Fine-grained Alloy Research Articles

Investigation of the heat-affected zone fracture in Ni3Al welds

Ternary Ni77Al23−yXy (X = Zr or Hf, y = 0.5 or 1) + 500 ppm boron compounds with various grain sizes were welded by a CO2 laser. Fractographic examinations of the heat-affected zone (HAZ) in the welds with or without postweld heat treatment (PWHT) were performed on the impact-fractured specimens. In laser welds, the fracture appearance of the HAZ was mixed transgranular/intergranular modes for fine-grained alloys and intergranular mode for coarse-grained materials. However, an entirely transgranular mode was observed in the base metal regardless of the grain size of the compounds. Boron desegregation at high temperatures during the thermal cycle of welding could be used to explain the fractographic change from originally ductile mode into less-ductile or even brittle fracture in the HAZ. Short-term PWHTs along with slow cooling provided sufficient time for boron segregation back to the grain boundary, resulting in a completely transgranular fracture mode in the fine-grained HAZ. Nevertheless, such a phenomenon was not observed in the HAZ of coarse-grained welds. Cracks in the HAZ of coarse-grained welds after long-term PWHT, if not so severe, could be healed by a sintering process.

Fine-grained tungsten-cobalt hard alloys based on plasmochemical tungsten

Fine-grained tungsten-cobalt hard alloys based on plasmochemical tungsten

Thermoelectric properties of fine grained (75% Sb2Te3-25% Bi2Te3)p-type and (90% Bi2Te3-5% Sb2Te3-5% Sb2Se3)n-type alloys

The thermoelectric properties of fine-grained alloys prepared by either cold pressing and sintering or hot pressing in the range 5–50 μm are compared with single-crystal best-direction values. It is shown that for thep-type alloy, almost the entire thermoelectric properties are recovered, i.e. the figure of merit for the finest grain size is almost the same as the best single-crystal value. The same trend is observed in then-type alloy except that 90% of the single-crystal figure of merit is recovered. These results are discussed in terms of a model which suggests that degradation of favourable thermoelectric properties by powdering the alloys is compensated by (1) decrease of thermal conductivity due to scattering of phonons at grain boundaries for grain sizes that are comparable to the mean free path of phonons; and (2) retention of some of the anisotropic properties of the single crystal in the fine-grained compacts.

Effect of alloy grain size and silicon content on the oxidation of austenitic Fe-Cr-Ni-Mn-Si alloys in a SO2-O2 gas mixture

Austenitic Fe-18Cr-20Ni-1.5Mn alloys containing 0, 0.6, and 1.5 wt.% Si were produced both by conventional and rapid solidification processing. The cyclic oxidation resistance of these alloys was studied at 900°C in a SO 2-O 2 gas mixture to elucidate the role of alloy microstructure and Si content on oxidation properties in bioxidant atmospheres. All the large-grained, conventionally processed alloys exhibited breakaway oxidation during cyclic oxidation due to their poor rehealing characteristics. The rapidly solidified, fine-grained alloys that contained less than 1.5 wt.% Si exhibited very protective oxidation behavior. There was considerable evidence of sulfur penetration through the protective chromia scale. The rapidly solidified alloys that contained 1.5 wt.% Si underwent repeated scale spallation that led to breakaway oxidation behavior. The scale spallation was attributed to the formation of an extensive silica sublayer in the presence of sulfur in the atmosphere.

Superplasticity in an AlZnMgCu alloy

Abstract The deformation behaviour of a fine-grained AlZnMgCu alloy was investigated in a broad strain rate interval at temperatures kbetween 672 K and 789 K. Superplastic characteristics were found at all temperatures used at strain rates between 10−4 s−1 and 10−3 s−1. The non-superplastic behaviour at low strain rates (in what is known as region I) was explained taking into account the existence of a threshold stress. Using a suitable value of the threshold stress, region I seems to be a continuation of the superplastic region II with nearly the same values of the strain rate sensitivity parameter m and activation energy. Possible sources of the threshold stress are discussed.

ON THE INTERRELATTON OF THE LUDERS DEFORMATION AND GRAIN BOUNDARY STRUCTURE IN AN ALUMINIUM ALLOY

Strain hardening of the fine-grained alloy AlMg6 at room temperature after annealing and preliminary superplastic deformation (SPD) at different strain rates was studied. On the base of the electron - microscopic investigations, measurements of the conventional yield stress and analysis of strain hardening it is shown that SPD in II and III regions (optimal and high strain rates) transforms the alloy into the state impeding the sink of lattice dislocations into grain boundaries. At the same time the equilibrium state of grain boundaries, observed after annealing and SPD in 1 region (low strain rates ), causes the light generation and sink of dislocations into grain boundaries, and the appearance of the Luders deformation.

Cavitation and fracture in the superplastic Al-33% Cu eutectic alloy

Experiments were conducted on the Al-33% Cu eutectic alloy in both an annealed and an as-extruded condition. For both conditions, the relationship between flow stress and strain rate is sigmoidal with maximum ductilities occurring at intermediate strain rates in the superplastic Region II. Specimens fail by necking at the faster strain rates in Region III, but the severity of necking is reduced with decreasing strain rate and the necks are very diffuse in Region II. There is extensive internal cavitation in the fractured specimens, especially at lower strain rates and in the vicinity of the fracture tip. It was observed that cavities form preferentially on theα-θ interphase boundaries. It is shown by calculation that the observed change from small rounded cavities to large cavities elongated along the tensile axis is reasonably consistent with the theories of cavity growth in fine-grained superplastic alloys

Extended Solubility and Stability in Vapor Quenched Ni3al-X Intermetallics.

ABSTRACTA series of fine-grain and amorphous Ni3Al-X alloys, where X is a solute of 1 to 45 at.% Hf, C or Hf+C in continuously graded compositions, was formed simultaneously into a 0.2 mm-thick disk by high-rate sputter co-deposition. The as-deposited Ni3Al-X alloys exhibited a disordered Ll2 structure and extended solid solubility limits beyond which amorphous phases were formed. Heat-treatment of the alloys at 900°C resulted in fine grain size of 0.5 to 1.5 μm. Ultra-fine dispersoids less than 500 Å in size were observed and they seemed resistant to coarsening.

Properties of Amorphous and Fine-Grained Alloys as Seen by Positron Annihilation

A brief review is given on positron annihilation (PA) results focusing on e+-trapping in metallic glassy alloys of various compositions. The influence of production parameters on the amount and distribution of the frozen-in “free volume” is pointed out. For several compositions a comparison of PA-data with results obtained by other experimental techniques (Mossbauer-effect, resistivity and density measurements, etc.) is given. Structural changes induced by heat treatment, recrystallization, mechanical stress, neutron and electron irradiation are also considered. Different structural models of amorphous alloys (dense random packing, microcrystalline-, quasi-crystalline-model, etc.) are briefly discussed from the point of view of PA. Microcrystallites as fine-grained alloys represent a class of solids of high practical importance. Due to the large specific surface fraction positron and positronium bound states – not existent in “homogeneous” bulk samples – are formed. The correlation between PA-parameters and surface properties of micro- and nanocrystallites is pointed out. Es wird eine Ubersicht der Ergebnisse der Positron-Annihilation (PA) insbesondere bezuglich des Positrontrapping in metallischen Glasern gegeben. Gezeigt wird der Einflus der Herstellungsparameter auf die Verteilung des „freien Volumens”. Fur verschiedene Glas-Zusammensetzungen werden die PA-Ergebnisse mit denen anderer experimenteller Techniken (Massbauereffekt, elektrischer Widerstand, Dichtemessungen, etc.) verglichen. Strukturanderungen als Folge von Warmebehandlung, Rekristallisation, mechanischer Spannung, Neutronen- und Elektronenbestrahlung werden gleichfalls betrachtet. Aus dem Blickwinkel der PA werden verschiedene Strukturmodelle (dichte statistische Packung, mikrokristalline, quasikristalline Modelle, etc.) kurz diskutiert. Mikrokristallite in Gestalt von feinkurnigen Legierungen stellen eine Gruppe von Festkurpern mit erheblicher praktischer Bedeutung dar. Aufgrund der grosen spezifischen Oberflache bilden sich gebundene Positron- und Positronen-Zustande, die in „homogenen”Materialien nicht auftreten. Diskutiert wird der Zusammenhang zwischen PA-Parametern und Oberflacheneigenschaften von Mikro- und Nano-Kristalliten.

The influence of load ratio on fatigue crack growth in 7090-t6 and in9021-t4 p/m aluminum alloys

The rate of fatigue crack growth has been studied over a wide range of growth rates in two high strength P/M aluminum alloys as a function ofR, with particular attention given to crack closure and near-threshold behavior. A principal difference between the two alloys was grain size, being less than oneµm for the alloy IN9021* and greater than five µm for the alloy 7090. No closure was found in the fine-grained alloy nor was there any dependency of the threshold level onR. In contrast, closure was observed in the coarser grained alloy and there was also a dependency of the threshold level onR. The observations are discussed in terms of current models for threshold behavior. Consideration is also given to the overall fatigue crack growth behavior of these P/M in comparison to that of a conventional high strength I/M aluminum alloy.

Anelasticity in fine grained materials

Large anelastic strains of magnitude up to 0.5% have been measured in fine grained (superplastic) alloys of AlCuZr, the AlCu eutectic and the ZnAl eutectoid. These anomalously large strains display similar features to those observed in an earlier study on the SnPb eutectic [J. H. Schneibel and P. M. Hazzledine, Acta metall. 30, 1223 (1982)] namely kinetics of the form ε a ∝ t n ( n is in the range 1 4 − 1 2 ), an approximate linearity in stress and inverse proportionality with grain size for anelastic strains measured at a fixed time after unloading. The magnitude and kinetics of the anelasticity can be explained qualitatively on the basis of the sintering of a distribution of grain boundary cavities given by n( R) = αR q ( q is in the range −2 to −3). Quantitative agreement with the observed effects can be obtained by measuring the grain boundary cavity size distribution in AlCuZr and AlCu eutectic after deformation. On the basis of the measurements of anelastic strain in ZnAl eutectoid information can be obtained on possible mechanisms of cavity nucleation and growth during deformation of this material.

Room-temperature diffusion-induced grain boundary migration in the fine-grained Pd side of CuPd diffusion couples

Transmission election microscopy (TEM) and Auger electron spectroscopy (AES) were used to study interdiffusion between fine-grained (≈150 A ̊ ) Pd films and Cu substrates. TEM demonstrated that a fine-grained alloy having a constant composition (65% CuPd) is formed at room temperature in the Pd side of the couple. Furthermore, the alloy grains are epitaxial to Pd grains at the interface region. The alloy phase is formed by the so-called diffusion-induced grain boundary (GB) migration. The diffusion of Cu atoms into Pd grain boundaries causes the epitaxial nucleation of the alloy phase on the side of the Pd GBs. The alloy phase then grows into the other face, while maintaining the high diffusive nature of the original GBs. The estimated size of the alloy phase was in good agreement with published GB diffusivity data for the diffusion of Cu into Pd GBs.

The composition of oxides grown on PbInAu films by rf oxidation

Oxides grown on PbInAu alloys using a combined thermal and rf oxidation process have been examined with x-ray photoemission spectroscopy (XPS or ESCA) and ellipsometry. The oxides were prepared on alloy films deposited at 300 K containing 13 and 26 at. % In in the PbIn phase and on fine-grain alloy films deposited at 95 K containing 13 at. % In. Following an in situ thermal oxidation at 75 °C, the films were rf-plasma oxidized while mounted on an electrode coated with either In or photoresist. The XPS spectra showed that both lead oxide and indium oxide were present, in agreement with previous findings that the bulk of such oxides consists of a layer of In2O3 overlying a layer of PbO. The binding energy of the Pb 4f peak from the oxide was shifted by 0.9–1 eV from the metal peak, a shift characteristic of PbO. Analysis of the XPS intensities based on a layered structure indicated that oxides on the films deposited at 300 K consisted of about 2.7 nm of In2O3 on top of 1.6 nm of PbO. Oxides on the fine-grained alloy were composed of about 4.3 nm of In2O3 and only about 0.4 nm of PbO. The ellipsometric determination of the total thickness agreed with the XPS thickness. The increased amount of In2O3 on the fine-grained film is attributed to the increased availability of In during oxidation as a result of enhanced diffusion through the grain boundaries of the metal film.

Unusual Combination of High Strength and Ductility in New Precipitation-Hardenable Nickel Silvers

A new series of fine-grained precipitation-hardenable copper alloy is presented. They are characterized above all by an excellent combination of high strength and ductility, and good resistance to atmospheric and tap-water corrosion. Since precipitation is sluggish, alloys do not have to be solution treated after final shape has been given, which reduces considerably the danger of distortion. Furthermore, because of a compensating mechanism in relaxation between matrix and precipitate, dimensional changes during aging are negligible. Finally, machining and cold forming tests under way suggest strongly that these forming processes are readily carried out prior to aging.

Effect of method of manufacture on the operating properties of a bearing material

1. A study was made of the frictional characteristics of specimens of a phosphorus-containing manganese-lead bronze produced by different methods. 2. It is demonstrated that the powder metallurgy technique offers a suitable means of improving the operating properties of copper-base bearing materials. 3. A correlation has been found between the structure and frictional properties of materials. As frictional materials, fine-grained alloys are more suitable for applications where good wear resistance is required, while coarse-grained materials should be employed in those cases where it is desired to reduce losses due to friction to a minimum. 4. The ability to regulate grain size in the manufacture of materials by the powder metallurgy method widens their range of application. 5. An oxidizing atmosphere has a deleterious effect on the operating performance of bearing materials in which lead acts as a solid lubricant.

Lead—strontium alloys for battery grids

The metallurgical properties of lead—strontium alloys were studied in relation to the properties required for lead—acid battery grids. The castability of all the alloys tested was as good as the corresponding alloys containing calcium. The mechanical properties of binary alloys containing 0.08% strontium or more and of ternary alloys containing 0.04% strontium or more indicated that these alloys could be handled and processed by normal production techniques. The creep and stress corrosion properties of the binary and ternary alloys were not good enough to consider them for use in deep cycled motive power batteries but the addition of up to 0.25% silver to the ternary alloys gave adequate creep and stress corrosion characteristics for this application. Ternary alloys containing 1–2% tin suffered from severe intergranular corrosion the effects of which could be overcome to some extent either by grain-refining the alloys with cerium mischmetal or by producing a fine-grained alloy grid by wrought techniques. Alloys which could be considered for battery application were lead-0.1% strontium for standby batteries, lead-0.5% tin-0.1% strontium for automotive batteries and lead-0.5% tin-0.01–0.25% silver-0.1% strontium for special motive power batteries if the cost could be afforded. A small addition of aluminium protected all the alloys from loss of alloying constituents in the molten state by oxidation.

Lead—strontium alloys for battery grids

The metallurgical properties of lead—strontium alloys were studied in relation to the properties required for lead—acid battery grids. The castability of all the alloys tested was as good as the corresponding alloys containing calcium. The mechanical properties of binary alloys containing 0.08% strontium or more and of ternary alloys containing 0.04% strontium or more indicated that these alloys could be handled and processed by normal production techniques. The creep and stress corrosion properties of the binary and ternary alloys were not good enough to consider them for use in deep cycled motive power batteries but the addition of up to 0.25% silver to the ternary alloys gave adequate creep and stress corrosion characteristics for this application. Ternary alloys containing 1–2% tin suffered from severe intergranular corrosion the effects of which could be overcome to some extent either by grain-refining the alloys with cerium mischmetal or by producing a fine-grained alloy grid by wrought techniques. Alloys which could be considered for battery application were lead-0.1% strontium for standby batteries, lead-0.5% tin-0.1% strontium for automotive batteries and lead-0.5% tin-0.01–0.25% silver-0.1% strontium for special motive power batteries if the cost could be afforded. A small addition of aluminium protected all the alloys from loss of alloying constituents in the molten state by oxidation.

The Influence of Composition and Structure on the Sliding Wear of Copper-Tin-Lead Alloys

The friction properties of two groups of copper-based bearing alloys, one with nominal compositions of 6% Sn, and 0, 5, 10, 15, 20, 25% Pb, and the other with 15% Pb and 0, 3, 6, 9, 12 and 15% Sn, were studied in argon and air as well as in various lubricants of 25 ± 1 C. Without lubrication, the presence of lead is essential in that the smeared Pb film, which is formed on the bronze surface by extrusion from the matrix, protects the steel shaft from severe damage. Up to 10%, Pb also lowers the wear of the bearing material itself appreciably. Up to 9%, Sn is beneficial. When a relatively viscous lubricant is applied, a running-in period is followed by a transition towards a fully hydrodynamic condition (f < 0.01). A “critical load” (Lc) exists, below which running-in wear is negligible and above which excessive wear occurs. Under these conditions, alloys with a coarse Pb distribution behave much better than alloys with finely divided Pb. In fine-grained alloys, addition of Pb is unfavorable as Pb lowers the value of Lc, drastically. Also, this effect is explained in terms of a lubricant adsorption mechanism in which are taken into account the lubricant adsorbing function of tin and the heat conductivity of the alloys.

The Effect of Hydrogen on the Work-Hardening and Fracture Behaviour of Titanium–Low-Aluminium–Low-Manganese Alloys

True stress/true strain curves have been determined for two fine-grained (α + β) titanium alloys containing various amounts of hydrogen, which was present either in solid solution or in solid solut...

Secondary recrystallisation in magnox AL 80

Exaggerated grain growth observed in the magnesium canning alloy Magnox AL 80 (Al/0.8 wt %, Be/0.004 wt %) is typical of the phenomenon classified by Burgers as “Secondary Becrystallisation”. Studies upon alloys of varying composition indicate that the presence of 40 ppm beryllium is of particular importance in promoting the effect. Metallographie observations indicate that both marked equilibrium segregation and precipitation occur at the grain boundaries of the fine-grained alloy. It is suggested that normal grain growth is restricted and the conditions for secondary recrystallisation realised, by the combined pinning action of the precipitates and equilibrium segregates. A rotation of 24 1 2 ° or 35 1 2 ° about the [0001]axis accompanies secondary recrystallisation, but the exact role of texture in relation to the phenomenon has not been clearly defined.