Effect of cobalt additions on the age hardening of Cu-4.5Ti alloy

The effects of 0.9 and 1.8 wt% cobalt additions on the age hardening behaviour of Cu-4.5Ti alloy have been investigated. It has been observed that though Co addition results in the refinement of grain size and the Cu-Ti-Co alloys exhibit age hardening (giving rise to peak hardness on aging at 400°C for 16 hours), the peak hardness as well as the corresponding yield and tensile strengths were found to decrease with increasing cobalt content. The electrical conductivities of 0.9 and 1.8 wt% Co alloys were found to be 6% and 10% International Annealed Copper Standard (IACS) and 7% and 14% IACS in solution treated and peak aged conditions, respectively. Like in the binary Cu-Ti alloys, precipitation of ordered, metastable and coherent Cu₄Ti(¹) precipitate was found to be responsible for maximum strengthening in these alloys. In addition, coarse intermetallic phases of Ti and Co, viz. Ti₂Co and TiCo particles have been observed in all the conditions studied. The inferior mechanical properties of Cu-Ti-Co alloys compared with those of the binary Cu-Ti alloys are attributed to the depletion of Ti from matrix, which is consumed to form Ti₂Co and TiCo phases.     

Decomposition of supersaturated solid solutions in Al–Cu–Mg–Si alloys

The Al–Cu–Mg–Si alloying system is a base for a diverse group of commercial alloys which acquire their properties after quenching and aging. Therefore, the knowledge of the phase composition of hardening precipitates and the conditions under which they are formed is very important. ast reference data were analyzed along with experimental results and calculations of phase equilibria. Different alloys were compared based on the composition of the supersaturated solid solution. It is shown that the phase composition of aging products in alloys with Mg : Si > 1 agrees well with the equilibrium phase composition at a temperature of annealing. However, the sequence of precipitation in the alloys with Mg : Si < 1 is more complicated. The hardening in these alloys occurs with precipitation of the and phases and their precursors. The former phase may contain copper and later transforms either to and (Mg₂Si) or to Q phase depending on the amount of copper and annealing temperature.     

Structure and mechanical properties of internally hydrided Mg-IIIa transition metal alloys

Magnesium alloys containing III_a transition metals, such as Sc, Y and Ho, respectively, were hydrogenated at 773 K and examined for microstructure, X-ray diffraction pattern, micro-Vickers hardness, and tensile properties at room and high temperatures. Results obtained are as follows:

1. The alloys, respectively, have been internally hydrided and have precipitated hydrides of the III_a transition metals as small flake-like particles in the matrix and at grain boundaries, as well as twin boundaries.
2. The dispersed hydride particles do not necessarily contribute to further hardening of the alloys at room temperature and up to near 673 K.
3. However, the dispersed particles are very stable and seem to improve mechanical properties of the alloys above 673 K.
4. Presumed relationships of crystallographic coincidence between the matrix and hydrides have been obtained.

     

Effect of impurities (graphite) on the high-temperature creep properties of NiAl

Previous high-temperature compression creep studies of NiAl have shown peculiar behaviour in the temperature range 700 to 900° C, which was perhaps due to precipitation of impurities in the matrix. To isolate the impurity effect, high-purity NiAl samples with 0.15 at% and 0.20 at% carbon (graphite) additions have been creep tested at four temperatures between 700 and 850° C. Addition of graphite has been shown to produce a significant reduction in the creep strength of the alloyS. However, alloys with higher graphite concentrations have shown better creep resistance than those with lower graphite concentrations. Transmission electron microscopy indicates the presence of competing softening and hardening mechanisms in the alloys. Softening is due to the graphite particles acting as a dislocation source. Hardening results from a grain-boundary hardening mechanism due to the graphite particles segregating at grain boundaries and a dislocation-impurity (fine graphite) interaction, developing a Cottrell-like atmosphere.     

Al-Zn base alloys with dual-phase microstructure

Al-55 wt % Zn alloys with copper or magnesium additions were subjected to various thermo-mechanical treatments and analysed by microscopy, hardness measurements, and tensile tests. A set of different microstructures was produced exhibiting characteristic features of a dual-phase structure: about 30 vol % of a harder zinc-rich phase are dispersed in a softer aluminium-rich matrix. Small amounts of additional compounds such as-(Al, Zn, Cu),-CuZn₄, or-(Al, Zn) could be obtained by various precipitation reactions. It is shown that the combination of conventional precipitation hardening with the new type of microstructure offers promising possibilities for the development of aluminium-base alloys with high deformability and strength.     

The effect of titanium additions onM s of β-CuZn

The effect of titanium additions to binary -CuZn alloys was investigated: the concentration range of at high temperatures (860° C), solid solution hardening of this phase, and the change in martensite temperature,M _s. Titanium in solution produces considerable solid solution hardening, both by replacing copper or zinc. Only replacement of zinc leads to a constant or increasingM _s, while replacing copper decreases it. Ageing of the -phase causes strong hardening and a decrease inM _s. The results have been interpreted considering the role of thermodynamic and mechanical properties in determiningM _s.     

The effect of hot-rolling on chill-cast Al,Al-2 wt % Ni and Al-4 wt % Ni alloys

The effect of hot-rolling on the mechanical properties and microstructure of directionallysolidified hypoeutectic Al-Al₃Ni alloys has been studied. Chill-cast hypoeutectic alloys were produced by casting into pre-heated mild-steel moulds placed on copper chills. The chill-cast Al-2 wt% Ni and Al-4 wt% Ni hypoeutectic alloys can be hot-rolled at 500 C to reductions of greater than 95%. Deformation is achieved by deforming the aluminium-rich dendrites in the rolling direction, followed by interpenetration of the Al₃Ni fibres into the dendrites resulting in a homogeneous microstructure. The variations of room-temperature tensile properties for the chill-cast hypoeutectic alloys were measured as a function of reduction of thickness during hot-rolling. The ultimate tensile strength and yield strength increase during rolling because of increasing Al₃Ni fibre alignment, homogeneous dispersion of the Al₃Ni fibres throughout the Al matrix, and work hardening in the Al matrix. The as-chill-cast alloys have strengths which agree with the composite law of mixtures for a combination of Al dendrites and Al-Al₃Ni eutectic. After hot-rolling, the alloy strengths can be predicted from discontinuous fibre reinforcement theory.     

Influence of aging heat treatment on mechanical properties of biomedical Ti–Zr based ternary alloys containing niobium

Titanium–zirconium based alloys containing a small amount of niobium were investigated in order to evaluate their possible use as biomedical materials. Zirconium, which belongs to the IVa group, is known to have good corrosion resistance and biocompatibility similar to titanium. As the titanium–zirconium system shows a complete solid solution, a wide variation of alloy design is available and large quantities of solid-solution hardening must be possible. Niobium, having a -phase stabilizing effect, was chosen as a ternary element in order to control desirably the microstructure. There have been no reports which suggest its harm to a living body. The alloys containing 2% or 3% niobium showed the highest hardness value after aging heat treatment at 773 K. In contrast to this, no alteration of hardness was seen in specimens aged at 1073 K. Through conventional X-ray diffractometry and in situ X-ray analysis using a hot stage, -phase precipitation in the A matrix was identified. From the above results, it is concluded that alloys containing 2%–3% niobium are hopeful candidates for new kinds of biomedical alloys, when they are heat treated under suitable conditions. © 1998 Kluwer Academic Publishers     

Ordering within precipitates in copper-nickel-titanium alloys

The precipitation process in alloys containing 4% Ti and 5 to 16% Ni was investigated using transmission electron microscopy, hardness and electrical conductivity measurements. After ageing, spherical -Ni₃Ti precipitates with ordered DO₂₂ superlattice were observed in alloys CuTiNi5 to 10 and the L1₂ superlattice in the CuTi4Ni16 alloy. With increasing nickel content the size of precipitates decreases; this has only a minor influence on the age hardening. The CuTiNi10 alloy has the best electrical conductivity approaching 18m^–1mm^–2.     

Carbide precipitation in certain alloys of the Co-Ni-Cr-C system

An investigation is reported of structural and hardness changes during the precipitation of Cr₂₃C₆ from f c c supersaturated solid solution in various alloys of the Co-Ni-Cr-C system containing 25 wt % Cr and 0.15 to 0.25 wt % C. The work extends results previously reported on a Co-25.3 wt % Cr-0.26 wt % C alloy, by studying the effect of adding nickel to replace cobalt (either wholly or completely) and hence changing the stacking fault energy. The addition of 10% nickel was found to increase the nucleation rate of matrix precipitation; precipitate particles also nucleated on partial dislocations with associated stacking fault formation. With approximately 55% nickel and 0.15% carbon, there was little matrix precipitation; instead precipitation occurred predominantly on dislocations, as rods growing along 1 1 0 directions; the main climb morphology was the formation of dislocation dipoles. Substantial precipitation hardening effects were obtained, particularly in the cobalt-rich alloys with 0.25% carbon. In a ternary Ni-25 wt % Cr-0.25 wt % C alloy, a non-uniform precipitate dispersion formed involving nucleation in the matrix and on dislocations. An investigation was also made of the effect of prior strain (1 to 10%) on the ageing of the Co-25.3 wt % Cr-0.26 wt % C alloy. Deformation of the f c c solution-treated material produced faulting and also the formation of h c p phase. On ageing, Cr₂₃C₆ precipitation occurred within the lamellae; at long ageing times after 10% deformation carbide particle coarsening, and matrix recrystallization occurred concurrently, and the recrystallization was accompanied by the transformation of the f c c matrix to h c p phase.     

Crystallization-induced superconductivity in amorphous Ti-Ta-Si alloys

Amorphous alloys containing 0 to 40 at% Ta and 15 to 20 at% Si have been produced in the ternary Ti-Ta-Si system by rapidly quenching the melts using a melt-spinning technique. The amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The superconducting transition temperature, T _c, increased with increasing tantalum content and showed the highest value of 7.6 K for the Ti₄₅Ta₄₀Si₁₅ alloy annealed for 1 h at 1073 K. An upper critical magnetic field, H _c2 of 4.7×10⁶ Am^–1 at 4.2 K and a critical current density, J _c, of 1.5×10⁴ A cm^–2 at zero applied field and 4.2 K were recorded for this alloy. Detailed electron microscopic studies of the crystallization behaviour of the amorphous alloys established that a supersaturated solid solution of tantalum in -Ti with a bcc structure forms first, followed by the precipitation of the bc tetragonal Ta₃Si compound. Since Ta₃Si is not superconducting above 4.2 K, it has been concluded that superconductivity in the crystallized alloys is due to the precipitation of -Ti(Ta) solid solution.     

A study on age hardening in cast Fe-Mn-Al-Si-C alloys

Phase transformations in a group of cast and solution treated (at 1100° C) Fe-Mn-Al-Si-C alloys during isothermal ageing have been investigated as a function of (a) ageing temperature (500 to 800° C), (b) aluminium (5 to 10%) and carbon (0.27 to 0.9%) contents for fixed manganese ( 30%) and silicon ( 1.5%) levels. Irrespective of the ageing temperature and carbon content, 5% Al alloys do not undergo appreciable ageing. Ageing tendency increases with increase in aluminium and carbon contents. Hardening is caused by the precipitation of Al(Fe, Mn)C _x phase inside the austenite grain and Mn₁₂Si₇Al₅ phase at the grain boundaries. The sequence of precipitation of the phases depends on the ageing temperature and composition. Cold working by rolling, after homogenization at 1100° C, accelerates ageing.     

Effect of manganese on the superconductivity of aluminum

The superconducting critical fields of pure aluminum and of two AlMn alloys, containing 440 and 900 ppm Mn, have been measured as a function of temperatureT from their transition temperaturesT _c down toT=0.07 K. The observedT _c depression can be understood in terms of Kaiser's theory for nonmagnetic localized states in superconducting alloys, with an effective Coulomb coupling constantN _d (0)U _eff equal to 0.43 for the AlMn alloys. Additional evidence that the Mn impurities do not possess a well defined magnetic moment in aluminum in this temperature range is obtained from the critical field results. In particular, the BCS law of corresponding states is obeyed by these alloys. We conclude that these AlMn alloys can be considered BCS superconductors, with a pairing interaction which is weaker than that in pure aluminum.Research supported by the National Science Foundation.     

Modeling of precipitation hardening in Mg-based alloys

This work deals with the development of Mg-based alloys with enhanced properties at elevated temperatures. This is achieved by precipitation of binary phases such as MgZn₂ and Mg₂Sn during the aging of these alloys. The aim of the present work is to develop and calibrate a model for precipitation hardening in Mg-based alloys, as different types of precipitates form simultaneously. The modified Langer-Schwartz approach, while taking into account nucleation, growth and coarsening of the new phase precipitations, was used for the analysis of precipitates’ evolution and precipitation hardening during aging of Mg-based alloys. Two strengthening mechanisms associated with particle-dislocation interaction (shearing and bypassing) were considered to be operating simultaneously due to particle size-distribution. Parameters of the model, R _N i and k _{σ i} , were found by fitting of calculated densities and average sizes of precipitates with ones estimated from experiments. The effective diffusion coefficients of phase formation processes, which determine the strengthening kinetics, were estimated from the hardness maximum positions on the aging curves.     

Superconductivity of copper containing small amounts of niobium

Superconductivity of copper containing small amounts of niobium has been investigated by measuring the electrical resistivity, superconducting volume fraction and by metallographic studies. Small amounts of niobium added to copper has a drastic effect on the low temperature resistivity of the alloys, The annealed alloy Cu_99.5Nb_0.5 shows zero resistance at a current density of 200 A cm^–2 below 3K. The estimated superconducting volume fraction of this alloy at 2K is about fifty times the physical volume fraction of the Nb in the alloy.When more Nb is added these effects unexpectedly become much smaller than those observed in the dilute alloys (< 1.5 at. % Nb). Metallographic results indicate that in all the Cu-Nb alloys studied there are two distinct types of Nb particles in the Cu matrix. The large particles (average size 10 m) randomly distributed in the alloy are probably formed at high temperature when the bulk of the alloy is still in the liquid state. The small Nb particles (size 1 m, interparticle distances < 0.2 m) probably form through a solid state precipitation. It has been found that the large precipitates are more abundant in the alloy containing more than 1.5 at. % Nb, than in alloys containing less than 1.5 at. % Nb. The observed superconducting properties of alloys Cu_99.8Nb_0.2 and Cu_99.5Nb_0.5 have been attributed to the proximity effect of the small Nb particles whose interparticle distances are compatible with the coherence length in the Cu matrix. The very wide superconducting transition shown in both the resistivity and the inductance measurements suggested a distribution in the Nb particle sizes as well as in the interparticle distances.     

Mechanical properties of unidirectionally transformed Cu-In alloys

Comprehensive hardness measurements and limited tensile tests have been made on eutectoid and off-eutectoid Cu-In alloys. The alloys were transformed by unidirectional heat-treatment techniques using a range of imposed growth rates which resulted in alignment of the pearlite microstructure and a range of interlamellar spacing. Room temperature hardness of the as-transformed alloys was found to vary linearly as a function of ^–1/2, where is the pearlite interlamellar spacing; the alloys were calculated to have cooled from the decomposition temperature at rates in the range 0.047 to 18.6 K min^–1, which could result in variations of precipitation/coarsening reactions in the pearlitic phases contributing towards strength, as well as the lamellae interfaces. Hardness was also measured as a function of temperature and indicated a change in strengthening mechanism at 550 K, thought to indicate the temperature above which no significant strengthening was contributed by the pearlitic interfaces. Tensile failure of the lamellar structure occurred in a manner identical to directionally aligned Al-CuAl₂ eutectics.     

Solidification and precipitation behaviour of Al-Si-Mg casting alloys

The effect of Mg content on the solidification and precipitation behaviour of both unmodified and Sr-modified Al-7Si-Mg casting alloys has been investigated at various solidification rates using cooling curve analysis, differential scanning calorimetry (DSC) and optical and electron microscopy. The Mg concentrations covered the range from 0.3 wt% to 0.7 wt%. The results indicate that increasing Mg content or cooling rate lowers the liquidus and binary Al-Si eutectic transformation temperatures. The latent heat of fusion of these alloys is strongly dependent on the level of Si present, but there is no observed dependence on Mg content. The solidification reactions observed under DSC are identified and it is noticed that the ternary eutectic solidification reaction L Al + Si + Mg₂Si is only observed at Mg levels of 0.6% and higher. The minor phases formed on solidification are identified and their response to solution heat treatment is examined. Increasing Mg content usually enhances precipitate hardening. However when Mg levels are increased above 0.5wt%, no apparent increase of yield strength with Mg is observed. This is correlated with dissolved Mg levels and energy released during reprecipitation.     

Thermal stability and mechanical properties of nanocrystalline Fe–Ni–Zr alloys prepared by mechanical alloying

The thermal stability of nanostructured Fe_100?x?y Ni _x Zr _y alloys with Zr additions up to 4 at.% was investigated. This expands upon our previous results for Fe–Ni base alloys that were limited to 1 at.% Zr addition. Emphasis was placed on understanding the effects of composition and microstructural evolution on grain growth and mechanical properties after annealing at temperatures near and above the bcc-to-fcc transformation. Results reveal that microstructural stability can be lost due to the bcc-to-fcc transformation (occurring at 700 °C) by the sudden appearance of abnormally grown fcc grains. However, it was determined that grain growth can be suppressed kinetically at higher temperatures for high Zr content alloys due to the precipitation of intermetallic compounds. Eventually, at higher temperatures and regardless of composition, the retention of nanocrystallinity was lost, leaving behind fine micron grains filled with nanoscale intermetallic precipitates. Despite the increase in grain size, the in situ formed precipitates were found to induce an Orowan hardening effect rivaling that predicted by Hall–Petch hardening for the smallest grain sizes. The transition from grain size strengthening to precipitation strengthening is reported for these alloys. The large grain size and high precipitation hardening result in a material that exhibits high strength and significant plastic straining capacity.     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.