Physical Metallurgy Research Articles

Some Aspects on Grain Refining Additions with Focus on Clogging during Casting

Some ideas of how to study optimum conditions for implementation of grain refining in liquid steel processing with focus on how to avoid clogging are discussed. It is assumed that the inclusions most beneficial for grain refining are known from studies by physical metallurgists. The challenge for a process metallurgist is how to provide a homogeneous distribution of grain refiners at the onset of solidification. Four different ways of providing information to succeed with this are discussed. Thermodynamic modeling can be used to predict what additions to make to create potential grain refiners, if relevant thermodynamic data is available. Mathematical fluid-flow modeling can be used to study where to add potential grain refiners. It is discussed that the tundish is the most appropriate reactor to add grain refiners, since enough time is given to a complete mixing of the grain refiner into the steel before the steel enters the mold. By using the scanning laser microscopy technique it is possible to study which potential grain refiners has the lowest attraction forces between each other. This is important in order to minimise growth of inclusions when they collide during transport in the tundish, which can lead to the formation of larger inclusions that do not serve as useful grain refiners. Finally, it is suggested that laboratory experiments are carried out in order to study the tendency for nozzle clogging, before the use of grain refiners is tested in industrial scale.

MODELLING TMCP OF HSLA STEELS BY PETRI NEURAL NETWORK TECHNIQUE

The optimization of the architecture of the Petri neural network (PNN) model for thermomechanically controlled processed (TMCP) high strength low alloy (HSLA) steel is done through the comparison of test errors with training errors at different model complexities. The development of the PNN models on the basis of phenomenological relationships between the input and the output variables of TMCP steels has been done. Physical metallurgy principles which govern the property response to the combined actions of chemical composition and process parameters and describe the operative strengthening mechanisms are embedded in the Petri neural network in such a way that the metallurgical reasoning are duly regarded in TMCP steels in predictive activities. It has been found that the Petri neural network is a good tool for modelling the effect of alloy composition and process parameters on the mechanical properties of HSLA steels with certain limitations.L’optimisation de l’architecture du modèle de réseau neuronal de Pétri (PNN) de l’acier haute résistance faiblement allié (HSLA), traité par contrôle thermomécanique (TMCP), est faite par comparaison des erreurs d’évaluation par rapport aux erreurs d’entraînement à différents niveaux de complexité du modèle. On a effectué le développement des modèles PNN en se basant sur les relations phénoménologiques entre les variables d’entrée et de sortie des aciers TMCP. Les principes de métallurgie physique qui gouvernent la réponse de la propriété aux actions combinées de la composition chimique et des paramètres de traitement et qui décrivent les mécanismes fonctionnels de renforcement sont inclus dans le réseau neuronal de Pétri de manière telle que le raisonnement métallurgique est proprement considéré dans les aciers TMCP dans les activités prédictives. On a trouvé que le réseau neuronal de Pétri était un bon outil pour la modélisation de l’effet de la composition de l’alliage et des paramètres de traitement sur les propriétés mécaniques des aciers HSLA, avec certaines limitations.

Enhancement of Ductility and Strength through Microstructural Refinement by FSP of Nickel Aluminum Bronze

Friction stir processing (FSP) is a severe plastic deformation (SPD) method that has been applied to as-cast NiAl bronze (NAB) materials, which are widely used for marine components. The thermomechanical cycle of FSP results in homogenization and refinement, and the selective conversion of microstructures from a cast to a wrought condition. The physical metallurgy of NAB is complex and interpretation of the effects of FSP on microstructure has required detailed analysis by optical and electron microscopy methods. Annealing and isothermal hot rolling have been employed to confirm microstructure-based estimates of stir-zone peak temperatures. The variation of mechanical properties was assessed by use of miniature tensile samples and correlated with microstructure for samples from stir zones of single and multi-pass FSP. Exceptional improvement in strength – ductility combinations may be achieved by FSP of NAB materials.

A coupling thermal mechanical and microstructural FE model for hot strip continuous rolling process and verification

Based on physical metallurgy rules and experiential equations, a module for calculating microstructural evolution in the metal hot-forming process has been developed in this article. Meanwhile, with practical technologies of some plant integrated, a coupling multi-variable simulation model for hot strip continuous rolling (HSCR) has been built using non-linear rigid-viscoplastic finite element method (FEM). After an actual 2050 mm HSCR process on JIS-SS400 was simulated using the above model, evolution laws of strain, temperature and microstructure were analyzed. Simulated results of rolling force, temperature and microstructure have a good agreement with the measured values.

The Application of Ti-15Mo Beta Titanium Alloy in High Strength Structural Orthopaedic Applications

Abstract Titanium-15Molybdenum beta titanium alloy is a generic alloy that has been around for a long time, but has never found wide use in the aerospace industry. Beta titanium alloys, in general, and corrosion-resistant biocompatible titanium alloys, in particular, attracted a lot of attention in the medical and surgical device industry in the late 1980s and early 1990s. Until that time, all of the titanium grades used for medical implant applications were based on ASTM “F” standards modeled from aerospace industry and AMS specifications. The CP titanium grades (CP-1, CP-2, and CP-4) and two α + β alloys, Ti-6Al-4V ELI and Ti-6Al-4V, were widely used in aerospace applications and found early acceptance to become the reference metallic materials in ASTM F 748. Two important factors held Ti-15Mo back from commercial applications and use: the physical metallurgy of the alloy and the inability of the then-current reactive metals melting and processing machinery to handle this unusual binary alloy. This paper will briefly review high-technology manufacturing advances and processing technology that have enabled the reliable production of large-scale lots of Ti-15Mo bar and rod product forms. Additionally, mechanical and metallurgical data for semi-finished high strength rod product suitable for highly stressed orthopaedic implant applications will be presented.

Physical metallurgy of high strength low alloy strip steel production using compact strip processing

The physical metallurgy of high strength low alloy (HSLA) steel production by compact strip processing or direct slab casting is reviewed. Specimens were extracted from 50 mm thickness cast Nb + Ti(+ V)HSLA slabs and quenched to room temperature, before and after the slab was heat treated in a tunnel furnace at 1150°C. These specimens were analysed to track changes in composition and microstructure during processing, in particular the formation of complex (Nb,Ti)(C,N) precipitates during solidification and subsequent heat treatment. It is shown that partial dissolution of the precipitates takes place during thermal equilibration, increasing the solute content of niobium and titanium before hot rolling. Tailoring of the industrial thermomechanical processing route, to improve the Charpy toughness and hardness of thin slab cast niobium HSLA linepipe and other grades, is also described.

The function of interstices in cubic unit cells

The occurrence of interstices, their number and position in cubic crystal structures is investigated from three viewpoints: physical metallurgy, crystallography and quantum-mechanical computations. Interstices are anything but free space left over. They play an important role in the stability or alteration of crystal structures, because they are able to include electronic charge density. Together with structural and thermal vacancies they are a universal means of reacting to the influencing control of those physical factors that govern the formation of the concrete structure. The description of the positions of interstices in a crystal structure can be accomplished in the simplest case by viewing the unit cell, by computing the Wigner–Seitz cells of the atoms in the unit cell or by computing the electronic charge density distribution. However, the latter two are very expensive. The interaction of the Fermi surface (‘necks’, from Pippard's experiment) with the {111} planes of the first Brillouin zone of copper and the non-occurrence of a structure alteration in this case is interpreted as a direct consequence of the charge-containing ability of the interstices.

Physical metallurgy for wrought gamma titanium aluminides: Microstructure control through phase transformations

This paper characterizes the phase equilibria among β/α(α 2)/γ phases and phase transformations involving β-Ti phase in Ti–Al–M (M is β stabilizer against α phase in pure Ti) ternary systems at elevated temperatures. Based on the knowledge, a novel method has been proposed to develop a new class of wrought gamma alloys having a good hot workability for processing and good toughness in service at around 900 K. The ternary systems exhibit the β+α(α 2)+γ three-phase tie triangle moving toward the Ti–Al binary edge with decreasing temperature, which results in a unique composition range with the transformation pathway of β+α→α→β+γ. The high-temperature β phase is effective for hot forming processes, whereas the decomposition (β+α→α) and precipitation (α→β+γ) of the β phase make it possible to produce a fully lamellar microstructure with very fine β particles dispersed within the lamellae.

Physical and mechanical metallurgy of premature failure of tie rods of a cable-stayed bridge

Tie rods of a newly constructed cable-stayed bridge failed after being in service only for six months. Failure occurred during winter months. Detailed metallographic analyses of the microstructure and fracture surfaces, supplemented by oblique section metallography, have been carried out. Results indicate that the final failure mechanism was cleavage due to lack of low temperature toughness of the steel used. Fatigue crack propagation due to cyclic and uneven loading was also an important contributing factor. The use of higher toughness steels with intrinsic weathering resistance has been suggested as a possible remedial measure.

2D calculations of the thermal effects due to femtosecond laser-metal interaction

Experimental results previously published have shown that the radial heat affected zone (HAZ) for a 500 nm thick Al sample in the femtosecond case is less than 2 μm [Appl. Phys. Lett. 80 (2002) 3886]. The spread of radial thermal effects for femtosecond pulses is calculated, by developing a two-dimensional version of the two temperature model (TTM). The case of an axi-symmetrical geometry is simulated by a finite element technique. A physical metallurgy approach is used to define and to estimate the radial HAZ width from the calculations. The evolution of the temperature as a function of time is studied, leading to a radial HAZ width of 220 nm for 500 nm thick Al samples. Similar calculations in the nanosecond regime are also performed and compared to the femtosecond case.

An integrated theory of whisker formation: the physical metallurgy of whisker formation and the role of internal stresses

Over 50 years of whisker research (cadmium, zinc, and tin) has not resulted in consensus about whisker formation fundamentals for metal films. New analytical tools have recently provided new insights into microstructural changes that occur during whisker formation. Integration of these newer observations with historical data led the authors to propose an Integrated Theory of Whisker Formation. The Integrated Theory incorporates physical attributes such as microstructure and internal stress states. Particular emphasis is placed on recrystallization, grain boundary diffusion, film-substrate interdiffusion (i.e., the Kirkendall effect), and stress gradients. The Integrated Theory does not require dislocation mechanisms for material transport to the whisker location. Material is transported to a whisker grain by the surrounding grain boundary network under the driving impetus of positive stress gradients. Transported atoms then move from the grain boundary network into the whisker grain. This movement into the whisker grain pushes the free surface of the whisker grain upward and, thereby, grows the whisker structure.

Modeling the Effect of Copper on Hardness of Microalloyed Dual Phase Steel through Neural Network and Neuro-fuzzy Systems

The effects of copper along with some microalloying elements and the processing parameters are modeled with artificial neural network and adaptive neuro-fuzzy inference system. Both the tools are found to be useful for modeling the effect of copper and other alloying additions along with the processing parameters on the hardness of microalloyed DP steels. In case of the neural network, the proposed committee of models is found to be effective in handling the problem of mapping the input-output relation in these steels. The increase in the number of rules is found to improve the predictability of the neuro-fuzzy inference system. The predictions made by both the models substantiate the knowledge of physical metallurgy principles.

Effects of working, heat treatment, and aging on microstructural evolution and crystallographic texture of α, α′, α″ and β phases in Ti–6Al–4V wire

Abstract Crystallographic texture was measured on Ti–6Al–4V wire at several stages of a process similar to that used in producing titanium products. As-received wire had a moderately strong fiber texture with prism plane normals aligned with the longitudinal wire axis. Wire specimens were cold worked by extrusion, solution heat-treated in the α + β phase field, water quenched, and then aged. Extrusion strengthened the as-received texture. After solutionization and quenching, a secondary fiber with basal plane normals aligned with the wire axis emerged at the expense of the initial texture, indicating that a strongly preferred variant selection occurred during quenching. After artificial aging, α″ and β phases were present, but subsequent natural aging for a year led to elimination of α″ and a reduction in the amount of the β phase. The effect of processing history on the orientations of α, α′, α″, and β (focusing particularly on the effect of extrusion and solutionization time) is discussed in the context of the known physical metallurgy of titanium alloys. The selection of preferred variants in the α → β → α transformation cycles is discussed in relation to shears imposed in the β phase due to differential thermal expansion between misoriented α regions, causing 90° rotations by variant selection.

Physical Metallurgy of Multi‐Phase Steel for Improved Passenger Car Crash‐Worthiness

The dynamic testing of high strength automotive steel grades is of great practical importance if their crash‐worthiness is to be evaluated. During forming operations, steels are processed in a controlled dynamic manner. In collisions, the deformation is different in the sense that the deformation is not controlled, i.e. both strain and strain rate are not pre‐determined. No clear standard testing procedures are currently available to test high strength steels dynamically, in order to evaluate their performance during car crashes. High tensile strength TRIP‐aided steels have been developed by the steel industry because of their promising high strain rate performance. The present contribution focuses on the effect of the strain rate and temperature on the mechanical behaviour of the low alloy high strength TRIP steel. The tests were carried out on the separated phases in order to determine their specific high strain rate deformation response. The temperature‐dependence of the transformation rate of the retained austenite is presented. It is argued that the adiabatic conditions present during high strain rate deformations have a beneficial effect on the behaviour of TRIP steel.

Study of transformation behavior in a Ti–4.4 Ta–1.9 Nb alloy

An alloy of composition Ti–4.4 wt.% Ta–1.9 wt.% Nb is being developed as a structural material for corrosion applications, as titanium and its alloys possess excellent corrosion resistance in many oxidizing media. The primary physical metallurgy database for the Ti–4.4 wt.% Ta–1.9 wt.% Nb alloy is being presented for the first time. Determination of the β transus, M s temperature and classification of the alloy have been carried out, employing a variety of microscopy techniques, X-ray diffraction (XRD), micro-hardness and differential scanning calorimetry (DSC). The β transition temperature or β transus determined using different experimental techniques was found to agree very well with evaluations based on empirical calculations. Based on chemistry and observed room temperature microstructure, the alloy has been classified as an α + β titanium alloy. The high temperature β transforms to either α′ or α + β by a martensitic or Widmanstatten transformation. The mechanisms of transformation of β under different conditions and characteristics of different types of α have been studied and discussed in this paper.

Feature Article: Influencing grain boundary properties by the application of mechanical stress fields

Our Feature Article [1] reviews the different reaction and motion of low- and high-angle grain boundaries in high-purity metals under mechanical stresses. The cover picture shows the microstructure at the surface of a polycrystalline Al–2%Mg alloy. Both figures illustrate the same sample area. In the upper left part the grains are coloured according to their orientation whereas in the underlying figure the grain boundaries are classi-fied due to their desorientation angles (black: high angles >15°, red: low angles <5°, blue: 5–15°). Myrjam Winning is head of the research group “Crystal Plasticity” at the Institute of Physical Metallurgy and Metal Physics of RWTH Aachen University. Her main research topics are grain boundary mechanics and dynamics as well as ultra-fine grained materials. Myrjam Winning is the winner of this year's Hertha Sponer prize sponsored by the German Physical Society. This issue also contains the Editor's Choice “Nanoparticle incorporated superconductor Bi-2212 tapes” by Raghu. N. Bhattacharya et al. [2]. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructural modification of as-cast NiAl bronze by friction stir processing

The application of friction stir processing (FSP) to a cast NiAl bronze (NAB) material is presented as a means for selective modification of the near-surface layers by converting as-cast microstructures to a wrought condition in the absence of macroscopic shape change. This may enable selective surface hardening of cast components. The complex physical metallurgy of the NAB is reviewed, and microstructure changes associated with FSP for a selected set of processing parameters are examined by optical microscopy (OM) and transmission electron microscopy (TEM) methods. Direct temperature measurement in the stir zone is infeasible and, so, these microstructure changes are used to estimate peak temperatures in the stir zone. The persistence of a Fe3Al phase (κii) indicates that peak temperatures are below the solvus for this phase, while the presence of transformation products of the β phase, including fine Widmanstatten α, bainite, and martensite, indicates that peak temperatures exceed the eutectoid temperature for the reaction β → α+κiii throughout the stir zone.

Effect of minor alloying additions on glass formation in bulk metallic glasses

This paper provides a comprehensive review of recent work on the effect of minor alloying additions on glass formation in bulk metallic glasses. Recently, minor alloying addition or microalloying technology has shown to have dramatic effects on the glass formation and thermal stability of many bulk metallic glasses. This technology basically involves adding small amounts of alloying additions (usually, less 2 at%) to existing bulk metallic glasses for the purpose of further improving their glass forming ability. Experimental evidences indicate that alloying additions of small atoms with atomic radius <0.12 nm (such as B and Si) or large atoms with radius >0.16 nm (such as Y and Sc) are most effective in enhancing glass forming ability. The beneficial effects of these elements are discussed in terms of physical metallurgy principles.

Physical metallurgy of high Nb-containing TiAl alloys

Abstract Intermetallic titanium aluminides exhibit attractive thermo-physical properties, which give them the potential for extensive use as lightweight structural components. Novel design concepts are based on alloys with the general composition (in at.%) Ti-45Al-(5–10) Nb, which were subjected to precipitation hardening. Optimized compositions have been identified that are capable of carrying stresses in excess of 700 MPa at service temperatures of 700°C and have superior creep properties. The alloys exhibit at room temperature yield stresses in excess of 1GPa combined with plastic tensile elongations of about 2%. Wrought alloys of this type can be an attractive alternative to the nickel-base superalloys in certain ranges of stress and temperature. The future and promise of these new TiAl alloys lies in innovative processing methods designed to achieve better performance.

Fe3Al-Fe3AlC intermetallics for high temperature applications: An assessment

The advantage of carbon containing dual-phase iron aluminides is that they exhibit relative improvement in high temperature strength as well as reduction in probability of hydrogen embrittlement (HE) over the single-phase iron aluminides, which were the main impediment for the commercialisation of these materials. Owing to its practical and scientific importance a study on the dual-phase Fe3Al-Fe3AlC intermetallics is presented, which deals with the current status of research and developments, understanding of physical metallurgy, HE, oxidation resistance and thermal stability; and its limitation and future challenges.