Showing papers in "Scripta Metallurgica Et Materialia in 1993"

Quasicrystals in as-cast Mg-Zn-RE alloys

Abstract: Since the epoch-making discovery of a quasicrystal (QC) in rapidly solidified Al-Mn alloys by Shechtman et al., numerous quasicrystalline alloys have been found. Most of these works were concerned with Al alloys. Rajasekharan et al. reported the quasicrystal in a rapidly solidified Mg-Al-Zn alloy in 1985, which was followed by work on the quasicrystal in rapidly solidified Ga-Mg-Zn alloy reported by Chen and Inoute. In this paper, the authors discuss the presence of quasicrystals in the Mg-Zn-RE system alloys.

The Lattice-Parameters of Austenite and Ferrite in Fe-C Alloys as Functions of Carbon Concentration and Temperature

Abstract: Despite its relevance for various calculations involving phase transformations in Fe-C alloys, little information is available on the lattice parameter of austenite at elevated temperatures and its dependence on the carbon concentration. Furthermore, severe scatter exists in the literature for the lattice parameter at high temperature. Most literature data were acquired using X-ray diffraction, although neutron diffraction seems to be a more suitable technique penetrate and probe a large volume of material, the advantages of neutron diffraction over X-ray diffraction are its reduced sensitivity to surface decarbonization and improved crystal statistics (both decarbonization and grain growth can occur during diffraction experiments at high temperatures requiring long exposure times). In this work in-situ neutron diffraction experiments on Fe-C alloys were performed to determine the lattice parameter of the austenitic and ferritic phases in a temperature region from just below to just above the bi-phasic austenite/ferrite region.

Ostwald ripening in multicomponent alloys

Abstract: The widespread use of second-phase dispersions in materials has stimulated the intensive study of the diffusion processes occurring in them at the last stage of a first-order phase transformation, termed Ostwald ripening or coarsening. As a result of the large surface area, the mixture is not in thermodynamic equilibrium. Relaxation of such a system is characterized by an increase in the scale size of the second phase and a decrease in the total energy. The authors analyze the coarsening process of an ensemble of particles, precipitation from a multicomponent alloy of arbitrary composition. The authors utilize the method of expansion of chemical potentials, which enables us to describe the coarsening rate through partial derivatives of the chemical potential, with respect to the mole fractions of different components. The latter can be easily calculated numerically, with the help of modern computer database programs such as ThermoCalc. The multicomponent interactions for diffusion are not considered, which implies rather special properties of the matrix of mobility coefficients.

New description of long period stacking order structures of martensites in β-phase alloys

Abstract: The martensitic transformations in {beta}-phase alloys have attracted much attention for many years, since they are associated with the unique shape memory and superelasticity effects, and the lattice instability or unusually soft elastic constant c{prime}[=c11-c12/2]. Besides, the structural changes are all similar to each other; i.e. ordered BCC parent (B2 type or DO{sub 3} type) structures transform to long period stacking order structures, with 2-dimensionally close-packed planes. Furthermore, these alloys in a single crystal state even exhibit martensite-to-martensite transformation among the long period stacking order structures. These structural changes became clear after the crystal structure determination of {beta}{sub 1}{prime} martensite in a Cu-Al alloy near the Cu{sub 3}Al composition by Nishiyama and Kajiwara. The authors discuss the structural change following their scheme. The structure of the martensite in {beta}-phase alloys was extensively re-examined. As a result, the displacements of stacking positions from the ideal a/3 positions and the monoclinicity of 18R(9R) martensites were confirmed in most of the alloys examined, such as Cu-Zn-Ga, Cu-Zn-Al, Au-Cu-Zn, and Cu-Al-Ni alloys. Thus, they can conclude that the structures of the martensites in {beta}-phase alloys are intrinsically monoclinic except for 2H martensites.

The shape memory effect in a Ti50Pd50 alloy

Abstract: Shape memory alloys (SMA) have in the past few decades attracted considerable attention as new functional materials, with commercial applications in pipe couplings, electrical connectors, various actuators, medical use etc. Among many shape memory alloys, Ti-Ni alloys are used mostly because of excellent mechanical properties. However, the maximum temperatures they can be used at is about 373K, due to the limitations of available transformation temperatures. Thus, a strong need exists to develop high temperature shape memory alloys, to widen the applications of SMA. As such candidates, Ti-Pd and Ni-Al alloys systems and a Ti-Ni-Zr alloy look most hopeful. The martensitic transformation in the Ti-Pd alloy was first found by Donkersloot and Van Vucht. Khachin et al. systematically studied the pseudo-binary TiPd-TiNi alloy system, and determined the transformation temperatures and the structures of the martensites as a function of composition.

Intrinsic ductility and environmental embrittlement of binary Ni3Al

Abstract: Polycrystalline, B-free Ni[sub 3]Al (23.4 at.% Al), produced by cold working and recrystallizing a single crystal, exhibits room temperature tensile ductilities of 3-5% in air and 13-16% in oxygen. These ductilities are considerably higher than anything previously reported, and demonstrate that the intrinsic' ductility of Ni[sub 3]Al is much higher than previously thought. They also show that the moisture present in ordinary ambient air can severely embrittle Ni[sub 3]Al (ductility decreasing from a high of 16% in oxygen to a low of 3% in air). Fracture is predominantly intergranular in both air and oxygen. This indicates that, while moisture can further embrittle the GBs in Ni[sub 3]Al, they persist as weak links even in the absence of environmental embrittlement. However, they are not intrinsically brittle' as once thought, since they can withstand relatively large plastic deformations prior to fracture. Because B essentially eliminates environmental embrittlement in Ni[sub 3]Al - and environmental embrittlement is a major cause of poor ductility in B-free Ni[sub 3]Al - it is concluded that a significant portion of the so-called B effect must be related to suppression of moisture-induced environmental embrittlement. However, since B-doped Ni[sub 3]Al fractures transgranularly, whereas B-free Ni[sub 3]Al fractures predominantly intergranularly,more » B must have the added effect that it strengthens the GBs. A comparison with the earlier work on Zr-doped Ni[sub 3]Al shows that Zr improves the ductility of Ni[sub 3]Al, both in air and (and even more dramatically) in oxygen. While the exact mechanism of this ductility improvement is not clear at present, Zr appears to have more of an effect on (enhancing) GB strength than on (suppressing) environmental embrittlement.« less

An explanation to the abnormal hall-petch relation in nanocrystalline materials

Abstract: According to the experimental results of the structural characteristics and the energetic state of the interfaces in the nanocrystalline (NC) materials that a reduction of grain size in nm regime would result in a decrease of interfacial excess volume and interfacial excess energy, a qualitative model explaining the abnormal Hall-Petch relationship in NC samples is presented. It can be deduced from this model that a thermal annealing to the as-prepared NC sample will relax the NC interfaces, and consequently increase the normal-abnormal H-P transition grain size, which is confirmed by the experimental observations in the literature. From this analysis one can see that the densification of grain boundary in the NC materials, which plays an important role in the mechanical behaviors, should be considered in any attempt to improve the properties of NC materials.

Introductory remarks to the viewpoint set on propagative plastic instabilities

Abstract: Readers will certainly be familiar with the phenomenon of deformation bands occurring during plastic deformation of alloys. Lueders bands, which have been known for more than a century, as well as somewhat younger Portevin-Le Chatelier (PLC) bands associated with discontinuous yielding have occupied the metallurgists all these years, as their detrimental influence on the mechanical performance of alloys was creating significant technological problems. Though different in their nature, the two types of bands have one thing in common: they propagate across a material, usually a dilute alloy. While a single deformation front travels along a specimen in the Lueders band phenomenon, repetitive bands with well-defined front and rear edges accompany the PLC effect. Ways to avoid the Lueders and the PLC regimes have been empirically found in forming technologies, but mechanisms of band propagation are still not completely explained by theory.

On the young's moduli of Ti6Al4V alloys

Abstract: In this paper, the authors will present an iterative approach to Young's modulus of multi-phase composites developed by Fan et al. The iterative approach will then be applied to Ti-6Al-4V alloys to predict their effective Young's moduli. It is hoped that the theoretical predictions will offer a quantitative explanation to the peculiar shape of the E[sup c][minus]f[sub [beta]] curve and will shed some light on controlling the Young's moduli of Ti-6Al-4V alloys by choosing the proper heat treatment procedure.

Effect of recrystallization on room temperature tensile properties of an Fe3Al-based alloy

Abstract: The purpose of the present study was to determine whether recrystallization alone had an effect on the room temperature tensile properties of a Fe[sub 3]Al-based alloy The best room temperature tensile strength and ductility were attained in specimens which had been heat treated to relieve stresses produced by the fabrication process but that had a minimum number of recrystallized grains The exact mechanism for this improvement is unclear, but could involve texturing effects or the enhancement of dislocation mobilities Also the elongated grain structure characteristic of as-rolled material provides a minimum of transverse cleavage planes (as well as a minimum of grain boundaries), and could simply be disrupting the path of atomic hydrogen entering the specimen during tensile stressing

Microstructural modification of MoSi2 through aluminum additions

Abstract: The objective of this work was to demonstrate the possibility of influencing one or more aspects of the microstructure of powder processed MoSi[sub 2] via in-situ displacement reactions Since silica is frequently present in MoSi[sub 2] produced by powder processing followed by consolidation and is generally considered as an undesirable phase, an in-situ process that could eliminate silica was selected There are three main routes to eliminate or control silica in powder processed MoSi[sub 2] namely (i) silica reduction via a suitable deoxidant, (ii) powder processing and consolidation in extremely clean conditions, and (iii) chemically etching the powder surface prior to consolidation under clean conditions Although there are a number of elements that can reduce silica, aluminum was selected in this work since it has some solubility in MoSi[sub 2] and, when dissolved in larger amounts, it promotes the formation of the hexagonal (C40), alumino-silicide Mo(SiAl)[sub 2] phase

Atom-probe investigation of the partitioning of interstitial elements in two-phase γ+α2 TiAl-based alloys

Abstract: Titanium aluminides based on TiAl are now receiving an increasing attention because of their low density and attractive properties at high temperatures, such as high specific mechanical strength and good oxidation resistance. However, these alloys suffer from a low ductility up to about 600 C. Compared to the single-phase [gamma] alloys, two-phase [gamma] + [alpha][sub 2] TiAl-based alloys containing about 48 atomic percent of aluminium exhibit a higher ductility. One of the possible explanations is that the presence of the [alpha][sub 2] phase enhances the ductility by scavenging interstitial impurities. It is therefore important to experimentally confirm the scavenging effect of the [alpha][sub 2] phase. The interstitial levels in each of the two phases is however difficult to quantify by the EELS technique. By contrast, the atom-probe field ion microscopy (APFIM) appears to be the most suitable technique for an accurate measurement of the concentrations. A recent study of Uemore et al. using the AP-FIM method has demonstrated the oxygen scavenging effect of the [alpha][sub 2] phase in a 50at. % Al alloy. For the sake of experimental convenience, however, these authors intentionally contaminated the alloy during melting in order to increase their oxygen content (>3 at.%). Such a highmore » oxygen concentration renders the experiments easier, but is expected to modify the equilibrium compositions of each phase. The present AP-FIM study reports on the interstitial scavenging capability of the [alpha][sub 2] phase in alloys of the normal impurity level ([approximately]2,500 at.ppmO). The concentration of both oxygen and carbon has been determined in each of the two phases, showing that the [gamma] phase is significantly purified by the minor [alpha][sub 2] phase. The reasons for such a behavior are discussed.« less

Propagative instabilities: An experimental view

Abstract: Propagating bands of localized deformation, such as Lueders bands and Portevin-Le Chatelier bands occur in virtually all alloys when deformed within certain regimes of temperature and strain rate. Such bands have attracted the attention of both theorists and experimentalists over several decades. Early theoretical studies were concerned with phenomenological treatments involving the cause of the instability and microscopic models of the dislocation processes involved. More recently attention has been focused on stability criteria and spatial aspects of band propagation. In view of the renewed theoretical interest in propagative instabilities it is important that the fundamental experimental features of band propagation. In view of the renewed theoretical interest in propagative instabilities it is important that the fundamental experimental features of band propagation be well characterized and understood. In this paper the authors review experimental measurements on deformation bands, with particular emphasis on geometrical aspects of band propagation associated with the Portevin-Le Chatelier effect. They also report new measurements of Portevein-Le Chatelier bands in an Al-Mg-Si alloy.

Mechanical properties of amorphous alloy compacts prepared by a closed processing system

Abstract: Recently, Al-based and Mg-based amorphous alloys have attracted much attention as light weight structural materials because of their high tensile strength and good ductility. The development of a process for consolidating amorphous alloy powders to amorphous alloy bulks is essential to the structural application of these new alloys. The consolidation of Fe-based and Co-based amorphous alloy powders has been carried out by a number of techniques, such as explosive compaction, dynamic compaction, static high pressure compaction, extrusion and rolling. More recently, it has been reported that mechanical properties of amorphous alloy compacts deteriorate in the presence of oxide films in interparticle boundaries. In the present study, a closed processing system for consolidation of amorphous alloy powders has been developed in order to suppress the formation of oxide layers in the interparticle boundaries and to avoid the dangers of explosions. In this system, a series of processing from the powder production to the extrusion of them can be performed in a vacuum or an inert gas atmosphere. The compressive strength of Al[sub 85]Ni[sub 10]Mm[sub 5] (Mm: mischmetal) amorphous compacts prepared by this process is examined and compared with that prepared by the ordinary one.

The temperature dependence of the flow and fracture of Fe-40Al

Abstract: In the present study, the authors set out to determine whether a peak in the yield strength occurs in Fe-40Al. In order to observe the lattice properties more clearly, they used large-grained material, for which the yield strength would be approximately the same as the lattice resistance, and they subjected the material to a long, low temperature anneal to minimize the vacancy concentration. The results clearly indicate a peak in the yield strength at a temperature similar to that observed by Chang in directionally-solidified material of the same composition.

Laser and electron beam welding of SiCp reinforced aluminum A-356 metal matrix composite

Abstract: This paper details the results of a comparative study of laser and electron beam welding of a cast aluminum metal-matrix composite (Al-MMC) reinforced with SiC particles (SiC[sub p]). Electron beam welding of 15% SiC[sub p] A356-MMC produces much less deleterious Al[sub 4]C[sub 3] than CO[sub 2] laser beam welding at the same input powers and travel speeds. The advantage of EB welding of 15% SiC[sub p] A356-MMC relative to laser beam welding due to the different mechanisms of energy absorption by the substrate material for the two processes. During laser beam welding, the SiC particles are dissolved into a homogeneous solution of liquid Al, Si, and C at high temperatures, and the Al[sub 4]C[sub 3] subsequently precipitates from this solution on cooling. During EB welding of SiC[sub p] Al-MMCs, neither phase preferentially absorbs the energy of the electrons, and heating is more uniform than with laser beam welding. As a result of the uniform heat transfer to each phase, dissolution and decomposition of the SiC is limited and little Al[sub 4]C[sub 3] is formed. It is recommended that sharp focus of the electron beam and high travel speeds be used to minimize Al[sub 4]C[sub 3] formation. The results of thismore » study can be more broadly interpreted to suggest that other high melting efficiency welding processes that allow for rapid welding cycles and minimal superheating can also be used to join SiC[sub p] Al-MMCs as long as the process is compatible with the material.« less

Ductilization of Ti-Ni-Pd shape memory alloys with boron additions

Abstract: Although high transition temperature shape memory alloys (SMAs) are desirable for certain applications, most usable SMAs have upper transition temperature ranges (TTRs) less than 100°C It has been reported that Pd substitutions for Ni in Ni-Ti alloys can increase the TFR to 500°C for more than 15 atomic pct Pd[1, 2]; however, in addition to higher cost, these alloys have poor ductility at room temperature[3, 4]

Cleavage-like fracture of austenite in duplex stainless steel

Abstract: Nitrogen alloying of stainless steel has proved to be extremely successful for mechanical and corrosion properties. For duplex stainless steel, which appears to be very promising for industrial application, the beneficial influence of nitrogen has also been established for contents of about 0.2 wt-%. Duplex structures are found in many alloys and systems as, for instance, in Cu- and Ti-alloys and in stainless steel. They all are characterized by a mixture of similar amounts of two phases whose plastic behaviors are different from one another. In the case of duplex stainless steel the matrix phase is usually ferrite. However, by increasing the nitrogen content and therefore the stability of austenite, it is shown in the present work that austenite can also become the matrix. Because the partition coefficient of nitrogen between [alpha] and [gamma] is large, it is also possible to harden the [gamma] phase more than [alpha]. The question this paper attempts to answer is whether or not nitrogen alloying can exchange the roles of the [alpha] and the [gamma] phases with respect to mechanical properties.

Influence of the microstructure on the fracture toughness and fracture mechanisms of forging steels microalloyed with titanium with ferrite-pearlite structures

Abstract: Titanium addition to vanadium microalloyed forging steels is one of the ways proposed to improve fracture toughness Fine TiN particles inhibit austenite grain growth after recrystallization at the high temperatures used to forge these steels TiN particles, however, can be formed in the liquid, and as their sizes exceed one micron, they could act as cleavage nucleation sites, impairing the fracture toughness The present work reports fracture toughness results obtained in Ti treated microalloyed forging steels, showing that in coarse microstructures cleavage is nucleated in coarse TiN particles, but that after refining the microstructure, voids originate at the same particles, resulting in ductile rupture