Formation Of GP Zones Research Articles

Effect of micro-nano hybrid SiCp on microstructure and mechanical properties of 7075Al alloy

In this study, micro-nano hybrid SiCp/7075Al composites were successfully prepared for the first time by ultrasonic-assisted semi-solid stirring casting method. The effects of micro-nano hybrid SiCp on the microstructure and mechanical properties of 7075 Al were studied. The results show that the distribution of SiCnp and micron SiCp in the microstructure was uniform. During the extrusion process, most of the micron SiCp were subjected to significant load and broken, which increased the contact area with the matrix and promoted the interface bonding. In addition, the enhanced interfacial interaction promoted the formation of MgAl2O4 phase, which significantly improved the mechanical properties. SiCnp not only had a significant effect on grain refinement, but also promoted the enrichment of Cu, accelerated the formation of GP zone, and promoted the formation of more nano-MgZn2 precipitates. The yield strength and the elastic modulus of micro-nano hybrid SiCp/7075 Al composites reached 450 MPa and 94 GPa, respectively. The synergistic effect of SiCnp and micron SiCp could produce composite strengthening effects such as load transfer strengthening, thermal mismatch strengthening and Orowan strengthening. Moreover, a large number of dislocation pile-ups were observed near the interface between micron and nano SiCp and Al matrix. This leaded to a higher work hardening rate of the composites as the particle content increases. The higher work hardening rate enhanced the strain hardening ability of the material during plastic deformation, thereby significantly improving the tensile strength and elongation of the material.

New insights into enhancing the mechanical properties of 6061-T6 aluminum alloy MIG welded joints by constructing bionic heterostructure

The trade-off between strength and toughness is a long-term challenge for engineering metal structural materials. However, natural materials overcome the inverse relationship between strength and toughness through their unique structures. Inspired by plant leaves and dragonfly wings, a ‘soft-hard’ alternating bionic heterostructure is prepared on 6061-T6 Al alloy MIG welded joints by laser surface remelting (LSR) technology. After LSR, the grain size of the laser treated zone is significantly refined, dislocation density is increased, and the formation of fine and stable GP zone and β′′ phase is promoted, enhancing microhardness and tensile properties of the LSRed sample. By adjusting the laser stripe spacing and distribution angle, stress transfer and redistribution can be optimized, effectively improving the tensile properties of the welded joint can be. Compared to as-welded sample, the tensile strength, yield strength and elongation of LSRed sample can be increased by 30.6 %, 13.6 % and 102 %, respectively. In addition, the bionic heterostructure composed of grain size gradient, dislocation gradient and precipitated phases gradient can effectively develop heterogeneous deformation induced (HDI) stress strengthening and HDI strain hardening, enabling synergistic strengthening of strength and toughness.

A study on the quantitative recrystallization of cold deformed copper affected by tin-lead solder

It has been observed that the fractional recrystallization characteristics of commercially pure copper is affected by the presence of individual or both the constituent elements of the Sn-Pb solder alloy. In order to design the experiment, commercially pure Cu, binary copper alloys (Cu-Sn and Cu-Pb) and ternary copper alloy, Cu-Sn-Pb are investigated. Cast alloys are homogenized, solution treated, and then quenched to complete the thermal treatment. In order to recrystallize, alloys are cold rolled to a 75% thickness and then annealed at 700°K isothermally for varying durations, up to 3600 seconds. In this experiment, the fractional recrystallization of annealed samples is evaluated as the normalized difference in microhardness recorded at various time steps. Additionally, in an attempt to verify the experimental results, the well-known Johnson-Mehl-Avrami-Kolmogorov equation is also used to predict the associated recrystallization behavior. From the study, it can be inferred that the presence of Sn-Pb solder-alloy elements have a positive impact on the recrystallization behavior of pure copper due to the solid solution strengthening, in which the effect of tin is greater than that of lead. Quantitative analysis indicates that recrystallization of pure Cu, Cu-Sn, Cu-Pb, and Cu-Sn-Pb alloys attains 99.4%, 95.4%, 98.4%, and 89.5%, respectively. Sn forms intermetallic with Cu but Pb does not. Additionally, Sn forms different intermetallic with impurities and has a BCC crystal structure dissimilar to the FCC of Cu and Pb. As a result, the formation of GP zones and the intermetallic phases during annealing show greater differences in the recrystallization behavior between the two approaches. By combination, microstructural studies of the cold-rolled alloys reveal the elongated grains of the second phases, and the alloys almost completely re-crystallized after 1800 seconds of annealing at 700°K.

Strengthening effects of GP zone induced by SiCp size variations in Al–Si–Mg matrix composites

In this work, SiCp reinforced Al–Si–Mg matrix composites were prepared by stirring casting with different SiCp sizes. The effect of SiCp sizes on the microstructure and mechanical properties was investigated, and different strengthening effects were systematically calculated and analyzed. With the decrease of SiCp sizes, the microstructure of composites is refined and the plasticity is improved significantly. The refined SiCp also leads to a more intense grain refinement strengthening and thermal expansion mismatch strengthening. However, an abnormal decrease of yield strength is discovered with the refinement of SiCp, which is due to the influence of SiCp on GP zones strengthening in the matrix. As SiCp sizes decreasing, the increased SiCp/Al interfaces capture more Mg atoms, and then inhibits the formation of GP zones in the α-Al. Therefore, the refined SiCp weaken the precipitation strengthening in composites. These acicular Mg–Si GP zones exhibit a preferentially growing direction along <100>Al and present a same structure as α-Al with coherent interfaces which pin dislocations and provide precipitation strengthening. The high density of GP zones also increases nano-hardness and elastic modulus of α-Al, which improves the wear resistance of SiCp/Al–Si–Mg composites. Therefore, it is suggested that the interaction between the SiCp and precipitates in matrix should also be considered to obtain a suitable strengthening effect in the design of SiCp/Al alloy composites.

Influence of temperature on the precipitation phase transformation behavior of RR350 aluminum alloy during artificial aging and related mechanisms

This study investigates the influence of artificial aging temperature on the evolution of precipitation phases and age hardening behavior of the RR350 aluminum alloy. The manner of nucleation and growth of different kinds of precipitates and phase transformation between them are analyzed, along with the corresponding mechanisms are discussed. The results reveal that the aging temperature affects little the precipitation process of precipitates which still follows the sequence of GP zones - θ″- θ′ in the RR350 aluminum alloy during artificial aging, but influences significantly the precipitation speed and phase composition, morphology, size and size distribution of the resultant precipitates in the alloy. An imperceptible relationship between the hardness of the sample and the composition of the precipitates were discovered. As the artificial aging temperature (160–180–200 °C) increases, the peak-aging hardness of the samples decreases and the time for reaching the peak-aging status shortens gradually. When the temperature is low, 160 °C for example, the precipitates in the peak-aging status are mainly composed of θ″ phase and some minor θ′ phase. As the temperature increases to 200 °C θ′ phase becomes prevalent. As a result, the alloy shows lower peak-aging hardness since the strengthening effect of the finer θ″ phase is much stronger than that of the θ′ phase. Increasing the aging temperature accelerates the competitive processes of nucleation of GP zones, growth of all precipitates, and the phase transformation. At higher temperatures, phase transformation from θ″ to θ′ overweighs the formation of GP zones, growth of both θ″ and θ′ phases, resulting in more amount of θ′phase in the peak-aging and over-aging statuses.

Modelling Age Hardening of Aluminium Alloys with Consideration of GP Zones or Clusters

The work reported here follows the theme of our previous research presented at the ICAA16 and ICAA17 conferences, where material models have been developed to predict the microstructure evolution and precipitation hardening of aluminium alloys during cooling and ageing. The models have two important features: one is the consideration of quenched-in vacancies and their effect on precipitation kinetics, which was discussed in past conferences; the other is the formation of GP zones or clusters and their transition to other hardening phases during ageing, which is the focus of this paper. GP zones or clusters are known to serve as precursor phases for various hardening phases. Depending on the ageing temperature being above or below the solvus temperature of the precursor phase, precipitates can either form on their own from the matrix (heterogeneous or HET type) or form on existing GP zones or clusters (homogeneous or HOM type). In the latter case, GP zones or clusters are allowed to form first from the matrix, which will later transform to their metastable counterparts. Both HET and HOM types of precipitates are considered in the current strengthening model for predicting age hardening curves for aluminium alloys. The age hardening model has been validated over a wide range of commercial aluminium alloys and demonstrated good agreement with experimental data in the form of age hardening curve and T5/T6 peak position and strength.

Synergy of Sn micro-alloying and thermomechanical processing on formability and precipitation behavior of Al-Mg-Si-Cu-Zn alloys

ABSTRACT The effect of Sn micro-alloying on microstructure evolution, formability and precipitation behaviour of Al-Mg-Si-Cu-Zn alloys were systematically studied by experimental techniques and theoretical calculations. Results show that Sn addition can accelerate both the precipitation and re-dissolution of the Fe-rich phase during casting and homogenising treatments, which thereby determined the final microstructure. A significant retarding effect to natural ageing precipitation was observed with increasing Sn content in quenching samples, but this effect was weakened in pre-aged samples, as explained by DSC and simulations. The different number densities of the strengthening phase β″at the same artificial aging state are mainly attributed to the changed activation energy of the β″ phase affected by the formed Sn-containing Mg-Zn clusters and Mg-Si clusters. Trace Sn participating in the formation of GP zones, Sn-containing MgZn2 phase and new precipitating sequences during ageing were proposed for the first time.

Effect of ageing treatment process on the microstructure development and mechanical properties of 6082 Al alloy

Different ageing treatment process was conducted to improve the mechanical properties of 6082 Al alloy. Tensile results show that as compared to the T6 sample, the ultimate tensile strength, yield strength and elongation of the T6I6 sample increase by 3.2 %, 3.5 %, and 47.3 %, respectively, the ultimate tensile strength, yield strength and elongation of the RD+T6I6 sample increase by 12.9 %, 11.8 % and 12.5 %, respectively. Microstructural results reveal that holding at 65 ºC for 336 h after pre-ageing promotes the formation of GP zones and stimulates the transformation of GP zones into β" precipitates during subsequent ageing at higher temperature. The introducing of the asymmetric rolling deformation after pre-ageing can reduce the average grain size, induce high-density dislocations and increase the average Taylor factor (M) due to the increasing of S and Brass texture. The dislocations have a good effect on the formation of fine β" precipitates during subsequent ageing. This microstructure is responsible for the improvement of the mechanical properties.

Hydrogen delaying the formation of Guinier-Preston zones in aluminium alloys

The consequences of hydrogen on the formation and growth of GP zones during natural ageing in an Al-5Cu alloy is investigated experimentally down to the atomic scale and numerically using ab initio calculations. As observed through scanning/transmission electron microscopy, the hardening kinetic is delayed due to a slower growth of GP zones during natural ageing when hydrogen is incorporated. According to ab initio calculations, the delayed hardening results from hydrogen trapped in vacancies that reduces significantly the diffusion coefficient of copper and the self diffusion of aluminium. This reduction of the diffusion coefficient is either due to hydrogen being in the path of the atom that exchanges with the vacancy (and increasing the energy barrier) or due to hydrogen being dissociated from the vacancy (and leading to a less stable state).

Effects of heat treatment and pre-stretching on the mechanical properties and microstructure evolution of extruded 2050 Al–Cu–Li alloy

Heat treatment process is one of the key methods to improve the mechanical properties of age-hardening Al–Cu–Li alloy, and pre-stretching can further increase nucleation sites of precipitates by introducing dislocations. In this study, the effects of solution treatment, artificial ageing, and 4% pre-stretching on the microstructure evolution, mechanical properties, and fracture mechanism of extruded 2050 Al–Cu–Li alloy were investigated. The results indicated that the residual secondary phase, including Al2Cu and Al2CuLi, precipitated in hot extrusion were dissolved and a non-recrystallization characteristic for the 2050 alloy was presented after solution treated at 530 °C for 90 min. During artificial ageing at 150 °C, the precipitation sequence of the alloy without pre-stretching was: SSS → GP zone + δ′/β'→ θ' + T1. Pre-stretching led to denser dislocations thus a higher nucleation rate of T1 phase in compared with the alloy without pre-stretching. Additionally, the segregations of non-Cu atom at dislocations inhibited the formation of GP zones in the early ageing state, thereby providing sufficient Cu atoms for accelerating the growth of T1 phases. After pre-stretched by 4% and aged at 150 °C for 12 h, a good combination of strength (YS = 671 MPa, UTS = 682 MPa) and ductility (EL = 10.3%) was obtained. The fracture mode changed from transgranular fracture to intergranular fracture with increasing ageing time due to the dislocation density and the distribution, size, and number density of the precipitates.

Optimizing strength and electrical conductivity of Cu-Cr-Zr alloy by two-stage aging treatment

Cu-Cr-Zr alloy can be strengthened by modulating the aging process. In this work, the effect of two-stage aging on the properties of Cu-Cr-Zr solid solution was studied. Hardness, tensile properties and electrical properties were tested, and the microstructures were examined by transmission electron microscope. The results show that pre-aging can facilitate the formation of GP zones in the matrix, and thus accelerate the precipitation of high-density nano-sized phase in a shorter time as compared to that in the one-stage aging process. The two-stage aging route harmonizes the strength, ductility and electrical conductivity of the alloy, and provides an accessible strategy to ameliorate the comprehensive properties of the Cu-Cr-Zr alloy.

The effect of the chemical distribution on the mechanical properties of the Al-5.5wt%Si-5wt%Zn-0.2wt%Mg alloy after thixoforging and solution heat treatment

The effect of heat and strain during the semisolid processing route on the microstructural evolution and mechanical performance of the Al-5.5wt%Si-5wt%Zn-0.2wt%Mg alloy was evaluated. The as cast alloy was heated and partially melted to 588 °C (fs = 0.40), thixoformed (in an open die forged) and solution heat-treated (SHT) at 525 °C for 1 to 6 h. The microstructural evolution and chemical distribution (SEM-EDS) of the main alloying elements were evaluated at each step of the processing route. The SHT for 2.5 h after thixoforming showed to be adequate considering (a) the spheroidization of Si crystals, (b) the formation of Fe- and Mn-rich platelet-like particles in the eutectic phase, (c) the Mg dissolution from the Mg2Si conglomerates and (d) the homogeneous Zn diffusion along the entire part. As a consequence of the higher circularity of secondary particles, the possibility of crack formation and propagation under stress was minimized. The possible formation of solute clusters and GP zones containing Zn, Mg, Si and Cu was enabled due to the dissolution and diffusion of secondary elements during SHT, which increased the alloy’s mechanical strength. Larger Si crystals and the homogeneous spatial spread of Zn resulted in the hardness increase of the SHT alloy.

Effect of Sc on solidification segregation and subsequent friction stir processing of aluminium alloy

Al-Zn-Mg alloys (7xxx series) are used in aerospace engineering due to their high strength-to-weight ratio and good mechanical properties. Al-Zn-Mg alloy is exhibited strong age-hardening properties due to the Sc (0.39 wt%) inoculation effects, which implies decomposition of the supersaturated solid solution and the formation of a fine distribution of coherent GP-zones precipitates, ή precipitates, η(stable) and Al3Sc(L12) precipitates. In addition, the ageing sequence consists of mainly three steps such as solutionizing at 465 °C for 1 h then immediately water quenching (T4), subsequently, room temperature ageing, and again proceed artificial ageing at 140 °C for 6 h (T6). The microstructure at peak strength is shown to depend on the mainly dissolution of GP zones, nucleation of ή and Al3Sc precipitates or formation of GP zones, nucleation of dislocations and dislocation with Al3Sc particles interaction. Thus, at higher temperature around 140 °C the ή transition phase is being formed and thermal stability of Al3Sc phase at > 300 °C. It has also identified those ageing treatments both operation eliminated micro-segregations and improved mechanical properties of this alloy. The influence of various process parameters on the microstructure and mechanical properties of the as-cast alloy and FSP of aluminium alloy are studied by OM, EPMA, FESEM, DSC, TEM, Vicker’s hardness measurements and tensile properties. The combined effect of Sc addition, FSP then later on post ageing treatment at 140 °C for 2 h significantly enhanced better tensile properties (278.8 MPa, ductility 6.7%) when compared to the T6 ageing treatment with Sc addition (340 MPa, ductility 4.8%). The aim of this paper is to investigate in details the ageing behaviour at T6 condition, and examine microstructural effect after optimized FSP parameters at 1000 rpm, traverse speed of 70 mm/min, specified tool design and induced heat input (1.07 kJ/mm) on Al-Zn-Mg-Sc alloy to enhances mechanical properties after post ageing treatment at 140 °C for 2 h.

Effect of various aging treatment on thermal stability of a novel Al-Zn-Mg-Cu alloy for oil drilling

Abstract Thermal stability plays an important role in practical application of Al-Zn-Mg-Cu alloy for oil drilling. In this work, a new four-stage aging treatment by employing a natural aging prior to re-aging is designed to improve thermal stability of Al-Zn-Mg-Cu alloy. The effect of T6, four-stage aging, three-stage aging and T74 on thermal stability was investigated by tensile testing, transmission electron microscopy (TEM) and atom probe tomography (APT). The results showed that after thermal exposure at 120 °C for 500 h, the tensile strength of the four-stage aged sample decreased only by 5.05%. APT analysis indicated that natural aging accelerates the diffusion of Mg atoms, and promotes the precipitation of GP zones during re-aging stage, and TEM observations also supports this result. Natural aging for 24 h prior to re-aging enhance the formation of GP zones, leading to an effective improvement of the strength properties and thermal stability of the studied alloy. In addition, precipitates in all aging samples gradually coarsen with the thermal exposure time increase. The influence of precipitates on mechanical properties of the investigative alloy after thermal exposure is also discussed.

Zn/Cu regulated critical strain and serrated flow behavior in Al–Mg alloys

Next-generation high performance aluminum alloys are required for lightweight strategies and advanced automotive applications. A current challenge for 5xxx series alloys is their poor strength and dreadful surface appearance caused by serrated yielding behavior during plastic deformation. Here we show that the serrated yielding behavior can be eliminated in Zn/Cu modified novel alloys, while simultaneously achieving above 100 MPa improvement of the strength at a pre-aging temper. The formation of GP zones of the precursor of T-Mg32(AlZnCu)49 phase during pre-aging treatment and the depletion of vacancies are mainly attributed to the elimination of dynamic strain aging. The critical strain shows a transformation from normal to inverse behavior with an increase of Zn content in the high-strength novel alloys. Such inverse behavior is because of the impediment of GP zones and solute atoms to the dislocations at lower strain rates. These results provide insight into the serrated yielding behavior of the aluminum alloys.

On the suppression of Lüders elongation in high-strength Cu/Zn modified 5xxx series aluminum alloy

The deterioration in surface quality due to the formation of Lüders lines is a typical phenomenon in 5xxx series Al–Mg alloys. Traditional efforts show that an increase of the grain size results in the suppression of the Lüders elongation. This paper presents that the suppression of the Lüders elongation can be realized at a smaller grain size by the addition of Zn/Cu elements in the 5xxx series alloys and by pre-aging treatment. The alloys with (Zn + Cu)/Mg ratios below 1.0 result in an increase of the strength by the precipitation of T-Mg 32 (AlZnCu) 49 phase, and quite different from 7xxx series Al–Zn–Mg alloys strengthened by η-MgZn 2 precipitates. The decisive measurement for suppressing Lüders elongation is based on the depletion of Mg atom concentrations in the matrix due to the formation of GP zones after pre-aging treatment. • The 5xxx series Al alloy is modified with (Zn + Cu)/Mg ratios below 1.0. • The Lüders elongation can be suppressed in a smaller grain size at T4P temper. • The Hall-Petch coefficient for Lüders increases with the concentrations of Mg atoms. • The suppression of Lüders elongation is due to the formation of T precipitates.

Quantitative analysis of the influences of pre-treatments on the microstructure evolution and mechanical properties during artificial ageing of an Al–Cu–Li–Mg–Ag alloy

The quantitative effects of pre-treatments, namely pre-stretching and natural ageing (NA), on the precipitation sequence, precipitation kinetics and mechanical properties of an Al–Cu–Li–Mg–Ag alloy during artificial ageing were systematically investigated, aiming to elucidate the mechanisms by which these effects proceed. At the early ageing stage, pre-stretching could inhibit the formation of GP zones by restricting the Mg–Cu co-clustering, and could result in a higher nucleation rate of the T1 precipitates due to an increase in the quantity of dislocations covered by solute segregations. The introduction of NA after pre-stretching could further increase the number density of T1 by providing more nucleation sites, which evolved from the much more Mg–Ag co-clusters forming during the NA process. With prolonging ageing time, the precipitation kinetics for T1 were strongly enhanced by pre-stretching due to a more efficient nucleation of T1 and a more rapid growth rate of this phase, which was attributed to the combination of the suppression effect on the GP zone precipitation and the pipe diffusion effect derived from the introduced dislocations. When applying NA after pre-stretching, a further acceleration of the precipitation kinetics was observed, which was related to the formation of much more T1 nuclei. At the peak-aged condition, both the two pre-treatments could not only refine the precipitates but also result in a higher T1 volume fraction. Mechanisms of the strength changes induced by the pre-treatments under various ageing conditions were discussed in detail in view of these experimental results.

Effects of Cu Addition on Age Hardening Response of Al-7Si-4Mg Alloys at 130 °C

The effects of Cu addition of 0.38, 3.82, and 6 wt. % in Al-7Si-4Mg alloy on hardness and age hardening response were discussed. Samples were solution treated at 495 °C for 2 h, quenched, and aged at 130 °C for 0 to 498 h. Characterization included hardness test, microstructural observation by an optical microscope (OM) and Scanning Electron Microscope (SEM) combined with Energy Dispersive X-Rays Spectroscopy (EDX), as well as Simultaneous Thermal Analysis (STA) testing. The results showed an increased in as-cast hardness along with addition of Cu. Peak hardness increased to 64.47, 65.8, and 70.1 HRB by addition of 0.38, 3.82, and 6 wt. % Cu, respectively. The addition of Cu promoted the formation of Al2Cu and Al5Cu2Mg8Si6 which contributed to higher as-cast hardness. Formation of GP-zone, θ”, and θ’ was observed after ageing at 48, 240, and 290 °C, respectively and no effects of Cu was detected on the formation temperatures of the precipitates.

(Al, Zn)3Zr dispersoids assisted η′ precipitation in anAl-Zn-Mg-Cu-Zr alloy

The influence of (Al,Zn)3Zr dispersoids on the precipitation of the main strengthening (Mg,Zn)-rich phases was investigated during isothermal aging of a model Al-Zn-Mg-Cu-Zr alloy. Upon homogenization of the alloy, dispersoids of (Al,Zn)3Zr with a L12 structure are present. Isothermal aging at 120 °C for 0.5 h leads to the homogeneous formation of spherical GP zones in the α-Al matrix and heterogeneous nucleation on (Al,Zn)3Zr dispersoids. After 2 h of aging, GP zones remain present in the α-Al matrix while the accelerated transformation of GP zones to plate-shaped (Mg,Zn)-rich ηʹ precipitates is shown on the {111} planes at the interface of the L12 dispersoids. Even at grain boundaries, the similar composite structure comprising ηʹ precipitates on the coarser Zr-dispersoid is observed, along with 10-nm wide precipitate-free zones around them. The composition and structure of pre-existing dispersoids, their role in the formation of the composite structure are discussed.

Effects of natural aging on the formation and strengthening effect of G.P. zones in a retrogression and re-aged Al–Zn–Mg–Cu alloy

In this work, a new four-stage aging treatment by employing a natural aging prior to re-aging is designed to promote GP zones formation and subsequent shear-able strengthening effect in an Al–Zn–Mg–Cu alloy. The effects of natural aging on the microstructural evolution, tensile properties and stress corrosion cracking (SCC) behavior of Al–Zn–Mg–Cu alloy are carefully explored by atom probe tomography (APT) technique, transmission electron microscope (TEM) and differential scanning calorimetry (DSC) analysis. The results show that increased GP zones distributed in the matrix of the alloy subjected four-stage aging processing, finally contributing to the greater tensile strength properties and elongation of four-stage aging treated alloy. The SCC test results reveal that four-stage aging condition can enhance the corrosion resistance of the Al–Zn–Mg–Cu alloy without sacrificing much strength.