Enhanced age-hardening behavior in Al–Cu alloys induced by in-situ synthesized TiC nanoparticles

The influence of in-situ synthesized TiC nanoparticles on age-hardening behavior of Al–Cu alloys was investigated in Al–4.5Cu–1.5TiC alloy. It was found that TiC nanoparticles decrease the peak-age time effectively, from about 20?h for Al–4.5Cu alloy decreasing to about 12?h for the Al–4.5Cu–1.5TiC. Mechanical property test shows that the age-hardening effect has been improved by the TiC nanoparticles. The increment of yield strength before and after aging is about 84?MPa for Al–4.5Cu, while, it reaches to about 113?MPa for the Al–4.5Cu–1.5TiC. After aging heat treatment, precipitates have been observed both in matrix and around TiC nanoparticles. Due to the difference of coefficient of thermal expansion between TiC and Al, high density dislocations in the Al–4.5Cu–1.5TiC were generated during water quenching after solution. Dislocations play a role of diffusion path for Cu atoms during aging, which reduces the peak-age time. Alpha-Al lattice distortion resulted from lattice mismatch of TiC/Al interface induces the precipitation of θ? phase around TiC nanoparticles, which increases the number density of θ? and improves the age-hardening effect. This finding is supposed to be also applicable to alloy systems of Al–Cu–Mg, Al–Cu–Mg–Li, Al–Cu–Mg–Ag, etc.     

Interfacial dislocations dominated lateral growth of long-period stacking ordered phase in Mg alloys

Understanding the interface between strengthening precipitates and matrix in alloys, especially at the atomic level, is a critical issue for tailoring the precipitate strengthening to achieve desired mechanical properties. Using high-resolution scanning transmission electron microscopy, we here clarify the semicoherent interfaces between the matrix and long-period stacking ordered(LPSO) phases, including 18 R and 14 H, in Mg–Zn–Y alloys. The LPSO/Mg interface features the unique configuration of the Shockley partial dislocations, which produces a near zero macroscopic strain because the net Burgers vectors equal zero. The 18 R/Mg interface characterizes a dissociated structure that can be described as a narrow slab of 54 R. There are two dislocation arrays accompanied to the 18 R/54 R and 54 R/Mg interface, resulting a slight deviation(about 2.3°). The 14 R/Mg interface exhibits the dislocation pairs associated with solute atoms. We further evaluate the stability and morphology of the corresponding interfaces based on elastic interaction, via calculating the mutual strong interactions between dislocation arrays, as well as that between the dislocations and solute atoms. The synchronized migration of interfacial dislocations and solute atoms, like move-drag behavior, dominates the lateral growth of LPSO phases in Mg 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 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.     

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.     

Mg segregations at and near deformation-distorted grain boundaries in ultrafine-grained Al–Mg alloys

The formation of Mg segregations at and near deformation-distorted grain boundaries (GBs) in ultrafine-grained Al–Mg alloys is theoretically described as a process enhanced by stress fields of extrinsic dislocations existing at such GBs. The equilibrium Mg concentration profiles near low-angle and high-angle GBs containing extrinsic dislocations are calculated. The results of the calculations explain the experimental observations (reported in the scientific literature) of spatially inhomogeneous Mg segregations characterized by high Mg concentrations at and near GBs in ultrafine-grained Al–Mg alloys processed by severe plastic deformation.     

Insights into microstructural interfaces in aerospace alloys characterised by atom probe tomography

Atom probe tomography (APT) is becoming increasingly applied to understand the relationship between the structure and composition of new alloys at the micro- and nanoscale and their physical properties. Here, we use APT datasets from two modern aerospace alloys to highlight the detailed information available from APT analysis, along with potential pitfalls that can affect data interpretation. The interface between two phases in a Ti–6Al–4V alloy is used to illustrate the importance of parameter choice when using proximity histograms or concentration profiles to characterise interfacial chemistry. The higher number density of precipitates and large number of constituent elements in a maraging steel (F1E) present additional challenges such as peak overlaps that vary across the dataset, along with inhomogeneous interface chemistries.     

热处理对Al-Mg-Ga-In-Sn合金微观结构和铝水反应的影响

制备不同镁含量的Al-Mg-Ga-In-Sn合金并对其进行固溶和时效热处理,用XRD和SEM分析和观察了显微结构和腐蚀表面,用AFM/SKPFM测量了合金不同晶界相与铝晶粒间的电势差,用排水法测量了在不同水温下合金的铝水反应.结果表明,热处理改变了合金低熔点界面相的种类、形态以及合金晶粒内Mg和Ga含量.热处理态Mg含...     

Creep and microstructure in ultrafine-grained 5083 Al

Creep and microstructure in ultrafine-grained (UFG) 5083 Al were investigated at 473 K. UFG 5083 Al was prepared by consolidating the cryomilled alloy powders via hot isostatic pressing followed by extrusion. The creep microstructure developed in the alloy was examined by means of transmission electron microscopy. The results show that the relationship between stress and strain is sigmoidal. Such a sigmoidal behavior is similar in trend to those reported for solid-solution alloys and superplastic alloys. An analysis of the mechanical data along with the consideration of several microstructural findings related to dislocation activity and configuration indicates that the alloy behaves as a superplastic alloy and not as a solid-solution alloy. Also, it is shown that the superplastic behavior of UFG 5083 Al is characterized by the presence of a threshold stress whose origin is most likely related to an interaction between impurities, which are able to segregate at nanoscale dispersion particles introduced as a result of processing, and dislocations, which are captured at the departure side of the particles.     

Microstructure characterization in cryomilled Al 5083

Nanocrystalline metals and alloys processed by severe plastic deformation (SPD) generally have improved mechanical strength compared with conventionally processed materials. In this work, we survey the microstructure of an Al 5083 alloy prepared by ball-milling powders at cryogenic temperatures (cryomilling) then consolidated by hot-isostatic pressing (HIPing) and extrusion into cylindrical billets. After milling, the particles are comprised of nanocrystalline grains, which are maintained following extrusion. We identify MgO, Al₆(FeMnCr), Al(MnFe)Si, AlCrMg, Mg₂Si, and SiO₂ phases as precipitates or dispersoids in the microstructure. This synthesis method results in a yield strength that is approximately twice that of typical wrought Al 5083 alloys. We find that the microhardness is essentially unchanged after annealing at temperatures up to 0.8T_m. The influence of the components of the microstructure on the measured mechanical properties is discussed.     

Effect of Mn on precipitation behaviour of NiAl phase in Fe–NiAl steels

ABSTRACT

The microstructural evolution of two model alloys, Fe–NiAl and Fe–NiAl–Mn, during aging at 500°C was investigated using atom probe tomography (APT). Vickers micro hardness measurements showed that the addition of Mn increased the hardness and decreased the time to reach the peak hardness. Both alloys exhibited a softening process at the initial stage of aging. The APT results showed that adding Mn increased the number density of NiAl nanoparticles significantly, and Mn atoms partitioned into the NiAl nanoparticles, occupying the Al sites preferentially. The precipitation of NiAl nanoparticles was accelerated by the addition of Mn because Mn increased the nucleation driving force for NiAl precipitates and reduced the strain energy for the nucleation.     

Mechanical properties of Al–Li alloys Part 1 Fracture toughness and microstructure

Abstract

Mechanisms influencing the ambient temperature mechanical properties of commercial Al–Li alloys 2090, 8090, 8091, and 2091 are examined as a function of plate orientation, with specific emphasis on the role of microstructure. In Part 1, results on the uniaxial tensile and plane strain fracture toughness properties are presented and the behaviour is discussed in terms of the role of the matrix and grain boundary precipitates, associated precipitate free zones (PFZs), and the occurrence of short-transverse delamination. It is seen that in general peak aged microstructures show an excellent combination of strength and toughness (L–T, T–L), equal to or exceeding that shown by traditional 2000 and 7000 series high strength aluminium alloys. The superior toughness of peak aged compared with naturally aged microstructures seems to be associated with widespread matrix precipitation of platelike precipitates (T₁ in Al–Li–Cu alloys and S in Al–Li–Cu–Mg alloys), β′-dispersoids and second phase particles which promote ductile (void coalescence) fracture, and with secondary cracking (through thickness delamination) caused by poor short transverse properties. By contrast, the deterioration in fracture toughness with overaging is primarily attributed to extensive grain boundary precipitation and corresponding formation of PFZs, similar to traditional aluminium alloys. All alloys show highly textured, predominantly unrecrystallised grain structures that render the properties to be strongly orientation dependent; specifically, fracture toughness values for the short-transverse orientations (S–L, S–T) are typically 50% lower than in the longitudinal and transverse orientations.

MST/926a     

Effect of Zn addition on the precipitation behaviors of Al–Mg–Si–Cu alloys for automotive applications

Effect of Zn addition on the precipitation kinetics and age-hardening response of Al–Mg–Si–Cu alloys was investigated by differential scanning calorimetry (DSC), hardness measurements, tensile tests and microstructural characterization. The results show that, compared with the Zn-free alloy, both the starting and peak temperatures in the DSC curve, and activation energy of β″ precipitation of Zn-added Al–Mg–Si–Cu alloy decrease significantly, corresponding to the greatly improved precipitation kinetics and age-hardening response, i.e., a hardness increment of 70HV after aging at 185 °C for 20 min. Moreover, the peak hardness and tensile properties can also be greatly enhanced after adding 3.0 wt% Zn even exhibiting a ductile fracture feature in the peak-aged state. No precipitates of the Al–Zn–Mg alloy system appear in the Zn-added Al–Mg–Si–Cu alloys after aging at 185 °C, and pre-β″, β″, and L precipitates are still the main precipitates in the two alloys after peak aging treatment. Finally, based on the microstructural evolution, a schematic diagram of precipitation in the Al–Mg–Si–Cu–Zn alloy is put forward, and the relationship between mechanical properties and microstructure is also established.     

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     

Strong and ductile Mg alloys developed by dislocation engineering

Dislocation engineering concept has been successfully employed to tackle the strength-ductility trade-off in steels, resulting in the development of high-strength high-ductility deformed and partitioned (D&P) steel. The present perspective proposes to employ such dislocation engineering concept to develop strong and ductile magnesium (Mg) alloys. High density of?<?c?+?a?>?dislocations could be generated at appropriate temperature and retained in the Mg alloy after quenching to room temperature. Those?<?c?+?a?>?dislocations inherited from the warm deformation could provide?<?c?+?a?>?dislocation sources when the Mg alloy is deformed at room temperature, resulting in good ductility. The high dislocation density generated at warm deformation provides dislocation forest hardening, leading to improved yield strength of Mg alloy.     

Deformation mechanisms in tensile fracture of Al foils containing Si precipitates

Recently, Kiritani et al. proposed a new mechanism of plastic deformation without involving dislocations in tensile fracture of metal foils. The paper reports transmission electron microscopy (TEM) study of tensile fracture of Al containing hard precipitates (Si) that are considered to act as obstacles to dislocation motion. In sawtooth-shaped thin-foils formed at the fracture tip (‘sawtooth portion’), tensile strain was as high as 10³, but only a few dislocations were pinned to precipitates. Instead, voids were formed at precipitate/matrix interface, elongated in the direction of tension, and broke up into several smaller voids, due to stress concentration around hard precipitates. The thicker area of the specimen (‘base portion’), where tensile strain was 30, did not contain voids but showed a dislocation cell structure. In tensile fracture of pre-thinned specimen, voids were formed in the sawtooth portion, despite the tensile strain also being 30. These results suggest that the sawtooth portion is formed by a new mechanism that does not involve dislocations.     

铸造Mg-3Zn-0.5Cu-0.6Zr镁合金的时效析出行为研究

利用金相显微镜、显微硬度计、X射线衍射仪、扫描和透射电镜研究了铸造Mg-3Zn-0.5Cu-0.6Zr镁合金铸态和固溶时效后的显微组织,初步确定了时效Mg-3Zn-0.5Cu-0.6Zr镁合金中主要合金相的种类和形态.合金铸态组织主要由初晶Mg基体和(Mg+Mg2Cu,CuMgZn)共晶组成.固溶后,晶界处大部分非平衡共晶组织溶解;180℃/20h时效后达到合金时效硬度峰值,此时晶内析出相主要有三类:(1)轴线垂直于(0001)Mg,板条状或棱柱状β2’-MgZn2,长度50nm～200nm,该相是合金的主要时效硬化相;(2)较粗大的、其轴线仍与基面垂直的六棱柱状β2’-MgZn2;(3)轴线平行于(0001)Mg,板条状或针状β-MgZn,长度50nm～150nm.     

Post-β″ phases and their influence on microstructure and hardness in 6xxx Al-Mg-Si alloys

Starting from solid solution (T4) or a condition with β″ precipitates (T6), three Al-Mg-Si alloys with similar total solute content (1.3 at%), but different Si/Mg ratios (2, 1.25 and 0.8) were isothermally heat-treated at 250 or 260°C and investigated by transmission electron microscopy. The result microstructure for all alloys and conditions consisted of metastable, needle-shaped precipitates growing along 〈100〉 directions in aluminium. Each of the phases β″-Mg₅Si₆, β′-Mg_1.8Si, U1-MgAl₂Si₂ and U2-MgAlSi could be identified as main precipitate in the alloy with its solute Si/Mg ratio closest to the same ratio in the composition of that particular phase: The highest Si content alloy produced coarse needles of the trigonal U1-phase coexisting with finer precipitates of hexagonal B′-phase. The most common phase in the Mg-rich alloy is coarse needles of hexagonal β′-type. The Si/Mg ratio of 1.25 in one alloy is similar to the Si/Mg ratio in β″. Here the microstructure changes from that of fine β″ needles to fine needles of the orthorhombic U2-phase. This material remains strongest during heat-treatment. Nucleation on dislocations, mainly by the B′-phase, was observed to be significant in the case of Si-rich alloys heat-treated from T4-condition.     

High cyclic fatigue behaviour of ultrafine grained Al 5083 alloy

Abstract

The effects of cryorolling (CR) and CR followed by warm rolling (WR) on high cycle fatigue behaviour of Al 5083 alloy have been investigated in the present work. The Al 5083 alloy samples were rolled for different thickness reductions of 50 and 85% at cryogenic (liquid nitrogen) temperature. Fifty per cent cryorolled samples were subsequently warm rolled at 175°C for 70% reduction (WR). Hardness, tensile strength and fatigue life using constant amplitude stress controlled fatigue were investigated. The microstructural evolution of the alloy was characterised using optical microscopy, SEM and TEM techniques. The cryorolled Al alloy after 85% thickness reduction exhibits high dislocation density due to suppression of dynamic recovery, whereas WR alloy has shown fine subgrain structure in the range of 100–150 nm, associated with dynamic recovery and dynamic aging during WR. The Al 5083 alloy after WR has shown significant enhancement in fatigue strength as compared to the coarse grained solution treated (ST) bulk alloy and CR material. It can be attributed to the formation of fine precipitates during WR, which inhibits dynamic recovery during cyclic deformation; precipitate–dislocation tangled zones inhibits the crack propagation in the alloy.     

Microstructure evolution and elemental diffusion of SiCp/Al–Cu–Mg composites prepared from elemental powder during hot pressing

15vol%SiCp/Al–Cu–Mg composites were fabricated by hot pressing method using pure elemental powders. Microstructure evolution and elemental diffusion of Cu and Mg were studied. The microstructure of as-hot pressed composites and the elemental distribution of the composites before and after solution treatment were also investigated. The results showed that there were two types of eutectic liquid phases with different compositions after the compact was heated to 580 °C. After the compact was held at 580 °C for 60 min, the eutectic liquid was absorbed into the Al matrix and some equilibrium liquid phases formed in the boundaries of the initial Al particles. Meanwhile, Cu was homogeneously distributed in the Al particles while Mg tended to be distributed near the boundaries of the initial Al particles and in the SiC clusters. The presence of Al₂Cu, Mg₂Si, and some oxides of Mg was identified in the as-hot pressed composite. After solution treatment, Al₂Cu dissolved into the Al matrix, however, some Mg-rich compounds (silicide and oxide of Mg) did not dissolve into the matrix completely.     

Composite strengthening in 6061 and Al-4 Mg alloys

Metal matrix composites using prealloyed 6061 Al (containing 1% Mg) and elemental blend Al-4 Mg alloys with 10 vol% SiC particulate reinforcements were fabricated using powder metallurgy techniques. The consolidation of the powders was effected by the section rolling process recently developed at the Defence Metallurgical Research Laboratory. This process involves the successive steps of cold isostatic pressing, vacuum sintering and special canning followed by section rolling. This resulted in a high-integrity composite product. An interfacial layer containing magnesium-rich precipitates observed in both the composites is suggested to be the major reason for the low (compared to the value predicted by the rule of mixtures) modulus and strength values in these composites. This layer also appeared to promote interfacial failure at the alloy/SiC interface. The Al-4 Mg alloy, which is known to be non-heat treatable, was found to respond to precipitation hardening heat treatment in the composite. The enhanced generation of dislocations due to the presence of SiC, promoting a more homogeneous precipitation of the second phase and the possibility of an inhomogeneous distribution of magnesium (as a result of elemental blending) are suggested to be the major factors responsible for rendering the Al-4 Mg alloy amenable to the precipitation hardening heat treatment.