Microstructure and mechanical properties of a NiAlMo eutectic composite

Post-solidification heat treatments are shown to have significant effects on tensile properties and high cycle fatigue resistance of a NiMo(31.2 wt%-Al(6.3 wt%) aligned eutectic. In the as-directionally solidified condition retained blocky γ is noted in the γ/γ′ matrix. Solution heat treatment at 1270°C produces a regular cubic network of γ/γ′. Subsequent aging at 850°C produces elliptical precipitates identified as αMo in the γ/γ′ matrix. These precipitates are responsible for a sharp drop in tensile ductility, accompanied by improved resistance to high cycle fatigue.     

Microstructures and mechanical properties of rapidly solidified CuCr alloys

Rapidly-quenched CuCr alloys have been prepared by melt-spinning and the microstructures and mechanical properties examined as a function of alloy content and subsequent annealing treatment. Mechanical properties have been successfully measured by tensile testing on the ribbon samples and it is shown that this method is more suitable than hardness testing for a proper evaluation of the materials. An alloy containing 2% Cr has been prepared in the totally solid-solution state, whilst a 5% Cr alloy contains large chromium particles distributed uniformly throughout the ribbon and fine-grain-sized CuCr solution. These large chromium particles are deduced to arise by a uniform, primary-solidification mode prior to, and independent of, the nucleation and growth of the copper from the lower ribbon surface. The increased solid solubility obtained allows extensive precipitation and strengthening following ageing: the primary chromium particles play an important role both in strengthening and in restricting grain coarsening.     

Microstructural influence of Mn additions on thermoelastic and pseudoelastic properties of CuAlNi alloys

The thermoelastic and mechanical properties of CuAlNiBMn shape memory alloys have been studied as a function of manganese concentration and of heat treatment. Below a limiting value of manganese content, the loss of thermoelastic and pseudoelastic properties has been observed, in particular in the quenched specimens. The partial transformation and its degradation during thermal cycling observed in the low manganese content alloy has been attributed to the lower degree of B2 order achieved during the quench, leading to slower kinetics of DO₃ ordering. The accomodation of strains between martensite variants and between the martensite/austenite phases appear to need dislocation accumulation at their interfaces. The presence of dislocations observed during the reverse transformation seem responsible for the degradation of the transformation and the loss of pseudoelastic properties of this alloy.     

Internal oxidation of NiAl and NiAlSi alloys at the dissociation pressure of NiO

NiAl and NiAlSi alloys were internally oxidized at temperatures of 1073–1273 K by the Rhines Pack method. For the NiAl alloy, the oxidation process follows parabolic law and the oxidation front was flat with severe integranular oxidation occurring at 1073 K and extensive grain boundary sliding at 1273 K. As for NiAlSi alloys, the oxidation rate increased with increase of Si content at 1073 K but the rate decreased at higher temperatures due to total or partial continuous oxide layer formation at the internal oxidation front. The depth of intergranular oxidation was also greatly reduced. For all samples, nickel was found to be transported out to the surface with the amount proportional to the Si content. Lattice diffusion (Nabarro-Herring creep) was believed to be the main cause for nickel transport in the NiAl alloy while dislocation pipe diffusion is the mechanism for NiAlSi alloys.     

Microstructure and mechanical properties of a 2091 AlLi alloy—III. Quantitative analysis of portevin le chatelier instabilities and relation to toughness in AlLi,AlCuMg and AlLiCuMg (2091) alloys

The microstructure and mechanical properties of a 2091 alloy are studied and compared to simpler AlLi and CuMg alloys. For ageing times between 6 and 24 h at 150°C, the 2091 alloy exhibits a toughness drop and a simultaneous change in PLC characteristics (as evidenced by a combination of local and total strain measurements), but no significant change in microstructure, except for the size of δ′ precipitation. SEM in situ tests show that plastic instabilities are always related to extra damage. A quantitative model accounts for the toughness drop, based on plastic dissipation by PLC active bands.     

High temperature properties of ductile CuAlNi shape memory alloys with boron additions

The use of polycrystalline CuAlNi alloys for high temperature applications is restricted to very small shape changes due to their brittle nature. Additions of alloying elements such as manganese and boron have been introduced to improve the ductility of the material. The behaviour of these alloys has been studied in terms of the influence of these elements on the stability of the microstructure after high temperature annealing or after room and high temperature deformation. The results show that the martensitic structure produced by quenching the alloy from the β-temperature has a lower degree of order than that obtained after further annealing at 300°C for up to an hour. Also, the alloys containing higher boron concentrations present a lower degree of order in all cases. Similarly, the ductility has been much influenced by the boron content. The ductility is greater, in particular at high temperatures, in the alloys with lower concentration of boron.     

Nitrogen solubility and nitride formation in FeCrNi alloys

A series of FeCrMnNi alloys was melted under high nitrogen gas pressure. The nitrogen concentration in the solidified metal was found to be experimentally related to the melt pressures that ranged from 0.1 to 200 MPa, to the alloy composition, and to alloy concentration. The nitrogen solubility followed Sievert's law. The activity coefficients determined at these higher pressures for the alloying elements Cr, Mn and Ni were similar to those previously obtained at lower pressures. At the higher nitrogen melt-pressures, the nitrogen concentration exceeded the interstitial nitrogen solubility resulting in the formation of metal-nitrides.     

The effect of microstructure and composition on the properties of vapour quenched AlCr alloys—II. Tensile properties

The effect of chromium and iron additions and of annealing and working on the microstructure and tensile properties of vapour quenched AlCr and AlCrFe alloys has been determined. Tensile strengths of the worked AlCrFe alloys were in the range 568–831 MPa. Chromium in solid solution or iron present as iron-rich precipitates increased the yield stress by 44.7 MPa/at.%Cr and 333 MPa/at.%Fe respectively. The contributions to the yield strength of AlCr alloys were solid solution 40% and dislocation density/cell size 60% and to the yield strength of AlCrFe alloys were solid solution 25%, iron-rich precipitates 42% and dislocation density/cell size 33%. Vapour quenching may allow the more efficient use of alloying elements in the strengthening of Al-alloys and greater flexibility in obtaining the desired combination of solute concentration, particle volume fraction and particle size.     

Grain coarsening in FeNiCr alloys and the influence of second phase particles

The effects of second phase particles, e.g. M₂₃C₆, MC and M(C, N) carbides on the grain growth phenomenon of FeNiCr alloys have been determined. Various theoretical models on grain coarsening have been compared. The grain size at all stages of grain coarsening was dependent on the undissolved carbide particle size (r), the volume fraction (f), and the nature of the carbides. The nature of M₂₃C₆ carbides varied, since Fe, Ni and Mo entered the M₂₃C₆ unit cell; and complex carbides such as (Cr₁₅Fe₄Ni₂Mo₂)C₆ were formed. Gladman's equation was verified for predicting the observed grain size values to a significant degree, and other models were less successful.     

Segregation structures of melt-spun CuSiB alloys and their high temperature deformation behaviour

Rapidly-solidified microcrystalline alloys typically possess a fine-grained matrix with solute distributed in a cellular arrangement, either in the form of enriched cell-wall regions or as second-phase particles at the original cell walls. It is unclear whether optimal strength would be achieved when a second phase is distributed as discrete particles or as a continuous film. The microstructures and mechanical properties of a series of CuSiB alloys were examined both at room temperature and at high temperature. Depending on the alloy considered, fairly stable amorphous films, or discrete particles, arranged in a cell structure, are formed by rapid solidification. The continuous second-phase film is shown to lead to higher average internal stresses and material deformation process, during hot testing, appears to be the same as that controlling deformation in the particle-strengthened materials.