Low cycle fatigue behavior of Ti-Mn alloys: Cyclic stress-strain response

The cyclic stress-strain response at constant total strain has been determined for a series of binary Ti-Mn alloys which have been heat treated at a constant temperature of 700 °C to produceα,α-β, and β phase microstructures where the chemical composition ofα andβ phase has been kept constant. The α phase hardens during cyclic straining and the amount of hardening increases with strain. Theβ phase initially softens but hardens with increasing number of cycles. Theα -β alloys show a mixture of these behaviors with softening at low strains and hardening at higher strains. Softening in theα -β alloys is not expected, particularly in the higherα alloys, because plastic strains are much higher inα than inβ at low strains. Transmission electron microscopy has indicated that this behavior is due to dislocation rearrangement inα phase. It has not been possible to ascertain the reason for softening ofβ phase, which may be due either to an increased mobile dislocation density or metallurgical instability of the alloy or both.     

Room-temperature deformation behavior of directionally solidified multiphase Ni-Fe-Al alloys

Directionally solidified (DS) β + (γ + γ′) Ni-Fe-Al alloys have been used to investigate the effect of a ductile second phase on the room-temperature mechanical behavior of a brittle 〈001〉-oriented β (B2) phase. The ductile phase in the composite consisted of a fine distribution of ordered γ′ precipitates in a γ (fcc) matrix. Three microstructures were studied: 100 pct lamellar/rod, lamellar + proeutectic β, and discontinuous γ. The β matrix in the latter two microstructures contained fine-scale bcc precipitates formed due to spinodal decomposition. Room-temperature tensile ductilities as high as 12 pct and fracture toughness (K _Q ) of 30.4 MPa √m were observed in the 100 pct lamellar/rod microstructure. Observations of slip traces and dislocation substructures indicated that a substantial portion of the ductility was a result of slip transfer from the ductile phase to the brittle matrix. This slip transfer was facilitated by the Kurdjumov-Sachs (KS) orientation relationship between the two phases and the strong interphase interface which showed no decohesion during deformation. In microstructures which show higher values of tensile ductility and fracture toughness, 〈100〉 slip was seen in the β phase, whereas 〈111〉 slip was seen in the β phase in the microstructure which showed limited ductility. The high ductility and toughness are explained in terms of increased mobile dislocation density afforded by interface constraint. The effect of extrinsic toughening mechanisms on enhancing the ductility or toughness is secondary to that of slip transfer.     

Cyclic creep-rupture behavior of three high-temperature alloys

Some important characteristics of the cyclic creep-rupture curves have been studied for the titanium alloy 6Al-2Sn-4Zr-2Mo at 900° and 1100°F (755 and 865 K), the cobalt-base alloy L-605 at 1180°F <910 K), and for two hardness levels of 316 stainless steel at 1300°F (980 K). Such curves have been used successfully in a previous investigation for the evaluation and prediction of strain-cycling life at elevated temperatures within the creep range. The cyclic creep-rupture curve relates tensile stress and tensile time-to-rupture for strain-limited cyclic loading and has been found to be independent of the total strain range and the level of compressive stress employed in the cyclic creep-rupture tests. The cyclic creep-rupture curve was always found to be above and to the right of the conventional (constant load) monotonic creep-rupture curve by factors ranging from 2 to 10 in time-to-rupture. This factor tends to be greatest when the creep ductility is large. Cyclic creep acceleration was observed in every cyclic creep-rupture test conducted. The phenomenon was most pronounced at the highest stress levels and when the tensile and compressive stresses were completely reversed. In general, creep rates were found to be lower in compression than in tension for equal true stresses. The differences, however, were strongly material dependent.     

Internal oxidation of dilute Cu-Ti alloys

Metallurgical and Materials Transactions A - The kinetics of internal oxidation of dilute Cu-Ti solid solutions, containing up to 1 wt pct Ti have been investigated over the temperature range 700...     

Internal oxidation of dilute Cu-Ti alloys

The kinetics of internal oxidation of dilute Cu-Ti solid solutions, containing up to 1 wt pct Ti have been investigated over the temperature range 700 to 900°C, and the oxide morphologies produced have been studied by electron metallography. Values of the solubility-diffusivity products for oxygen in copper (N_o ^(s)D_o) are in good agreement with those obtained for pure copper by electrochemical techniques. The TiO₂ particles formed in the Cu-matrix are extremely small (50-200?) and appear to be ellipsoidal in shape. The oxide particle size has been observed to increase linearly with distance below the specimen surface. In addition, electron metallography reveals TiO₂ particles both in the matrix and at grain boundaries which exhibit interesting fringe contrast or “stripes” which are perpendicular to the major axis of the ellipsoids. The origin and possible significance of these regions in terms of coherency of the TiO₂/Cu interfaces are discussed. The interfacial energies for incoherent TiO₂ /Cu interfaces have been found to be quite low, on the order of 400 to 700 ergs/cm². D. ADAMONIS is formerly Graduate Student Assistant     

Internal oxidation of dilute Co-Ti alloys

The kinetics of internal oxidation of Co-Ti alloys and the crystal structures and morphologies of the internal oxide particles have been investigated in the temperature range 900 to 1080°C. Three oxides were observed:δ-TiO, TiO₂ (rutile) and CoTiO₃. The permeabilities of oxygen in Co were calculated from the observed internal oxidation kinetics using an analysis due to Meijering as well as an analysis based on a more rigorous solution of the diffusion equations. The factors affecting the size and morphology of internal oxide particles are discussed.     

Electrochemical oxidation of copper-nickel metal-sulfide alloys

The electrochemical dissolution of the copper-nickel converter matte is considered as one of the ways of its processing. It is shown that regularities are inherent to the anodic oxidation of this material that are characteristic of its phase components, namely, nickel and copper and nickel sulfides. The sequence of oxidation of phases and the effect of the scan rate of the potential on the limiting oxidation currents and features of formation of the passivation film are established.     

Microplasma oxidation of metals and alloys

Institute of Physical Chemistry, Russian Academy of Sciences. Translated from Metallurg, No. 8, pp. 34–35, August, 1994.     

Internal oxidation of dilute Co-Ti alloys

The kinetics of internal oxidation of Co-Ti alloys and the crystal structures and morphologies of the internal oxide particles have been investigated in the temperature range 900 to 1080°C. Three oxides were observed:δ-TiO, TiO₂ (rutile) and CoTiO₃. The permeabilities of oxygen in Co were calculated from the observed internal oxidation kinetics using an analysis due to Meijering as well as an analysis based on a more rigorous solution of the diffusion equations. The factors affecting the size and morphology of internal oxide particles are discussed. Formerly graduate student assistant, Department of Metallurgical and Materials Engineering, University of Pittsburgh, Pittsburgh, PA 15261 This paper is based, in part, on a thesis submitted by J. Megusar in partial fulfillment of the requirements for the Ph.D. Degree, University of Pittsburgh, April, 1974.     

Slip transfer and dislocation nucleation processes in multiphase ordered Ni-Fe-Al alloys

Directionally solidified (DS) alloys with the nominal composition Ni-30 at. pct Fe-20 at. pct Al having eutectic microstructures were used to study slip transfer across interphase boundaries and dislocation nucleation at the interfacial steps. The slip transfer from the ductile second phase, γ(fcc) containing ordered γ′(L1₂) precipitates, to the ordered β(B2) phase and the generation of dislocations at the interface steps were interpreted using the mechanisms proposed for similar processes involving grain boundaries in polycrystalline single-phase materials. The criteria for predicting the slip systems activated as a result of slip transfer across grain boundaries were found to be applicable for interphase boundaries in the multiphase ordered Ni-Fe-Al alloys. The potential of tailoring the microstructures and interfaces to promote slip transfer and thereby enhance the intrinsic ductility of dislocation-density-limited intermetallic alloys is discussed.     

Electrochemical oxidation of nickel sulfide metallic alloys

The sequence of the electrochemical oxidation of phase components of the converter matte is confirmed and variations in the composition of its surface during the anode oxidation are revealed. It is shown that regularities of this process that are characteristic of its phase components, namely, nickel and its sulfides, are inherent to the converter matte. The oxidation of nickel sulfides in sulfide-metallic alloys starts at lower potentials (1.1–1.2 V) than that of synthesized nickel sulfide (1.2–1.3 V). During the oxidation of the converter matte, metal nickel initially oxidizes and, only as this stage finishes, the oxidation of sulfides accompanied by the isolation of elemental sulfur is developed. Copper impurities weakly affect the oxidation of the converter matte, although the shift of potentials takes place as the Cu/Ni ratio increases.     

Microstructure of alumina-forming oxidation resistant Al-Ti-Cr alloys

• 1.1) The protective alumina-forming Al-Ti-Cr alloys identified by Perkins et al. [2] are based primarily on the τ (L1₂) and TiCrAl laves phases.
• 2.2) The τ phase in the multiphase protective alumina-forming Al-Ti-Cr alloys is not stable at 1073K and decomposes to r-Al₂Ti and Cr₂Al.
• 3.3) Future coating alloy development in the Al-Ti-Cr system should focus on the γ/TiCrAl laves two phase field.

     

The fracture toughness of niobium-based,in situ composites

The fracture resistance of Nb-Cr-Ti alloys orin situ composites of three different compositions, Cr₂Nb, and a Nb-10Siin situ composite was studied at ambient temperature. The crack-tip deformation and fracture behaviors were characterized using near-tip measurement techniques and fractographic analyses. The relevant fracture and toughening mechanisms were identified and related to the microstructure. Despite fracture by a combination of cleavage and slip band decohesion, the Nb solid-solution alloy exhibited a resistance-curve behavior with a relatively high toughness and local ductility. The source of toughness was modeled and explained in terms of a cracking process that involved alternate slip band decohesion and cleavage. Thein situ composites, on the other hand, exhibited cleavage fracture but considerably lower toughness with little or no resistance-curve behaviors. The difference in the fracture behavior appears to arise from two factors: (1) the presence of a high constraint in the Nb solid-solution matrix in thein situ composites, and (2) the lack of plastic flow associated with cleavage of the constrained Nb solid-solution matrix.     

Numerical modeling of multiphase diffusion in the process of homogenizing in binary alloys

A general model to treat multiphase diffusion in an impregnated semi-infinite couple, a finite/semi-infinite couple, and a finite/finite couple, under conditions of zero zurface flux, is presented. The model is based both on the numerical solutions previously presented for the multiphase diffusion in semi-infinite and infinite media to which Boltzmann's transformation can be applied and on the finite difference solutions with a variable grid for the multiphase diffusion to which the transformation cannot be applied. Not only critical layer widths for the occurrence of a new phase and the disappearance of a pre-existing phase but also criteria to calculate or not a change in concentration for a terminal phase are introduced in the model in order to diminish the number of iterations, in the calculation. Typical examples to which the model has been applied are discussed.     

Numerical modeling of multiphase diffusion in the process of homogenizing in binary alloys

A general model to treat multiphase diffusion in an impregnated semi-infinite couple, a finite/semi-infinite couple, and a finite/finite couple, under conditions of zero surface flux, is presented. The model is based both on the numerical solutions previously presented for the multiphase diffusion in semi-infinite and infinite media to which Boltzmann’s transformation can be applied and on the finite difference solutions with a variable grid for the multiphase diffusion to which the transformation cannot be applied. Not only critical layer widths for the occurrence of a new phase and the disappearance of a pre-existing phase but also criteria to calculate or not a change in concentration for a terminal phase are introduced in the model in order to diminish the number of iterations in the calculation. Typical examples to which the model has been applied are discussed.     

Oxide dispersed copper alloys by surface oxidation

A series of Cu-Al and Cu-Ni-Al alloys, inert gas atomized, as -44 μ powders, were converted to submicron thick flakes covered by copper oxide and aluminum oxide in varying amounts, depending on the initial aluminum content. The flake powders were either hydrogen reduced to eliminate copper oxide, or were reacted in a hot inert gas to increase the Al₂O₃ content by internal oxidation. A range of alloy compositions were produced by subsequent hot extrusion, yielding excellent room temperature tension and 450°and 650°C stress rupture properties; the alloys are structurally stable from 900°to 1000° C and higher in certain cases. The process is attractive for its simplicity, reproducibility, and resultant excellent properties. Formerly Research Assistant