Precipitation-hardened Alloys Research Articles

Quantitative prediction of the mechanical properties of precipitation-hardened alloys with special application to Al–Mg–Si

Proper design of the heat treatments of metallic materials is crucial for creating a suitable microstructure resulting in the desired properties. In this paper, we present a multi-component simulation procedure for the quantitative prediction of yield strength, fracture strain, and fracture toughness of precipitation-hardened alloys, with special emphasis on Al–Mg–Si alloys. The procedure combines thermokinetic and micromechanical models, taking the microstructural information in terms of different populations of various second-phase particles into account. The predictions are compared to the results of experiments performed on AA6082 samples subjected to different artificial aging times, and good quantitative agreement is demonstrated.

Heterogeneous precipitation behavior and stacking-fault-mediated deformation in a CoCrNi-based medium-entropy alloy

Combining high strength and good ductility is highly-desired yet challenging for conventional structural materials. Newly emerging concentrated multi-component alloys with face-centered-cubic structure provide an ultra-ductile matrix, and the precipitation hardening based on these alloys provides a very effective way to achieve a superior strength-ductility combination. Here, we report a high-strength CoCrNi-based medium-entropy alloy hardened by nanoscale L12-(Ni, Co, Cr)3(Ti, Al)-type particles with mixing heterogeneous and homogeneous precipitation behaviors. Compared to the single-phase CoCrNi medium-entropy alloy, the yield strength and the tensile strength of the precipitation-strengthened CoCrNi medium-entropy alloy were increased by ∼70% to ∼750 MPa and ∼44% to ∼1.3 GPa, respectively, whereas a good ductility, ∼45%, was still achieved. The underlying deformation micro-mechanisms were systematically investigated using transmission electron microscope. In the single-phase CoCrNi medium-entropy alloy, the deformation mode was dominated by mechanical twinning. In the precipitation-hardened medium-entropy alloy, however, a high density of stacking faults prevailed. We revealed that the absence of mechanical twinning in this low stacking fault energy precipitation-strengthened medium-entropy alloy could relate to the increasing critical twinning stress affected by the channel width of the matrix. We further calculated that the increment of the yield strength was substantially from precipitation strengthening. Our present findings provide not only a fundamental understanding of the deformation micro-mechanism of the precipitation-strengthened CoCrNi medium-entropy alloy but also a useful guidance for the development of precipitation-hardened concentrated multi-component alloys in the future.

Experimental Investigation on Metallurgy of High Vacuum Electron Beam Welded Ni Base Alloy Inconel 718

The excellent weld quality of Inconel 718, among the class of Ni, Cr precipitation hardenable alloys is well recognised. Electron Beam Welding (EBW) (especially the high vacuum) is the most preferred technique, to impart the optimum level of properties, as is demanded for many of its critical applications in the fields of Nuclear power and Aerospace. This paper examines in detail the microstructure of weldments obtained in different material conditions i.e. with parent material in solution treated condition, in double aged condition and the ones weld in solution treated condition and administered varied post weld heat treatments, along with compositional analysis i.e. EDAX, and the effect of weld heat on grain structure. This paper also highlights and contrasts evaluation of weld properties of Inconel 718 by Electron Beam Welding in comparison with laser beam welding (LBW).

Experimental Investigation on Metallurgy of High Vacuum Electron Beam Welded Ni Base Alloy Inconel 718

The excellent weld quality of Inconel 718, among the class of Ni, Cr precipitation hardenable alloys is well recognised. Electron Beam Welding (EBW) (especially the high vacuum) is the most preferred technique, to impart the optimum level of properties, as is demanded for many of its critical applications in the fields of Nuclear power and Aerospace. This paper examines in detail the microstructure of weldments obtained in different material conditions i.e. with parent material in solution treated condition, in double aged condition and the ones weld in solution treated condition and administered varied post weld heat treatments, along with compositional analysis i.e. EDAX, and the effect of weld heat on grain structure. This paper also highlights and contrasts evaluation of weld properties of Inconel 718 by Electron Beam Welding in comparison with laser beam welding (LBW).

Microstructure, accumulated strain, and mechanical behavior of AA6061 Al alloy severely deformed at cryogenic temperatures

The combination of Severe Plastic Deformation (SPD) and cryogenic temperatures can be an efficient way to obtain metals and alloys with very refined microstructure and thus optimize the strength-ductility pair. However, there is still a lack of studies on cryogenic SPD process and their effects on microstructure and mechanical properties, especially in precipitation-hardenable aluminum alloys. This study describes the effect of low temperature processing on microstructure, aging kinetic and tensile properties of AA6061 Al alloy after cryo-SPD. Samples of AA6061 Al alloy in the solutionized state was processed by Equal-channel angular pressing (ECAP) at 77 K and 298 K, up to accumulate true strains up to 4.2. Results indicated that the aging kinetic is accelerated when deformation is performed at cryogenic temperature, dislocation density measurement by x-ray and diffraction analysis at TEM achieved a saturation level of 2×1015 m−2 by ECAP at 298K and 5×1015 m−2 after cryogenic ECAP plus precipitation hardening. The same level of yield strength was observed in both deformation procedures but an improvement in uniform elongation was achieved by cryogenic ECAP followed by a T6 treatment

Friction stir welding of aluminium alloys: An overview of experimental findings – Process, variables, development and applications

The never ending appetite of the mankind to produce more and more competitive products results in continuous development of newer and newer manufacturing processes. One of such a kind, a solid state welding process highly appreciated for joining of a variety of aluminium and copper alloys, is friction stir welding. The process is also an accomplished method for joining dissimilar materials efficiently. The process finds its major application for joining hard-to-weld metals, especially the precipitation hardenable aluminium alloys and is widely adopted by industries for the welding of such aluminium alloys. However, the process has still not found an economical way for welding of steels and hence found limited applications in industries for welding steels. This paper aims at providing a comprehensive review of the work undertaken in the field of friction stir welding and provides an insight into the friction stir welding of aluminium alloys. The article pays critical attention and analytical evaluation of classification of aluminium alloys, friction stir welding process parameters, the mechanical testing and properties of the friction stir welding joints, macrostructure and microstructure evolution during friction stir welding, friction stir welding defects and industrial applications of the process. The friction stir welding process variants are discussed as well. Special accentuation has been given to (i) effect of friction stir welding parameters on the microstructure evolved and thus the ultimate mechanical properties (viz. tensile strength, hardness, fatigue strength, fracture toughness and residual stresses), (ii) the texture formation, microstructure refinement and the role of intermetallics. However, studies related to welding of dissimilar aluminium alloys, temperature, and heat transfer modeling and material flow are out of the scope of this paper. Finally, the directions of future research are examined.

Microstructure and property evolution of Fe-N ferrite undergoing early-stages of precipitation

The precipitation-hardening phenomenon is well-known in nitrogen-supersaturated ferrite and its precipitation sequence has received extensive attention. Thus far the α″-Fe16N2 phase has been known as the main hardening precipitates in the alloy upon ageing at low temperatures. Here we report that distinct precursors (pre-α″) of the α″-phase exist and they also play a crucial role in the precipitation-hardened alloy. Using (in-situ) high-resolution transmission electron microscopy, property characterization and first-principle energy calculations in association with varying thermal processes, it is shown that lying on the {001}α planes, the pre-α″ precipitates have a plate-like morphology and are the major hardening precipitates in natural aged Fe-N ferrite. They are stable without much change even after tempering at 60°C for 2h. Furthermore, the plate-like pre-α″ precipitates typically consist of needle-like domains, due to energy minimization. The in-situ observations demonstrate that the precipitation sequence upon ageing is as follows: N-supersaturated α-Fe→N-rich clusters→pre-α″ (GP zones)→α″, where GP zones stands for Guinier-Prestone zones. The pre-α″ precipitates can have even more significant effects on properties of the alloy, as compared with the well-known α″ precipitates.

Origin of unusual fracture in stirred zone for friction stir welded 2198-T8 Al-Li alloy joints

Friction stir welded (FSW) joints of conventional precipitation-hardened aluminum alloys usually fracture in the lowest hardness zone (LHZ) during tension testing. However, all of the FSW joints of a 2198-T8 Al-Li alloy fractured in the stirred zone (SZ) instead of the LHZ with the welding parameters of 800rpm-200mm/min and 1600rpm-200mm/min under the condition that no welding defects existed in the SZ. The experiment results revealed that lazy S was not the dominant factor resulting in the unusual fracture. The SZ consisted of three subzones, i.e., the shoulder-affected zone, the pin-affected zone, and the transition zone between them. While the former two zones were characterized by fine and equiaxed recrystallized grains, incompletely dynamically recrystallized microstructure containing coarse elongated non-recrystallized grains was observed in the transition zone. The transition zone exhibited the lowest average Taylor factor in the SZ, resulting in a region that was crystallographically weak. Furthermore, obvious lithium segregation at grain boundaries was observed in the transition zone via time-of-flight secondary ion mass spectroscopy analysis, but not in the shoulder-affected zone or the pin-affected zone. The combined actions of both the two factors resulted in the appearance of preferential intergranular fracture in the transition zone and eventually caused the failure in the SZ. The lithium segregation at grain boundaries in the transition zone was closely associated with both the segregation in the base material and the partially dynamically recrystallized microstructure resulting from the inhomogeneous plastic deformation in the SZ.

Investigation on Mode I Propagation Behavior of Fatigue Crack in Precipitation-Hardened Aluminum Alloy with Different Mg Content

The influence of excess Mg on the Mode I propagation of fatigue crack was examined in newly developed precipitation-hardened Al alloy containing Zr and excess Mg. The aim of this study was to evaluate the underlying factor affecting fatigue crack growth rate in the stage II region. For this purpose, the rotating bending fatigue tests were performed in constant amplitude loading, and replication technique with an optical microscope was used to measure the crack growth in the Al alloys. Through analyses of the crack propagation on the specimen surface and striation formation of the fracture surface, the effects of excess Mg in the Al alloys were clarified to promote the occurrence of mode I fatigue crack, and decelerate the fatigue crack propagation. These facts suggest that the dynamic strain aging of Mg induces the formation of fatigue striation and reduce the driving force of the crack propagation. The findings were supported by the fractographic observations in the fatigue crack propagation region.

High-temperature plastic flow of a precipitation-hardened FeCoNiCr high entropy alloy

In this work, we systematically investigated flow behavior of a high entropy alloy (HEA) strengthened by coherent γ′ precipitates in the temperature range of 1023–1173K. In contrast to the single-phase FeCoNiCrMn HEA, this precipitate-hardened alloy, i.e., (FeCoNiCr)94Ti2Al4, exhibited large reduction of the steady-state strain rate (by ~2 orders of magnitude) or drastic enhancement in flow stress, indicating significant improvement in high-temperature properties. Our results showed that the deformation could be divided into two regimes. At temperatures below 1123K, coherent γ′ precipitates effectively blocked the dislocation motion, thus resulted in a threshold stress effect. Above 1123K, however, γ′ particles dissolved and the deformation was controlled by the ordinary dislocation climb mechanism. In addition, we conducted transmission electron microscopy to characterize dislocation-precipitate interaction to provide microstructural evidences to support our conclusion of the specific deformation mechanisms in the two temperature regimes.

Monotonic and cyclic behavior of Cu-Ni-Si precipitation-hardened alloy used for magnetic pulse technologies

The compromise mechanical characteristics-electrical conductivity comes first to determine the employment possibilities for precipitation hardening copper alloys. The Cu-Ni-Si alloy as a precipitation hardened alloy exhibits high mechanical characteristics and moderate electrical conductivity. The present work is based on two parts, in the first we studied alloy tensile behavior, in the second we are interested in material response at high cycle fatigue. Cu-Ni-Si has a very good tensile strength of 646 MPa; it displays a very wide plasticizing phase in monotonic stress. Furthermore, the Cu-Ni-Si alloy gives good fatigue resistance in law cycle fatigue as well as in high cycle fatigue with a fatigue limit of 210 MPa

Direction and location dependency of selective laser melted AlSi10Mg specimens

In recent years, additive manufacturing has gained importance, especially since the full melting of the raw material in the selective laser melting process enables the fabrication of directly deployable components. However, the multiple directional dependencies involved result in an anisotropic material behavior. The raw material under investigation in this study was the precipitation-hardenable AlSi10Mg alloy, with the main focus on the positioning and inclination effects, which were studied on six characteristic orientations. In addition, the superimposed effects based on the surface condition and thermal post-treatments were taken into account. The examination contained: comprehensive tensile tests with strain gauges, detecting strains in two directions; detailed surface hardness investigations in various conditions; and micro-section investigations. Major direction dependencies were revealed and the tensile strength and the surface hardness results, coupled with annealing procedures, exhibited consistent results, explaining the encountered findings. The Young’s modulus varied between 62.5GPa to 72.9GPa with the Poisson’s ratio fluctuating between 0.29 and 0.36. Regarding the tensile strength, the UTS ranged from 314MPa to 399MPa with breaking elongations spanning from 3.2% to 6.5% in the non-heat-treated condition.

Microstructure and mechanical properties of the PM precipitation-hardening alloy N07626: effect of HIP temperature and solution annealing

The microstructure and mechanical properties of the precipitation-hardening powder metallurgy Ni-base alloys UNS N07626 are investigated. The material was compacted by hot isostatic pressing and solution treated adopting distinct conditions to reveal the effect of processing temperature on material properties. The microstructure is analysed by scanning electron microscopy, optical microscopy and electron backscatter diffraction and energy-dispersive X-ray spectroscopy. The prior particle boundaries (PPBs) are related to the presence of thermally stable nanometric Nb-carbides and aluminium oxides. An increase of processing temperature conditions is associated to a slight decrease of mechanical strength and to an appreciable improvement of toughness, as suggested by Instrumented V-Notch Charpy experiments. This behaviour is attributed to the reduction of the density of PPBs precipitates acting as void-nucleating sites in a micro-ductility mechanism. The precipitates’ density is related to a limited grain growth and the resulting separation of PPBs from grain boundaries.

Measurements and Modeling of Stress in Precipitation-Hardened Aluminum Alloy AA2618 during Gleeble Interrupted Quenching and Constrained Cooling

Solutionizing and quenching are the key steps in the fabrication of heat-treatable aluminum parts such as AA2618 compressor impellers for turbochargers as they highly impact the mechanical characteristics of the product. In particular, quenching induces residual stresses that can cause unacceptable distortions during machining and unfavorable stresses in service. Predicting and controlling stress generation during quenching of large AA2618 forgings are therefore of particular interest. Since possible precipitation during quenching may affect the local yield strength of the material and thus impact the level of macroscale residual stresses, consideration of this phenomenon is required. A material model accounting for precipitation in a simple but realistic way is presented. Instead of modeling precipitation that occurs during quenching, the model parameters are identified using a limited number of tensile tests achieved after representative interrupted cooling paths in a Gleeble machine. This material model is presented, calibrated, and validated against constrained coolings in a Gleeble blocked-jaws configuration. Applications of this model are FE computations of stress generation during quenching of large AA2618 forgings for compressor impellers.

Thermal Modeling of Al-Al and Al-Steel Friction Stir Spot Welding

This paper presents a finite element thermal model for similar and dissimilar alloy friction stir spot welding (FSSW). The model is calibrated and validated using instrumented lap joints in Al-Al and Al-Fe automotive sheet alloys. The model successfully predicts the thermal histories for a range of process conditions. The resulting temperature histories are used to predict the growth of intermetallic phases at the interface in Al-Fe welds. Temperature predictions were used to study the evolution of hardness of a precipitation-hardened aluminum alloy during post-weld aging after FSSW.

High-strength copper-based alloy with excellent resistance to hydrogen embrittlement

High-pressure components are generally designed with safety factors based on the tensile strength (TS) of the material; accordingly, materials with higher TS permit designed components with thinner walls, which reduce the weight and cost of the parts. However, many high-strength metals are severely degraded by hydrogen. To this point, efforts to develop a high-strength metal with a TS far beyond 1000 MPa and excellent resistance to hydrogen embrittlement (HE) have failed. This study introduces a high-strength metal with an excellent HE resistance, composed of a precipitation-hardened copper-beryllium alloy with the TS of 1400 MPa. Slow strain rate tensile (SSRT) tests of both smooth and notched specimens were performed in 115-MPa hydrogen gas at room temperature (RT). The alloy had a relative reduction in area RRA ≈ 1 and a relative notch tensile strength RNTS ≈ 1, without degradation in either characteristic.

Precipitation in a mixed Al–Cu–Mg/Al–Zn–Mg alloy system

The precipitation-hardened alloy systems Al–Cu–Mg (2xxx) and Al–Zn–Mg (7xxx) were compositionally combined from a base alloy containing 2% Cu and 1% Mg, added 1–4% Zn (weight fractions). Precipitates in these alloys were studied in their peak-aged states by scanning transmission electron microscopy, focusing on qualitative and quantitative compositional measurements using energy-dispersive X-ray spectroscopy mapping. An alloy with 2.5 wt% Zn contained both the hardening S phase from the 2xxx system and η-type phases from the 7xxx system, while alloys with 1% and 4% Zn were dominated by respectively S and η-type precipitates. The elements Mg, Cu and Zn were always present in both phases, suggesting a modification of the compositions they assume in their “native” alloy systems. Density functional theory calculations were used to investigate which atomic sites were most prone to an elemental replacement. For the η-type precipitates, calculations show that a Cu → Zn substitution leads to a decrease in formation enthalpy up to Zn/(Cu + Zn) ≈ 25%, which agrees well with the experimental results. For the S phase, an Al → Zn substitution was found to be the most favorable, although this increases the formation enthalpy slightly.

Improvement of tensile properties of pure Cu and CuCrZr alloy by cryo-rolling process

The present study investigates the effect of cryo-rolling process, i.e. cold-rolling at liquid-nitrogen temperature followed by heat treatment, on tensile properties of pure copper and precipitation-hardened CuCrZr alloy. The cryo-rolling process resulted in a simultaneous improvement of strength and ductility of pure copper. On the other hand, a cryo-rolled CuCrZr alloy showed higher tensile strength but comparable ductility with a conventional cold-rolled CuCrZr alloy. Microstructural analysis indicates that the drastically-beneficial effect of cryo-rolling on pure copper may be due to its heterogeneous size distribution of grains which consist of cryo-rolled fine grains, residual cryo-rolled grains and recrystallized coarse grains. The modest but certain benefit of cryo-rolling on CuCrZr alloy can be explained by different texture formation compared with conventional cold-rolling. Effect of neutron irradiation on tensile properties of cryo-rolled CuCrZr alloy is also examined.

Temperature Effects on the Tensile Properties of Precipitation-Hardened Al-Mg-Cu-Si Alloys

Because the mechanical performance of precipitation-hardened alloys can be significantly altered with temperature changes, understanding and predicting the effects of temperatures on various mechanical properties for these alloys are important. In the present work, an analytical model has been developed to predict the elastic modulus, the yield stress, the failure stress, and the failure strain taking into consideration the effect of temperatures for precipitation-hardenable Al-Mg-Cu-Si Alloys (Al-A319 alloys). In addition, other important mechanical properties of Al-A319 alloys including the strain hardening exponent, the strength coefficient, and the ductility parameter can be estimated using the current model. It is demonstrated that the prediction results based on the proposed model are in good agreement with those obtained experimentally in Al-A319 alloys in the as-cast condition and after W and T7 heat treatments.

Acceleration of volume decomposition of supersaturated Al + 4 wt.% Cu solid solution under irradiation with Ar+ ions

Effect of irradiation with Ar+ ions on the decomposition processes of model precipitation-hardening alloy Al + 4 wt.% Cu has been studied. Using X-ray diffraction, high-resolution electron microscopy methods and micro hardness measurements, it was established, that, already at low temperatures (T < 60 °C), ion irradiation causes accelerated decomposition of solid solution, with precipitation of θ′- and θ-phase particles at a depth greatly exceeding the Ar+ ions projected range.