure formed by dark gray bands is also produced during sliding. Considerable plastic flow occurs on an amorphous alloy surface with sliding and the flow film of the alloy transfers to the aluminium oxide pin surface. Multiple slip bands due to shear deformation are observed on the side of the wear track. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris and/or powdery and whiskery oxide debris. The wear rate of Fe 67Co 18B 14Si 1 was 5 × 10 -9 mm 3 N -1.

Non-destructive evaluation of hardening depth using surface-wave dispersion patterns

This paper describes a non-destructive test method for surface treatments for steels — induction hardening, carburizing (and then quenching) and nitriding — by means of ultrasonic surface waves. Velocity measurements with the pulse-overlap technique show the frequency dependence of the surface-wave velocity, demonstrating that these treatments form a well-defined surface layer of uniform thickness, having different elastic properties to the matrix. Assuming a one-layered structure, a conventional technique for the estimation of the most probable layer parameters from the surface-wave dispersion pattern is proposed. The resultant thicknesses are interpreted in terms of the destructive observation of micro-Vickers hardness variation with depth. The discussion here is restricted to the observational aspects and does not go into consideration of the underlying metallurgical micro-structures.

Microstructural and surface modification of an aluminum alloy by rapid solidification with a pulsed laser

The interaction of a microsecond-duration CO 2 laser pulse (10.6 μm) in vacuum with an aluminum alloy containing 1.2% Mn causes a dramatic change in the metallurgical microstructure in the laser-melted region. Second phases which are normally present as discrete particles in the alloy resolidify in the near-surface region in straited bands parallel to the free surface. The rapid solidification process also modifies the chemical composition of the surface, depleting the surface of the alloying elements manganese, silicon and iron. The surface also contains a network of ripples caused by rapid melting and solidification.

The effect of solution treatment temperature on stress corrosion susceptibility of 7075 aluminium alloy

The effect of varying solution heat treatment temperature of the stress corrosion behaviour of 7075 aluminium alloy was investigated using double cantilever beam self-stressed test specimens. Concentrations of Mg, Cu and Zn at grain boundaries for different heat treatments were determined by scanning Auger microscopic analysis of grain boundary fracture surfaces. It was observed that grain boundary concentrations of the above elements and the stress corrosion plateau crack velocity varied in a similar fashion with the solution treatment temperature. It is suggested that changes in solution treatment temperature affect the stress corrosion susceptibility by changing the grain boundary composition and the metallurgical microstructure.

Mechanical properties and acoustic emission in A 533 B steel in temperature range 24–288°C

The mechanical properties of A 533B C11 steel in uniaxial tension have been investigated as a function of specimen orientation throughout the temperature range 24–288°C. Simultaneously, measurements of the acoustic emission output have been made and this has been shown to be a sensitive function of the metallurgical microstructure. Attention has been focused on the relationship between yield stress and strain ‘energy’ released as acoustic emission at general yielding. The results demonstrate two distinct trends, one for longitudinal and transverse specimens, the other for short-transverse specimens. Signal amplitude analysis did not discriminate between different specimen orientations, but it proved to be useful in the detection of load instabilities the microstructural origin of which has been explored by examinations of the fracture surfaces using a scanning electron microscope.

Laser glazing – a new process for production and control of rapidly chilled metallurgical microstructures

A new process for producing rapidly chilled metallurgical microstructures under reproducible cooling-rate conditions has been developed. Processing is accomplished by rapidly traversing a continuou...

Estimation of surface-layer structure from Rayleigh-wave dispersion. III. Sparse data case—Interpretation of experimental data

Elastic surface waves penetrating into solids to a depth proportional to the wavelength are expected to be dispersive in the presence of gradients in physical properties such as density, chemical composition, internal stress, or metallurgical microstructure. This paper presents the results of studies to use this phenomenon to determine physical property profiles in a nondestructive manner. In a previous paper, a mathematical model was presented giving, for discrete data sets, a probabilistic description of the possible results of measurement, including measurement errors. The model also yields auxiliary measures pertaining to bias, data-vs-model dominance, resolution, and a posteriori variance. Here, the model is applied to actual experimental data consisting of the phase velocities of Rayleigh surface waves measured as a function of frequency for three typical profiles: a thin layer on a thick substrate, a thin layer embedded near the surface of a thick substrate, and a smoothly and monotonically varying profile. The estimated profile is compared with independent (destructive) measurements. As a test, the theory is also applied at a discrete set of frequencies to a set of synthetic data calculated from an assumed profile. The above auxiliary measures giving properties of the estimator are also discussed.