Effect of aluminum on precipitation hardening in Cu–Ni–Zn alloys

The effect of aluminum on the precipitation hardening of Cu–Ni–Zn alloys with varying aging temperatures and times was investigated in this article, in the hope to achieve better mechanical properties. Vickers hardness, tensile, and electrical conductivity tests were carried out to characterize the properties of the Cu–Ni–Zn alloys with or without an addition of aluminum subjected to different aging treatments. The results show that an addition of 1.2 wt% aluminum can play an influential role in the precipitation hardening of the Cu–Ni–Zn alloys. For example, it can increase the peak hardness from 58 Hv for the solution-treated Cu–10Ni–20Zn alloy to 185 Hv for the solution-treated Cu–10Ni–20Zn–1.2Al alloy during aging at 500 °C. The yield strength, tensile strength, and electrical conductivity of the Cu–10Ni–20Zn–1.2Al alloy subjected to suitable treatments under prior cold-rolled and aged conditions can reach 889 MPa, 918 MPa, and 10.96% IACS, respectively, being much higher than those of the relevant alloy without aluminum and comparable to those of the Cu–Be alloys (C17200 and C17510). According to the transmission electron microscope observations, it was found that formation of nanosized precipitates with the L1₂-type ordered lattice results in precipitation hardening, and an orientation relationship of [011]_\textp//[011]_\textm [011]_{\text{p}}//[011]_{\text{m}} and (100)_\textp//(200)_\textm (100)_{\text{p}}//(200)_{\text{m}} exists between the precipitates and the α-Cu matrix.     

Hexagonal martensite decomposition and phase precipitation in Ti–Cu alloys

The mechanical behavior of Ti–Cu alloys can be improved by controlling Ti₂Cu precipitation. In eutectoid alloys, such precipitation can be achieved by the decomposition of martensite in response to aging heat treatment. The purpose of this work is to discuss the evolution of precipitates during the decomposition of hexagonal martensite in Ti–Cu alloys. First, samples with near-eutectoid compositions were prepared in an arc furnace equipped with a non-consumable tungsten electrode and water-cooled copper hearth under a high purity argon atmosphere. After chemical homogenization at a temperature in the beta field, the samples were water-quenched and examined by differential scanning calorimetry and high-temperature X-ray diffraction. The results indicate that rapidly quenched near-eutectoid Ti–Cu alloys present Ti₂Cu precipitates. Regardless of the cooling rate applied, such precipitation is unavoidable. No evidence of beta phase stabilization was found in the rapidly quenched samples. Precipitation temperatures of coherent and incoherent phases of 415 °C and 550 °C, respectively, were determined from the differential scanning calorimetry measurements. Ti₂Cu precipitation was examined in situ by high temperature X-ray diffraction experiments. The total decay of martensite was found to occur above 575 °C. Vickers hardness testing of aged samples revealed a correlation between phase precipitation and hardening.     

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.     

Effect of prior cold work on age hardening of Cu-4Ti-1Cd alloy

This research is part of a project whose scope was to develop high strength ternary alloys based on Cu-Ti system with the primary aim of substituting them for toxic and expensive Cu-Be alloys. In this pursuit, age hardening behaviour of Cu-4Ti-1Cd alloy has already been investigated and the present paper reports the investigations on the influence of prior cold work by rolling of 50, 75 and 90% on the age hardening of a Cu-4Ti-1Cd alloy using hardness and tensile tests and optical as well as transmission electron microscopy. As a result of cold work followed by aging, hardness of the alloy increased from 237 Hv in solution treated condition to 425 Hv on 90% cold work and peak aging. Similarly, yield and tensile strengths of the alloy reached maxima of 1037 and 1252 MPa respectively on 90% deformation and peak aging. The microstructure of the deformed alloy exhibited elongated grains and deformation bands. The maximum strength on peak aging was obtained due to precipitation of ordered, metastable and coherent β^l, Cu₄Ti phase in addition to high dislocation density and deformation twins. Both hardness and strength of the alloy decreased on overaging due to the formation of incoherent and equilibrium β, Cu₃Ti phase. However, the morphology of the discontinuous precipitation was changed to globular shape due to large deformations and overaging.     

Microstructural change and precipitation hardening in melt–spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified bymelt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400%C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200%C in the Mg–Al–Si–Caalloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200%C for 1h.     

Development of microstructure in cast Mg–AI–rare earth alloys

Abstract

The microstructures and age hardening behaviours of a series of Mg–Al–rare earth (RE) alloys that had been either pressure die cast or permanent mould cast were investigated by SEM and analytical TEM. Two types of phases, Al₄MM and Al₁₂Mg₁₇, were found in the as cast alloys and no pseudoternary Mg–Al–RE phases were present. The Al₄MM phase was thermally stable during solution treatment at temperatures as high as 500°C, whereas Al₁₂Mg₁₇ partially dissolved in the α-Mg matrix during solution treatment at 420°C. No rare earth containing precipitates formed during heat treatment of the investigated alloys but two types of Al₁₂Mg₁₇ precipitation took place. Colonies of discontinuous precipitation containing alternate lamellae of α-Mg and Al₁₂Mg₁₇ formed preferentially in regions α-Mg with high aluminium content. Spheroidisation and coarsening of the discontinuous precipitates occurred after aging at 200°C. Continuous precipitation of Al₁₂Mg₁₇ also occurred and these precipitates had a rodlike morphology and grew in preferred crystallographic directions.

MST/3382     

Accelerated precipitation behavior of cast Mg-Al-Zn alloy by grain refinement

This study demonstrates that the precipitation behavior of β-Mg₁₇Al₁₂ phase during aging and the resultant variation in hardness and mechanical properties of cast Mg-Al-Zn alloy are strongly dependent on initial grain size. Grain size reduction accelerates discontinuous precipitation at the early stage of aging treatment by increasing the area fraction of grain boundaries that can act as nucleation sites for discontinuous precipitates (DP), but it does not influence DP growth rate. Grain refinement also prematurely terminates continuous precipitation because the formation of a large number of DP reduces the amount of Al dissolved in the matrix, which is required for the formation of continuous precipitates (CP). This promotion of DP formation and early termination of CP formation significantly decrease the peak-aging time to one-third. The enhanced precipitation behavior also leads to an additional hardness improvement in the aged alloy, along with an increase in hardness owing to grain boundary strengthening by grain refinement. The amount of increase in hardness changes with aging time, which is determined by the variation of three variables with aging time: DP fraction difference between refined and nonrefined alloys, hardness difference between DP and matrix, and matrix hardness difference between the two alloys. Grain refinement improves both tensile strength and ductility of the homogenized alloy owing to grain boundary strengthening and suppression of twinning activation, respectively. However, the loss of ductility after peak-aging treatment is greater in the refined alloy because of the larger amount of DP acting as a crack source in this alloy.     

Constitution and age hardening of Al-Sc alloys

Aluminium-rich alloys from the Al-Sc system were examined to determine the form of the equilibrium phase diagram and to obtain information on age hardening of chill cast alloys. Samples containing up to 8.75wt% Sc were examined using thermal analysis and optical microscopy. This work indicated a eutectic type of phase diagram with a eutectic temperature of about 665° C and a eutectic composition of about 0.6wt% Sc. The scandium-rich primary phase was found to be ScAl₃ which is f c c with a lattice parameter of 0.4105nm. Chill cast samples of a 1 wt% Sc alloy were examined for their age hardening behaviour over the temperature range of 225 to 360° C. A maximum hardness of 77 VHN was obtained after ageing at 250° C for 3 days. This hardness was retained after ageing for a total of at least 12 days. The hardening precipitates were ScAl₃ which were observed to form via a discontinuous precipitation mechanism. The ScAl₃ precipitates were observed to have a parallel orientation relationship with the matrix.     

Yield strength of twin-belt cast Al–Mg–Sc–Zr alloy after annealing

AA5xxx aluminium alloys are used in the automotive and packaging industries owing to their high strength and ductility. The addition of Sc and Zr to these alloys has shown promise for improving high temperature stability and therefore broadening the range of applications. This high temperature stability is due to the formation of fine Al₃(Sc, Zr) precipitates during aging. In this work, two twin-belt cast Al–3%Mg alloys, one with 0·4% Sc and the other without Sc, were annealed at 300 and 400°C. Hardness, tensile yield stress and electrical resistivity measurements were used to examine the evolution of microstructure and strength of the alloys. These results were then utilised to develop a yield stress–precipitation model to describe simultaneous precipitation hardening and recovery.     

Characterization of the evolution of the volume fraction of precipitates in aged AlMgSiCu alloys using DSC technique

Differential scanning calorimetry is used to quantify the evolution of the volume fraction of precipitates during age hardening in AlMgSiCu alloys. The calorimetry tests are run on alloy samples after aging for various times at 180 °C and the change in the collective heat effects from the major precipitation and dissolution processes in each run are used to determine the precipitation state of the samples. The method is implemented on alloys with various thermal histories prior to artificial aging, including commercial pre-aging histories. The estimated values for the relative volume fraction of precipitates are compared with the results from a newly developed analytical method using isothermal calorimetry and a related quantitative transmission electron microscopy work. Excellent agreement is obtained between the results from various methods.     

Microstructural control of maraging steel C300

Abstract

The microstructure evolution and precipitation kinetics of maraging steel C300 have been studied in the aging temperature range from 400 to 600°C. The relation between mechanical properties and precipitation hardening response is explained, and modelling is used to optimise the properties. Ultrafine needle shaped Ni₃Ti phase is the main strengthening precipitate in maraging C300, and it shows very high resistance to coarsening. A spherically shaped Fe₂Mo phase is formed at higher temperatures and in the overaged condition. Inter- and intralath reverted austenite nucleates at higher temperature (～600°C). Rolling and aging treatment can produce the highest hardness by a combination of work hardening and precipitation strengthening. Microstructural evolution simulation using Monte Carlo modelling has been applied to this alloy, and the modelling has been validated by the experimental results.     

Joint effect of quasicrystalline icosahedral and L12-strucutred phases precipitation on the grain structure and mechanical properties of aluminum-based alloys

Dispersoid hardening is a key factor in increasing the recrystallization resistance and mechanical strength of non-heat treatable aluminum-based alloys.Mn and Zr are the main elements that form dispersoids in commercial Al-based alloys.In this work,the annealing-induced precipitation behavior,the grain struc-ture,and the mechanical properties of Al-3.0Mg-1.1 Mn and Al-3.0Mg-1.1 Mn-0.25 Zr alloys were studied.The microstructure and the mechanical properties were significantly affected by annealing regimes after casting for both alloys.The research demonstrated a possibility to form high-density distributed quasicrystalline-structured I-phase precipitates with a mean size of 29 nm during low-temperature annealing of as-cast alloys.Fine manganese-bearing precipitates of Ⅰ-phase increased recrystallization resistance and significantly enhanced the mechanical strength of the alloys studied.The estimated strengthening effect owing to Ⅰ-phase precipitation was 150 MPa.Due to the formation of L12-structured Al3Zr dispersoids with a mean size of 5.7 nm,additional alloying with Zr increased yield strength by about 90 MPa.The L12-phase strengthening effect was estimated through the dislocation bypass looping and shearing mechanisms.     

Mechanical properties of Cu–Cr system alloys with and without Zr and Ag

The effects of addition of Zr and Ag on the mechanical properties of a Cu–0.5 wt%Cr alloy have been investigated. The addition of 0.15 wt%Zr enhances the strength and resistance to stress relaxation of the Cu–Cr alloy. The increase in strength is caused by both the decrease in inter-precipitate spacing of Cr precipitates and the precipitation of Cu₅Zr phase. The stress relaxation resistance is improved by the preferentially forming Cu₅Zr precipitates on dislocations, in addition to Cr precipitates on dislocations. The addition of 0.1 wt%Ag to the Cu–Cr and Cu–Cr–Zr alloys improves the strength, stress relaxation resistance and bend formability of these alloys. The increase in strength and stress relaxation resistance is ascribed to the decrease in inter-precipitate spacing of Cr precipitates and the suppression of recovery during aging, and to the Ag-atom-drag effect on dislocation motion. The better bend formability of the Ag-added alloys is explained in terms of the larger post-uniform elongation of the alloys.     

Precipitates,zones and transitions during aging of Al-Zn-Mg-Zr 7000 series alloy

Abstract

Age hardening of an industrial 7000 series alloy in the temperature range 70-150° C has been followed by mechanical testing, electrical conductivity measurement, differential scanning calorimetry and extensive electron microscopy (TEM). The property changes during aging are interpreted in terms of structural transformations that involve two types of Guinier-Preston (GP) zones (I and II), the metastable hardening precipitate η′ and the stable phase η-MgZn₂, as well as coarsening, changes of composition and internal order within zones and precipitates. Time-temperature ranges of the transformations during aging, and its dependence on quenching temperature, are estimated from TEM observations. The role of the GP(II) zones in the aging of alloys quenched from temperatures above 450°C is emphasized.     

The effect of heat treatments on the creep–rupture properties of a wrought Ni–Cr heat-resistant alloy at 973 K

The effect of heat treatments on the creep–rupture properties was investigated on a wrought Ni–Cr heat-resistant alloy at 973 K. Short-time aging (aging for 3.6 ks (1 h) at 973 K) was made on the solution-treated specimens with different grain sizes. The fine-grained specimen (the grain diameter, d = 45.2 μm) produced by short-time solution treatment exhibited almost the same rupture life and superior creep ductility as those of the medium-grained specimen (d = 108 μm) produced by normal solution treatment. The fine-grained specimen and medium-grained specimen showed the longer rupture life compared with the specimen with recommended aging. The principal strengthening of specimens was attributed to the precipitation hardening by γ′ phase particles. The fine-grained specimen had the highest hardness, and the increase of the hardness was observed in both the fine-grained and the medium-grained specimens during creep at 973 K. However, coarse-grained specimen (d = 286 μm) with high-temperature long-time solution treatment exhibited significantly short rupture life owing to insufficient precipitation hardening after the short-time aging and during creep. Ductile intergranular fracture with dimples occurred in the fine-grained specimen, while brittle intergranular fracture was observed in the medium-grained specimen and in the specimen with recommended aging. Both transgranular fracture and brittle intergranular fracture were observed in the coarse-grained specimen. A simple heat treatment composed of short-time solution treatment and short-time aging is applicable to high-temperature components of wrought Ni–Cr alloys.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

Precipitation evolution and creep strength modelling of 25Cr20NiNbN austenitic steel

25Cr-20Ni-Nb-N is a high strength and oxidation-resistant austenitic stainless steel intended for Ultra-Supercritical (USC) power plants. In this work, the precipitation evolution, and creep strength at 650 and 750°C for up to 100?000?h are predicted. Six precipitates are considered in the thermokinetic calculation by MatCalc: M₂₃C₆, η (Cr₃Ni₂SiN), σ, G, Z, Nb(C,N). For the creep strength prediction, three hardening mechanisms are taken into account: dislocation, precipitation, and solid solution hardening. Both matrix composition and precipitation evolution, calculated with MatCalc, are used for modelling the precipitation and solid solution hardening. It is found that the dislocation hardening, followed by precipitation hardening gives the largest contribution to the creep strength. The most important precipitates strengthening phases are found to be Z-Phase and Nb(C,N), which are nucleated at the dislocations. The model for the creep rate can represent how the creep exponent is raised with increasing applied stress and reduced temperature.     

Precipitation hardening in Al-Zn-Mg-Al2O3(p) composite

The precipitation hardening of a Al-Zn-Mg-Al₂O₃(p) composite is explored. It is found that the peak hardness achieved is almost double that of precipitation hardening of Al-Zn-Mg alloy or dispersion strengthening of Al-Zn-Mg with 5% Al₂O₃(p). Toughness is marginally improved and tensile strength is one and half times that of precipitation hardened Al-Zn-Mg alloys. The ageing time for peak hardness is reduced due to acceleration of formation of precipitate.     

Grain size-dependent Mg/Si ratio effect on the microstructure and mechanical/electrical properties of Al-Mg-Si-Sc alloys

Al-Mg-Si-Sc alloys with different Mg/Si ratio (<1.73 in wt.% vs>1.73 in wt.%) and different grain size (coarse grains vs ultrafine grains) were prepared, which allowed to investigate the grain size-dependent Mg/Si ratio effect on the microstructural evolution and concomitantly on the hardness and electrical conductivity when subjected to aging at 200 °C. In the coarse-grained Al-Mg-Sc-Sc alloys, the β″ precipitation within the grain interior and also the precipitation hardening were highly dependent on the Mg/Si ratio, while the electrical conductivity was slightly affected by the Mg/Si ratio. A promoted β″ precipitation was found in the case of Si excess (Mg/Si ratio <1.73), much greater than in the case of Mg excess (Mg/Si ratio>1.73). While in the ultrafine-grained Al-Mg-Si-Sc alloys, the electrical conductivity rather than the hardness was more sensitive to the Mg/Si ratio. The alloy with Si excess displayed electrical conductivity much higher than its counterpart with Mg excess. This is rationalized by the grain boundary precipitation promoted by Si, which reduced the solute atoms and precipitates within the grain interior. Age softening was found in the ultrafine-grained alloy with Si excess, but the ultrafine-grained alloy with Mg excess held the hardness almost unchanged during the aging. The hardness-conductivity correlation is comprehensively discussed by considering the coupling effect of Mg/Si ratio and grain size. A strategy to simultaneously increase the hardness/strength and electrical conductivity is proposed for the Al-Mg-Si-Sc alloys, based on present understanding of the predominant factors on strengthening and conductivity, respectively.     

Precipitation and aging in high-conductivity Cu–Cr alloys with additions of zirconium and magnesium

Abstract

The precipitation reactions responsible for age hardening in a high-conductivity Cu–Cr–Zr–Mg alloy have been investigated by analytical transmission electron microscopy and compared briefly with the processes that occur in simpler Cu–Cr and Cu–Cr–Mg alloys. Aging at low temperatures (400°C) results in the formation of Guinier–Preston zones. Peak hardness, obtained by aging for 24 h at 450°C, is found to be a result of the fine scale precipitation of an ordered compound, possibly of the Heusler type, with the suggested composition CrCu₂(Zr, Mg). Overaging results in the formation of coarse precipitates of Cr and CU₄Zr. The intergranular precipitate which forms in the Cu–Cr–Zr–Mg alloy is Cu₄Zr. This phase precipitates both as discrete particles on the grain boundaries and as thin ( ~ 5 nm) continuous intergranular films.

MST/89