Showing papers on "Alloy published in 1988"

Metal matrix composites

Abstract: A ceramic-reinforced aluminum matrix composite is formed by contacting a molten aluminum-magnesium alloy with a permeable mass of ceramic material in the presence of a gas comprising from about 10 to 100% nitrogen, by volume, balance non-oxidizing gas, e.g., hydrogen or argon. Under these conditions, the molten alloy spontaneously infiltrates the ceramic mass under normal atmospheric pressures. A solid body of the alloy can be placed adjacent a permeable bedding of ceramic material, and brought to the molten state, preferably to at least about 700° C., in order to form the aluminum matrix composite by infiltration. In addition to magnesium, auxiliary alloying elements may be employed with aluminum. The resulting composite products may contain a discontinuous aluminum nitride phase in the aluminum matrix and/or an aluminum nitride external surface layer.

A study of the transient liquid phase bonding process applied to a Ag/Cu/Ag sandwich joint

Abstract: Transient liquid phase (TLP) bonding is a process currently used for joining heat resistant alloys, for example nickel- and cobalt-based superalloys. It involves the formation of a liquid layer between two adjoining pieces and the formation of a solid bond as the liquid disappears during annealing at a suitable constant temperature. In the present study, a model Ag/Cu/Ag sandwich joint associated with a simple eutectic phase diagram was used to study the different stages of this process. The results confirm that the TLP bonding is a diffusional process occurring in clearly distinctive stages. The two most important stages are the widening and homogenization of the previously dissolved liquid interlayer, and the subsequent solidification and shrinking of the interlayer. Whereas the former stage involves diffusional processes both in the liquid phase and in the adjoining solids, the latter is controlled mainly by the diffusion in the solid phase. A modeling approach has been explored which shows that in most eutectic systems there exists an optimal bonding temperature corresponding to the shortest time needed for complete solidification. The results of a study on a Ag/Ag-20 wt pct Cu/Ag sandwich joint provide evidence that the use of an alloy close to the eutectic composition as an interlayer material shortens the TLP process substantially.

Mechanical alloying of brittle materials

Abstract: Mechanical alloying by high energy ball milling has been observed in systems with nominally brittle components. The phases formed by mechanical alloying of brittle components include solid solutions (Si + Ge → SiGe solid solution), intermetallic compounds (Mn + Bi → MnBi), and amorphous alloys (NiZr2 + Ni11Zr9 → amorphous Ni50Zr50). A key feature of possible mechanisms for mechanical alloying of brittle components is the temperature of the powders during milling. Experiments and a computer model of the kinetics of mechanical alloying were carried out in order to esti-mate the temperature effect. Temperature rises in typical powder alloys during milling in a SPEX mill were estimated to be ≤350 K using the kinetic parameters determined from the computer model. The tempering response of fresh martensite in an Fe-1.2 wt pct C alloy during milling was consistent with the maximum results of the computer model, yielding temperatures in the pow-ders of ≤575 Ki.e., ΔT ≤ 300 K). Thermal activation was required for mechanical alloying of Si and Ge powder. No alloying occurred when the milling vial was cooled by liquid nitrogen. The pos-sible mechanisms responsible for material transfer during mechanical alloying of brittle components are considered.

Oxygen Reduction at Pt0.65Cr0.35, Pt0.2Cr0.8 and Roughened Platinum

Abstract: Oxygen reduction in has been investigated at , and at surfaces produced from them by selective dissolution of the Cr component. Rotating disk electrodes (RDE) were used to examine the oxygen reduction reaction (ORR). The published (1–2) surface analysis using x‐ray photoelectron spectroscopy (XPS) combined with sputter profiling and Rutherford backscattering spectrometry (RBS) convincingly demonstrated that the surface region can be selectively depleted of the Cr component by electrochemical excursions to potentials positive of ~+1.25V vs. RHE. For the more Cr‐rich alloy very severe depletion (>500A) occurs upon prolonged potential excursions above +1.25V. For the sample, the surface depletion extends about 2–3 monolayers into the surface. Electrochemical characterization by cyclic voltammetry also confirms that the surface becomes progressively roughened as the potential exceeds the region where chromium is passive. Extensive roughening of the surface results in a significant increase of the measured ORR current per geometric area of the RDE. Tafel slopes of the ORR for moderately rough surfaces (roughness factor, ), produced from either starting alloy, are nearly identical to pure smooth Pt. For rougher surfaces (produced only at the more Cr‐rich starting alloy) the Tafel slopes determined from a mass‐transfer corrected plot of are also nearly identical to those obtained at smooth Pt. It is concluded that the measured increase of ORR current following the roughening of the RDE alloy electrode is due solely to the increase of Pt surface area. The mechanistic and practical implications of this effect in fuel cell electrochemistry are discussed.

The compctition between martensite and omega in quenched Ti-Nb alloys

Abstract: A careful experimental study of the phase transformations which occur in annealed β phase Ti-Nb alloys during quenching has been completed. The compctition of the α″ and ω phases to form in alloys of 20 to 70 at. pct Nb was investigated as a function of quench rate and alloy composition. Particular attention was paid to the interstitial content and chemical homogeneity of the alloys. The martensitic α″ phase was found only in 20 and 25 at. pct Nb alloys, and then only using fast water quenches of ~300 °C/sec. Under slower quench conditions,e.g., ~0.3 to 3 °C/sec, ω phase precipitates were found in these alloys and in 30 and 35 at. pct Nb alloys. Evidence of “diffuse” ω phase precipitation was observed in alloys up to 50 at. pct Nb. Only alloys of 60 and 70 at. pct Nb were found to retain the single phaseβ structure upon quenching. These results constitute the first part of a study of the stable and metastable equilibria of the Ti-Nb alloy system.

Design and development of an experimental wrought aluminum alloy for use at elevated temperatures

Abstract: A new wrought aluminum alloy has been designed having high room temperature strength (e.g., 0.2 pct P.S. 520 MPa) combined with improved creep resistance at temperatures in the range 150° to 220 °C. The alloy is A1-6.3 pct Cu-0.5 pct Mg-0.5 pct Ag-0.5 pct Mn-0.2 pct Zr and it is hardened by a new precipitate which forms on the {111} planes and appears to be highly stable at elevated temperatures. Details are given of the principles underlying the development of the alloy and of the preliminary assessment that has been made of mechanical properties. The alloy, or compositions close to it, also has potential for welded applications.

Effects of sic whiskers and particles on precipitation in aluminum matrix composites

Abstract: The age-hardening precipitation reactions in aluminum matrix composites reinforced with discontinuous SiC were studied using a calorimetric technique. Composites fabricated with 2124, 2219, 6061, and 7475 alloy matrices were obtained from commercial sources along with unreinforced control materials fabricated in a similar manner. The 7475 materials were made by a casting process while the others were made by powder metallurgy: the SiC reinforcement was in the form of whiskers or particulate. It was found that the overall age-hardening sequence of the alloy was not changed by the addition of SiC, but that the volume fractions of various phases and the precipitation kinetics were substantially modified. Precipitation and dissolution kinetics were generally accelerated. A substantial portion of this acceleration was found to be due to the powder metallurgy process employed to make the composites, but the formation kinetics of some particular precipitate phases were also strongly affected by the presence of SiC. It was observed that the volume fraction of GP zones able to form in the SiC containing materials was significantly reduced. The presence of SiC particles also caused normally quench insensitive materials such as 6061 to become quench sensitive. The microstructural origins of these effects are discussed.

A melting and solidification study of alloy 625

Abstract: The melting and solidification behavior of Alloy 625 has been investigated with differential thermal analysis (DTA) and electron microscopy. A two-level full-factorial set of chemistries involving the elements Nb, C, and Si was studied. DTA results revealed that all alloying additions decreased the liquidus and solidus temperatures and also increased the melting temperature range. Terminal solidification reactions were observed in the Nb-bearing alloys. Solidification microstructures in gastungsten-arc welds were characterized with transmission electron microscopy (TEM) techniques. All alloys solidified to an austenitic (γ) matrix. The Nb-bearing alloys terminated solidification by forming various combinations of γ/MC(NbC), γ/Laves, and γ/M6C eutectic-like constituents. Carbon additions (0.035 wt pct) promoted the formation of the γ/MC(NbC) constituent at the expense of the γ/Laves constituent. Silicon (0.4 wt pct) increased the formation of the yJLaves constituent and promoted formation of the γ/M6C carbide constituent at low levels (<0.01 wt pct) of carbon. When both Si (0.4 wt pct) and C (0.035 wt pct) were present, the γ/MC(NbC) and γ/Laves constituents were observed. Regression analysis was used to develop equations for the liquidus and solidus temperatures as functions of alloy composition. Partial derivatives of these equations taken with respect to the alloying variables (Nb, C, Si) yielded the liquidus and solidus slopes t(m L , m S ) for these elements in the multicomponent system. Ratios of these liquidus to solidus slopes gave estimates of the distribution coefficients (k) for these same elements in Alloy 625.

Effects of oxygen and heat treatment on the mechanical properties of alpha and beta titanium alloys

Abstract: Two alloys, Ti-6Al-2V and Ti-2Al-16V, simulating the alpha and beta phases of Ti-6A1-4V, respectively, were prepared with oxygen concentrations from 0.07 to 0.65 wt pct (0.20 to 1.83 at. pct). Their microstructure, deformation behavior, and strength were investigated with X-ray diffraction, microscopy, and mechanical tests to determine the effects of oxygen concentration and heat treatment. In both alloys the hardness increases in identical fashion with the square root of oxygen concentration. The alloys' strengths also depend on heat treatment, but in different ways. Whereas the alpha alloy is non-age-hardenable, the beta alloy's strength can be doubled by aging. The hardening effect of oxygen is generally unaffected by heat treatment, except for the alloys with the highest oxygen concentrations. During aging of the alpha a small amount of Ti3Al can form, and slight age-hardening occurs. The ductility of the alpha alloy is little affected by aging. On the other hand, oxygen causes a change from good ductility at low oxygen concentration (0.07 wt pct) to total brittleness at 0.65 wt pct oxygen, independent of heat treatment. In the beta alloy there are complex phase transformations depending on heat treatment. Its deformation behavior varies from very ductile in solutiontreated and quenched (STQ) condition to totally brittle in aged conditions. The aging embrittlement appears to be caused by alpha and some omega precipitation. Decoration of the beta grain boundaries with precipitates accounts for the intergranular brittle fracture. Oxygen, on the other hand, is not an embrittler, although it reduces the ductility of the beta alloy.

Aluminium nitride in steel

Abstract: Aluminium nitride (AlN) nucleates with difficulty in steel, unless precipitation is enhanced by thermal or mechanical treatments. This important characteristic determines the precipitation kinetics of AlN, and accounts for the wide variations in precipitate morphology achieved following different processing histories. The most significant effect of AlN in steel is on grain size control, which directly influences hardenability, hot ductility, texture development, and mechanical properties. In addition, the accompanying removal of nitrogen from solid solution affects the strain aging characteristics, weldability, mechanical properties, and creep performance. Aluminium nitride also exerts important second order effects through its influence on the precipitation of other alloy nitrides. The precipitation of AlN can cause embrittlement and cracking phenomena in castings, continuously cast products, ingots, and rolled or forged products, However, the basic mechanisms are well understood, and the occurre...

Phase transformations and modulated microstructures in Ti-Al-Nb alloys

Abstract: The decomposition of alloys roughly based on the composition Ti3Al with ternary additions of Nb has been studied through the use of transmission electron microscopy and selected area electron diffraction. It has been shown that a wide variety of transformation behavior in the metastable high temperature β phase can be produced through varying composition, cooling rates, and heat treatment. Transformations observed during quenching from the β phase field include the formation of hcp martensite in alloys with low Nb contents, the occurrence of B2(CsCl)-type ordering over a wide composition range in alloys richer in Nb, and the formation of an “ω-type≓ phase in the parent matrix subsequent to B2 ordering. In many of the as-quenched alloys a “tweed-like≓ or modulated micro-structure is observed with accompanying elaborate networks of rel rods and local diffuse maxima in electron diffraction patterns. During aging the decomposition proceeds through a complex sequence of reactions the nature of which will be discussed below. Many of the anomalies observed in these alloys are quite similar to behavior observed during decomposition of other ordered β(B2, DO3)-type phases in a wide variety of other alloy systems.

An XPS Investigation into the Passivity of an Amorphous Ni‐20P Alloy

Abstract: Anodic polarization experiments in and electrolytes show that amorphous Ni‐20P alloy passivates at potentials between −0.3 and about 0.2V (SCE) and dissolves transpassively above 0.2V (SCE). In , in which crystalline Ni pits readily, Ni‐20P resists pitting and passivates as effectively as in . Analysis by x‐ray photoelectron spectroscopy of Ni‐20P surfaces after polarization indicates an enrichment of elemental P on the alloy surface, the presence of hypophosphite anions and the absence of oxidized nickel species. Thus, the Ni‐20P alloy does not passivate by development of a three‐dimensional oxide film, such as the film that forms on nickel. A chemical passivation process is proposed that is controlled by the formation and surface retention of hypophosphite anion, which acts as a barrier between the alloy and the electrolyte.

Commercial scale production of Fe-6.5 wt. % Si sheet and its magnetic properties

Abstract: Commercial scale production of a Fe‐6.5 wt. % Si sheet has been successfully developed. Presently manufactured sheets are in coil form, whose thickness ranges from 0.1 to 0.5 mm with a maximum width of 400 mm. Magnetic properties of the manufactured sheet have been investigated. The permeability of Fe‐6.5 wt. % Si sheet is about 10 times higher than the conventional nonoriented silicon steel sheet. The core losses are less than half the conventional, and even less than that of the grain‐oriented silicon steel sheet at frequencies over 400 Hz. Superior soft magnetic properties are attributed to the low magnetostriction and high electric resistivity of this alloy. It is well known that the Fe‐6.5 wt. % Si alloy has poor ductility in conventional mechanical work. But investigation of the forming conditions has enabled the stamping and bending of alloy sheets. Low core losses and high permeability make Fe‐6.5 wt. % Si sheet adequate for motor cores, transformer cores operating at high frequencies, and magnetic shielding. Application to the micromotor core shows that Fe‐6.5 wt. % Si sheet reduces the consumption of no‐load electric current by 25% in comparison with the conventional silicon steel.

Effects of aging on the microstructure of 17-4 PH stainless steel

Abstract: Microstructures developed in commercial 17-4 PH precipitation-hardened stainless steel after different heat treatments have been studied. The martensite formed in this alloy has been found to be of a lath morphology having a dislocated substructure. Transmission electron microscopy studies have revealed the formation of a copper-rich phase in the samples aged at temperatures below 550°C. However, at higher temperatures, in addition to the formation of these precipitates, austenite phase reversion has been noticed, primarily at the lath boundaries. The reversion process has been found to be accompanied by a sharp decrease in the hardness of the alloy. The morphologies of the transformation products and the crystallography of the transformation have been discussed.

The effect of plasma‐sprayed calcium phosphate ceramic coatings on the metal ion release from porous titanium and cobalt‐chromium alloys

Abstract: Bone tissue ingrowth in porous materials is enhanced by the deposition of bioactive calcium phosphate ceramic linings onto the pore walls. These bioactive coatings can be deposited using several methods which yield a variety of coating efficiencies and thereby influence the mechanisms and kinetics of ion release from the metal. We analyzed the effect of plasma-spraying hydroxyapatite onto titanium and cobalt-chromium alloys by measuring the release of Ti, Al, V, Co, and Cr in vitro. Plasma-sprayed coatings significantly reduced the Ti and Al release from titanium-based alloy specimens. The tendencies of release from the cobalt-based specimens are less pronounced. The data substantiate that neither localized enhanced passive dissolution of metal ions nor ceramic shielding of the metal occurs. The Scanning Auger Electron Microprobe Spectroscopic data suggest that the dissipation of thermal and kinetic energy of the ceramic particle at the time of impact can produce compositional and structural changes in the metal surfaces. The resulting effects are significant for the titanium alloy but less significant for the Co-Cr alloy system.

Catheter apparatus employing shape memory alloy structures

Abstract: An improved catheter of the type used for mechanically removing arterial plaque employs shape memory alloy structures in the form of an integral portion of the catheter housing for compression against the artery wall and storage of pieces of plaque and at least one actuatable door for selectively opening and closing a window in the housing. The shape memory alloy material is heated by a remotely located current source connected to the catheter by conductors for selective activation of the alloy for a reversible change in shape such as expansion and elongation.

The stable and metastable Ti-Nb phase diagrams

Abstract: The phase transformations which occur in the Ti-Nb binary alloy system have been discussed in two recent papers. The phase relationships were investigated by varying alloy composition and thermal history. In this paper, these results are summarized in complete and thermodynamically consistent calculations of the stable and metastable phase diagrams. The calculations of the metastable equilibria are relevant to the Ti-V and Ti-Mo systems, as well as to several other titanium and zirconium-based transition metal alloy systems.

Initial Development of Thermal and Stress Fields in Continuously Cast Steel Billets

Abstract: A mathematical model has been developed to compute the thermomechanical state of the shell of continuously cast steels in a round billet casting mold. The model determines the temperature distributions, the stresses in and the gap between the casting mold and the solidifying strand. The effect of variations in steel carbon content and mold taper on the thermal, displacement, and stress fields are examined. Comparisons with available experimental observations verify the predictions of the model. The model demonstrates that the thermal shrinkage associated with the phase change from delta-ferrite to austenite in 0.1 Pct C steel accounts for the decreased heat transfer observed in that alloy, as well as its susceptibility to cracking.

Segregation and ordering at surfaces of transition metal alloys: the tight-binding Ising model

Abstract: From the electronic structure of the disordered alloy we derive an effective Ising Hamiltonian for segregation and ordering processes at transition metal alloy surfaces. In this tight-binding Ising model (TBIM), the Hamiltonian contains a linear term, quasi-concentration-independent, which proves to be very close to the difference in surface tensions between pure constituents and a quadratic one involving effective pair interactions larger at the surface than in the bulk. The former explains the success of the popular phenomenological approaches based on surface tension arguments and the latter could be of prime importance in surface ordering processes.

Calorimetry study of the synthesis of amorphous Ni-Ti alloys by mechanical alloying

Abstract: We synthesized amorphous Ni33Ti67 alloy powder by ball milling (a) a mixture of elemental nickel and titanium powders and (b) powders of the crystalline intermetallic NiTi2. We characterized the reaction products as a function of ball-milling time by differential scanning calorimetry and X-ray diffraction. The measurements suggest that in process (a) the amorphous alloy forms by a solid state interdiffusion reaction at the clean Ni Ti interfaces generated by the mechanical attrition. In process (b), the crystalline alloy powder stores energy in the form of chemical disorder and lattice and point defects. The crystal-to-amorphous transformation occurs when the stored energy reaches a critical value. The achievement of the critical stored energy competes with the dynamic recovery of the lattice.

Effect of large boron additions to magnetically hard Fe‐Pt alloys

Abstract: Magnetic properties were investigated for the ternary (Fe0.6Pt0.4)100−xBx and (Fe1−yPty)83B17 alloy ribbons prepared by the twin roll technique. Boron changes the compositions of the ordered FePt (γ1) phase, which coexists with Fe2B at various alloy compositions so that the actual Pt content of γ1 in the ternary alloy goes up with increasing boron content. Coercive force increase due to boron addition is remarkable, in particular at low Pt concentration. Annealing time to get high coercivity becomes much shorter by boron addition. iHc=5.6 kOe and 4πMs=7.8 kG were obtained in the (Fe0.65Pt0.35)83B17 ribbon.

Orientation dependence of β 1 → β 1 ' stress-induced martensitic transformation in a Cu-AI-Ni alloy

Abstract: A systematic investigation was carried out to settle some critical issues on the orientation dependence of the stress-induced martensitic transformation. The transformation we picked up is the βl (DO3) → β1′ (18R) stress-induced transformation in a Cu-14.1 mass pct Al-3.9 mass pct Ni alloy. By utilizing tensile tests and two surface analyses coupled with X-ray diffraction, the following clear results were obtained. Concerning the variant selection rule under stress, it was found to be the shape strain which interacts with an applied stress. The critical resolved shear stress for the martensitic transformation was found to have orientation dependence, the shear stress increasing with the normal stress. The observed transformation strains were consistent with those calculated from the shape strain in all orientations. The strong orientation dependence of the Young’s modulus was consistent with that predicted by the elastic constants in the parent phase.