Characterization of INCONEL alloy MA 6000 powder

Mechanically alloyed powders of INCONEL* alloy MA 6000 have been characterized in order to understand the mechanical alloying process. Various analytical techniques including transmission electron microscopy, X-ray diffraction, electron microprobe analysis, and Auger electron spectroscopy were used to reveal the microstructural changes, chemical homogeneity, degree of mechanical working, and surface chemistry of powders during the powder processing. The current investigation has brought about a better understanding of the basic mechanism of the mechanical alloying process as well as a guideline for quality control of mechanically alloyed powder.     

Void formation in INCONEL MA-754 by high temperature oxidation

Subsurface void formation in oxide dispersion strengthened MA-754 caused by high temperature oxidation was investigated at temperatures of 1100, 1150, and 1200 °C for times of 1, 10, 50, and 100 hours. Material exposed at 1200 °C was examined using microprobe, SEM, and optical microscopy techniques. After exposure in air at 1200 °C for 100 hours, chromium depletion by as much as 10 wt pct was observed near the surface, and voids of various sizes up to 15 μm in diameter were found to depths of 300 μm. The fraction of voids increases with exposure time and, with the exception of anomalous values near the surface, decreases with depth. The maximum area fraction of voids observed was approximately 8 pct. Correlation of the void area fraction profile with the measured chromium depletion through a diffusion analysis shows that void formation is due to vacancy injection. Similar void formation in Ni-Cr alloys without oxide dispersions suggests that void formation is not dependent upon the presence of oxide dispersions. The diffusion coefficient for chromium in MA-754 at 1200 °C was computed from microprobe data to be 4 × 10^-10 cm² per second.     

Elevated temperature compressive steady state deformation and failure in the oxide dispersion strengthened alloy MA 6000E

A study of the compressive flow strength-strain rate behavior of the oxide dispersion strengthened (ODS) alloy MA 6000E has been conducted between 1144 and 1366 K. Specimens taken in the longitudinal, long transverse, and short transverse bar directions were tested at strain rates ranging from 2.1 × 10⁻⁵ s⁻¹ to 2.1 × 10⁻⁷ s⁻¹. The inherent compressive strength of MA 6000E was essentially independent of orientation. Testing at the higher temperatures and slower strain rates produced large scale cracking. Such cracks formed and propagated in bands in which slip had dissolved and redistributed the γ’ precipitates. Steady state deformation could be described through use of a threshold stress model of creep where threshold stresses were calculated as functions of temperature and orientation from the relatively fast flow stress-strain rate data and the assumption that the effective stress exponent was 3.5.     

Void formation during tensile testing of dual phase steels

The effects of martensite volume fraction (MVF) and strain state on necking behavior, post-uniform elongation, and the nucleation and growth of voids in thin sheet dual phase steel, strained in tension, were investigated. Steel containing, in weight percent, 0.08C, 1.45Mn, and 0.21Si, was cold rolled 50 pct and intercritically annealed to produce dual phase microstructures. The effects of MVF were evaluated with a series of constant geometry tensile samples with martensite volume fractions between 5 and 40 pct. The effects of strain state within the neck were evaluated with a series of constant thickness samples with 20 pct MVF and with width variations between 3 and 25 mm. A transition from diffuse to localized necking, as well as a decrease in post-uniform elongation, occurred with both an increase in MVF and sample width. Metallographic analysis of deformed samples revealed that the void nucleation occurs primarily at martensite particles by three distinct mechanisms. The void size and density in the necked region increased toward the fracture surface in all samples and the void density was significantly higher for the samples which exhibited localized necking. However, independent of neck geometry, voids were nucleated uniformly throughout the samples, and were associated with the martensite. The difference in void size and density between the samples with different necking behavior indicates that void growth is a consequence of the strain gradient while the shape of the voids depends on both the strain state and strain gradient. The implications of the void structure analysis are interpreted based on the dual phase microstructure. Formerly Graduate Research Assistant, Colorado School of Mines.     

In situ observations of crystalline oxide formation during aluminum and aluminum alloy oxidation

Anin situ morphological study of the oxidation of electron transparent specimens of aluminum and aluminum alloys containing zinc and magnesium has been carried out in the temperature range 400 to 520°C using the hot stage of a 1 MeV transmission electron microscope. The structure and morphology of the crystalline oxide produced in each alloy has been carefully examined by selected area electron diffraction and stereomicroscopy. In pure aluminum, oxidation takes place after a temperature dependent induction period, by the nucleation of crystalline γ-Al₂O₃ at the amorphous oxide/metal interface. This process is delayed by additions of zinc which modify the structure of the oxide. In alloys containing magnesium, oxidation takes place by the rapid nucleation and growth of MgAl₂O₄ or MgO, with a secondary form of magnesia developing from the reduction of the amorphous γ-Al₂O₃ surface layer.

     

High-cycle fatigue properties of the ODS-alloy MA 6000 at 850 °C

The high cycle fatigue (HCF) and cyclic crack growth rate (CCGR) properties of the dispersion strengthened ODS-alloy MA 6000 were investigated with smooth bars and with fracture mechanics samples at 850 °C. The material was very coarse grained with the grains elongated in the rolling direction. Fatigue crack initiation and crack propagation were studied parallel and perpendicular to the rolling direction and a pronounced influence of orientation was found. The fatigue limit of sam-ples cut parallel to the grain elongation direction (p-samples) was almost a factor of 2 higher than the one of samples cut transverse to the elongation direction (t-samples). Inclusions were found to be responsible for crack initiation. For p-samples a reasonable agreement between particle size, fatigue limit, and crack growth behavior was found. For t-type samples such an agreement also exists provided differences in the crack growth behavior of short cracks and long cracks are taken into consideration. The low fatigue strength of t-samples could be linked with low Young's modulus in this direction. The crack propagation rate of long cracks is lower in t-samples than in p-samples due to crack branching along the grain boundaries. HCF-strength of MA 6000 is high compared to conventional cast alloys mainly because of reduced size of crack nucleation sites and higher fatigue threshold stress intensity range ΔK_th, as a result of higher Young's modulus.     

Oxide formation at the initial stages of oxidation of a eutectic Pb-Bi alloy

The results of electron diffraction study of the oxidation of a eutectic Pb-Bi alloy during heating at various partial oxygen pressures in the gas phase are presented. It is revealed that only the oxide phases of lead form at the initial stages of oxidation, which occurs from α-PbO₂ through intermediate oxides nPbO₂ · mPbO and Pb₃O₄ to the β-PbO modification.     

Contact formation during the electric-pulse sintering of a titanium alloy powder

Translated from Poroshkovaya Metallurgiya, No. 7(331), pp. 20–23, July, 1990.     

Contact formation during the liquid-phase sintering of Fe-Cu and Cu-P alloy powders

Conclusions During the liquid-phase sintering of Fe-Cu and Cu-P alloy powders a dominant role in the formation of particle contact structure is played by capillary forces, which ensure migration of the liquid phase along the grain boundaries of the solid phase. In the course of prior heat treatment of these alloy powders emergence of the liquid phase to their surfaces under the influence of excess intergranular pressure is favored energetically.Translated from Poroshkovaya Metallurgiya, No. 12(276), pp. 33–37, December, 1985.     

Influence of gravity on the dispersoids during the melting of an oxide dispersion-strengthened alloy (INCONEL MA754)

Formerly Graduate Student at Columbia University, New York, NY.     

Carbide formation in alloy 718 during electron-beam solid freeform fabrication

In electron-beam solid freeform fabrication (EBSFF), material in wire form is fed into a melt pool maintained on the surface of the part by the electron beam, and controlled rapid movement of the part deposits metal selectively in a layer-by-layer fashion. Solidification occurs at a high rate, forming a fine dispersion of primary carbides in Alloy 718. Growth of the carbide particles has been modeled, assuming diffusion control and a linear change in the driving force for precipitation with temperature. The model predicts the maximum carbide size as a function of EBSFF operating parameters and alloy composition. For the material and conditions used experimentally in this work, the model predicts a maximum diameter of approximately 1.0 μm. Extraction-replica transmission electron microscopy of EBSFF samples identified carbides in the 300 to 600 nm range, consistent with a population having the predicted maximum size. Control material from a conventional vacuum-arc remelted ingot was also examined, and platelike carbides up to 40 μm in length were noted. This is an indication of the potential of EBSFF to refine the carbide morphology and size distribution.     

The formation of AIN crystals in an Fe-Mn-Al-C alloy during the nitriding process

We observed the formation of AIN crystals in an Fe-Mn-Al-C alloy during a nitriding process. The composition of the alloy was Fe-30.4 wt pct Mn-8.7 wt pct Al-1.0 wt pct C. The nitriding process consisted of heating the Fe-Mn-Al alloy in pure nitrogen at 1000 °C for 1 hour. During nitriding, AIN crystals formed in the regions near the surface layer of the alloy. The aluminum nitride formed along specifically preferred orientations of the metal matrix. We discovered that the formation of the secondary arms of the AIN crystals was related to the direction of flux of the nitrogen, and we proposed a growth mechanism for the formation of secondary arms on the primary AIN plates. As the supply of nitrogen continued from the surface, the AIN Widmanstätten side plates formed the secondary arms growing on the leeward side of the primary AIN plates.     

Isothermal oxidation of TiAl alloy

Isothermal oxidation behavior of Ti-48.6 at. pct Al alloy was studied in pure dry oxygen over the temperature range 850 °C to 1000 °C. The oxidation was essentially parabolic at all temperatures with significant increase in the rate at 1000 °C. Effective activation energy of 404 kJ/mol was deduced. The oxidation products were a mixture of TiO₂ (rutile) and α-Al₂O₃ at all temperatures. An external protective layer of alumina was not observed on this alloy at any of the temperatures studied. A layered structure of oxides was formed on the alloy at 1000 °C.     

Elevated temperature strength of fine-grained INCONEL alloy MA754

Elevated temperature tensile and creep-rupture tests were performed on INCONEL alloy MA754 in an as-rolled, fine-grained condition. Tensile tests were performed at 25 °C, 800 °C, 900 °C, and 1000 °C; creep-rupture tests were performed at 800 °C, 900 °C, and 1000 °C. the elevated temperature strength in the fine-grained condition was approximately 25 pct of that the coarse-grained, annealed condition. While good ductility was observed in tensile tests at a nominal strain rate of 1 × 10⁻³s⁻¹, ductility in creep-rupture tests was very low, with failure elongations less than 5 pct and no reduction in area. Creep deformation appeared to occur primarily by cavity formation and growth.     

MA956 ODS合金微晶涂层对1Cr18Ni9Ti不锈钢氧化性能的改善

采用高频电脉冲沉积技术在1Cr18Ni9Ti不锈钢表面上沉积了厚度约为50μm的MA956 ODS合金纳米微晶涂层、涂层与基体之间呈现良好的冶金结合.在1000℃静态空气中对1Cr18Ni9Ti试样与表面施加涂层试样进行了总计100h的恒温氧化实验.用AFM,SEM,EDX和XRD分别对涂层和氧化后试样的形貌。成分和组成进行了观察和分析.结果显示:1Cr18Ni9Ti表面沉积MA956ODS。合金纳米微晶涂层促进了Cr的选择氧化,提高了1Cr18Ni9Ti表面氧化膜的粘附性,改善了1Cr18Ni9Ti抗高温氧化性能.     

The mechanisms of formation and prevention of channel segregation during alloy solidification

Conditions for the formation of macroscopic segregation channels have been examined in the ammonium chloride-water and lead-tin systems, using base chilled molds. Such channels develop when the rejected solute is less dense than the solvent and are therefore a result of density inversion, but slow (≺5 rpm) rates of mold rotation, about axes inclined to the vertical by 20 deg to 30 deg, throughout the time of solidification, effectively prevent the formation or propagation of these channels. Artificially created channels or those momentarily blocked fail to continue and are overgrown, but channels can be initiated by drawing liquid upward from close to the growth front in fine capillaries. Examination of these effects leads to the conclusion that channels originate at the growth front, rather than within the dendritic array, and that their formation is necessarily preceded by a liquid perturbation from the less dense boundary layer into the supernatant, quiescent bulk liquid. Intermittent ‘solute fingers’ are then fed by dendritic entrainment to produce stable convective plumes and concomitant channels. It is considered that the effects of mold precession are primarily caused by translation of bulk liquid across the dendritic growth front, shearing off convective perturbations from the boundary layer before they have time to develop. The nature of the liquid movements is discussed and shown to be a function of the mold dimensions. The inclination of the gravitational vector within the solid-liquid, dendritic array is considered to be of secondary importance to the formation or prevention of channels. This paper is based on a presentation made at the symposium “Fluid Flow at Solid-Liquid Interfaces” held at the fall meeting of the TMS-AIME in Philadelphia, PA on October 5, 1983 under the TMS-AIME Solidification Committee.     

Nugget formation and growth during resistance spot welding of aluminium alloy 5182

Abstract

Surface interaction at the worksheet/worksheet interface during resistance spot welding of aluminium alloy 5182 with spherical tip electrodes was investigated. Oxide layer cracking and nugget formation were focused. Both experimental work and finite element analysis were employed to explain the contact behaviour at this interface. It was found that sheet separation and thus bending occurred during the squeezing phase of the resistance spot welding process and suggested a profound influence on nugget formation. The sheet separation caused enlarged and aligned cracks in the surface oxide layers which led to a good metal‐to‐metal contact near the periphery of the faying surface. High current densities which occurred at the beginning of the current phase caused significant heat generation in this zone. Consequently, the melting at the faying surface started near the periphery and moved in towards the central zone of the contact region to produce a ‘doughnut shaped’ nugget with a filled‐in but thin central region.

On a étudié l'interaction de la surface à l'interface feuille de travail‐feuille de travail lors du soudage par points par résistance de l'alliage d'aluminium 5182 avec des électrodes à bout sphérique. On s'est concentré sur la fracture de la couche d'oxyde et sur la formation du noyau. On a utilisé tant le travail expérimental que l'analyse par éléments finis pour expliquer le comportement de contact à cette interface. On a trouvé que la séparation de la feuille, et donc le pliage, se produisait lors de la phase de compression du procédé de soudage par points par résistance, suggérant une influence profonde sur la formation du noyau. La séparation de la feuille résultait en fissures agrandies et alignées dans les couches d'oxyde de la surface, ce qui amenait un bon contact de métal à métal près de la périphérie de l'aire de contact. Des densités élevées de courant, qui se produisaient au début de la phase de courant, résultaient en un dégagement important de chaleur dans cette zone. Conséquemment, la fonte de l'aire de contact commençait près de la périphérie et se déplaçait vers la zone centrale de la région de contact, produisant un noyau en forme d'anneau avec une région centrale remplie, mais mince.