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.     

Effect of Sc and Zr additions on microstructures and corrosion behavior of Al-Cu-Mg-Sc-Zr alloys

The effects of adding the alloy element Sc to Al alloys on strengthening, recrystallization and modification of the grain microstructure have been investigated. The combination of Sc and Zr alloying not only produces a remarkable synergistic effect of inhibition of recrystallization and refinement of grain size but also substantially reduce the amount of high-cost additional Sc. In this work, the microstructures and corrosion behavior of a new type of Al-Cu-Mg-Sc-Zr alloy with Sc/Zr ratio of 1/2 were investigated.The experimental results showed that the Sc and Zr additions to Al-Cu-Mg alloy could strongly inhibit recrystallization, refine grain size, impede the segregation of Cu element along the grain boundary and increase the spacing of grain boundary precipitates. In addition, adding Sc and Zr to Al-Cu-Mg alloy effectively restricts the corrosion mechanism conversion associated with Al2 Cu Mg particles, which resulted in the change of the cross-section morphology of inter-granular corrosion from an undercutting to an elliptical shape. The susceptibility to inter-granular corrosion was significantly decreased with increasing Sc and Zr additions to the Al-Cu-Mg alloy. The relationships between microstructures evolution and inter-granular corrosion mechanism of Al-Cu-Mg-Sc-Zr alloys were also discussed.     

Effects of Cr,Mn, Si,Cu and Zr on microstructure and mechanical properties of high carbon Fe–16Al alloy

Abstract

Effects of alloying elements Cr, Mn, Si, Cu and Zr on the microstructure and mechanical properties of Fe₃Al (Fe–16Al) based alloy containing ～0·5 wt-%C have been investigated. Six alloys were prepared by a combination of air induction melting with flux cover and electroslag refining (ESR). ESR ingots were hot forged and hot rolled at 1373 K and were further characterised with respect to microstructure and mechanical properties. The base alloy and the alloys containing Cr, Mn, Si and Cu exhibit a two phase microstructure of Fe₃AlC_0·5 precipitates in Fe₃Al matrix whereas the alloy containing Zr exhibits a three phase microstructure, the additional phase being Zr rich carbide precipitates. Cr and Mn have high solubility in Fe₃AlC_0·5 precipitates as compared to Fe₃Al matrix whereas Cu and Si have very high solubility in Fe₃Al matrix compared to Fe₃AlC_0·5 precipitate and Zr has very low solubility in both Fe₃Al matrix and Fe₃AlC_0·5 precipitate. No significant improvement in room and high temperature (at 873 K) strengths was observed by addition of these alloying elements. Furthermore, it was observed that addition of these alloying elements has resulted in poor room and high temperature ductility. Addition of Cr, Mn, Si and Cu has resulted in marginal improvement in creep life, whereas Zr improved the creep life significantly from 22·3 to 117 h.     

Laser-based directed energy deposition of novel Sc/Zr-modified Al-Mg alloys: columnar-to-equiaxed transition and aging hardening behavior

The control of grain morphology is important in laser additive manufacturing(LAM), as grain morphology further affects the hot cracking resistance, anisotropy, and strength–ductility synergy of materials. To develop a solidification-control solution and achieve columnar-to-equiaxed transition(CET) in Al-based alloys during LAM, Sc-and-Zr-modified Al-Mg alloys were processed via directed energy deposition(DED).CET was achieved by introducing high potent primary Al_3(Sc,Zr) nucleation sites ahead of the solidification interface. Furthermore, the relationship between the solidification control parameters and precipitation behavior of primary Al_3(Sc,Zr) nucleation sites was established using the time-dependent nucleation theory. Then, the CET was studied according to the Hunt criterion. The results indicated that coarse columnar grain structure was still obtained at the inner region of the molten pool at low Sc/Zr contents owing to the effective suppression of the precipitation of the primary Al_3(Sc,Zr) nucleation sites via rapid solidification during DED. In addition, the relatively low melt temperature at the fusion boundary unavoidably promoted the precipitation of primary Al_3(Sc,Zr) nucleation sites, which resulted in a fine equiaxed grains band at the edge of the molten pool. As the Sc/Zr content increased, the solidification cooling rate was not sufficient to suppress the precipitation of the primary Al_3(Sc,Zr) nucleation sites, and a fully equiaxed grain structure was obtained. Furthermore, the effect of the layer-by-layer manufacturing process on the subsequent precipitation strengthening of secondary Al_3(Sc,Zr) precipitates was discussed.Both the remelting and subsequent aging during thermal cycling should be considered to achieve greater precipitation strengthening.     

Characterization of microstructure and mechanical properties of Al–Cu–Mg–Ag–(Mn/Zr) alloy with high Cu:Mg

This study examines the effects of the addition of Mn or Zr on the microstructure and mechanical properties of Al–Cu–Mg–Ag alloy using optical microscopy, scanning electron microscopy, X-ray diffraction, differential scanning calorimetry, transmission electron microscopy, tensile test, and tear test. The results showed that the alloy with Zr exhibited the highest strength and the lowest fracture toughness, which may be attributed to the segregation of the secondary phases containing the Zr element on the recrystallization grain boundaries. The alloy with Mn exhibited strength that is roughly equal to the Al–Cu–Mg–Ag alloy and the highest fracture toughness, which may be due to the formation of secondary phases containing Mn and Fe elements. Mn or Zr addition also has no remarkable influence on the characteristics of the precipitates. The Ω phase and a small quantity of θ′ phases dominated the microstructure of the three alloys after aging.     

Laser powder bed fusion of high-strength Sc/Zr-modified Al-Mg alloy:phase selection,microstructural/mechanical heterogeneity,and tensile deformation behavior

Laser powder bed fusion (L-PBF) of Sc/Zr-modified Al-based alloys has recently become a promising method for developing a new generation of high-performance Al alloys.To clarify the modification roles of Sc/Zr elements,an Al-4.66Mg-0.48Mn-0.72Sc-0.33Zr (wt.％) alloy was processed using L-PBF.The ef-fect of the local solidification condition of the molten pool on the precipitation behavior of primary Al3(Sc,Zr) was analyzed based on time-dependent nucleation theory.it was found that primary Al3(Sc,Zr)inevitably precipitated at the fusion boundary,while its precipitation could be effectively suppressed in the inner region of the molten pool.This subsequently induced the formation of a heterogeneous α-Al matrix.After direct aging,the heredity of solidification microstructure introduced heterogeneous secondary Al3(Sc,Zr) precipitates within α-Al matrix.Owing to the inverse relationship between grain boundary strengthening and precipitation strengthening,the direct-aged sample with dual heterogeneous structures exhibited reduced mechanical heterogeneity,resulting in lowered hetero-deformation-induced hardening.The low strain-hardening capability in the direct-aged sample promoted necking instability while inducing a large Lüders elongation,which effectively improved the tensile ductility.     

含钪Al-Cu-Li-Zr合金的组织与性能

为了研究微量Sc对Al-3.5Cu-1.5Li-0.12Zr合金组织与性能的影响,采用铸锭冶金法,制备了4种不同Sc含量的Al-3.5Cu-1.5Li-0.12Zr合金.采用室温拉伸力学性能实验、金相显微镜和透射电镜研究了微量Sc对Al-3.5Cu-1.5Li-0.12Zr合金微观组织和拉伸性能的影响.结果表明:添加0.1%Sc能消除铸态合金的枝晶组织,有效地抑制了再结晶的发生,具有一定的强化作用和明显的增塑效应;添加O.15%Sc和0.25%Sc能显著细化合金铸态的晶粒组织,但添加0.15%Sc不能抑制合金固溶过程中再结晶;添加0.25%Sc会促进合金固溶过程中的再结晶,从而降低合金的强度.合金中较适宜的Sc加入量为0.10%～0.15%,此时合金既具有较高的强度,又兼具较好的塑性.     

Effects of grain refinement on age hardening behavior and precipitation kinetics of Al–Si–Cu–Mg alloys

The influences of grain refinement on precipitation kinetics were investigated for an Al–11 wt% Si–1.5 wt% Cu–0.3 wt% Mg casting alloy doped with B and and with La–B respectively by microstructure observation, hardness test and Johnson–Mehl–Avrami (JMA) equation. Co‐alloying of La–B facilitates the faster hardening response with higher hardness value for the alloy. The calculated Avarmi exponent indicates that the nucleation of θ′‐Al₂Cu precipitates occurs on grain boundaries for the refined alloys. The activation energies for the precipitation are of 42 kJ/mol and 30 kJ/mol for B‐doped and La–B co‐doped alloys, respectively.     

Impurity effects on the nucleation and growth of primary Al3(Sc,Zr) phase in Al alloys

Impurity effects on the nucleation and growth of primary Al₃(Sc,Zr) phase have been investigated in high purity Al alloys and commercial purity Al alloys, respectively. In the case of high purity Al alloys, primary Al₃(Sc,Zr) phases were found to be pushed to grain boundaries ahead of the solidification front. Such type of primary Al₃(Sc,Zr) phase did not contribute to the heterogeneous nucleation, and thereby the grain refinement of Al alloys. In the case of commercial purity Al alloys, the presence of Fe, Si, Cu, Mg, Ti, and other impurities significantly enhanced the heterogeneous nucleation of primary Al₃(Sc,Zr) phase. Most primary Al₃(Sc,Zr) phases were found to be located within the α-Al matrix, and kept an identical orientation relationship with the α-Al matrix. Furthermore, the presence of the impurities also changed the growth mode on the primary Al₃(Sc,Zr) phase. In the case of commercial purity Al alloys, a peritectic to eutectic reaction was induced due to the presence of the impurities. A layered growth was observed leading to a narrow particle size distribution. In contrast, in the case of high purity Al alloys, a featureless structure was observed. This investigation demonstrates that impurities and their concentrations are important factors affecting the nucleation and growth of primary Al₃(Sc,Zr) phases, and thereby for the successful grain refinement in Al-based alloys.     

微量Sc和Zr对Al—Az—Mg合金组织与性能的影响

采用铸锭冶金法制备了Al-6.2Zn-2.0Mg-0.25Zr和Al-6.2Zn-2.0Mg合金,测试不同处理态的拉伸力学性能。利用金相显微镜和透射电子显微镜研究其不同处理态的显微组织,结果表明：添加微量Sc和Zr可明显细化合金的铸态晶粒,并显著提高Al-Zn-Mg合金的力学性能,其作用机理主要为Al3(Sc,Zr)造成的细晶强化,亚结构强化和弥散强化。     

Yield Strength Enhancement Without Ductility Loss Through Controlling the Intercritical Annealing Time in Medium-Mn Steels

Herein, the effect of shortening the intercritical annealing (IA) time in a two-step process “intercritical annealing and tempering (IAT)” on the microstructure and the mechanical properties of medium-manganese steel (MMnS) made of Fe–0.05C–7Mn–1.5Cu–1.5Ni–1.5Al–1.5Si–0.5Mo (wt%) and containing copper-rich (CRP) and Ni(Al/Mn) precipitates is investigated. The atom probe tomography (APT), electron backscattering diffraction (EBSD), and the synchrotron X-ray diffraction (SYXRD) are used to study precipitation, phase microstructure evolution, the austenite stability, and deformation mechanisms. Shortening the IA step, which is carried out at 700 °C, from 2 min (IAT-2) to 1 min (IAT-1), results in a yield strength (YS) increment of around 218 MPa with less than 1% loss of ductility. While the enhanced yield strength in IAT-1 is attributed to the four times higher precipitates’ number density (n), the insignificant loss of ductility is attributed to the enhanced austenite stability factor from 4.5 to 9.2 in IAT-2 and IAT-1, respectively. The simultaneous increase in YS without ductility loss reflects that controlling the IA time is a promising strategy to overcome the yield strength and ductility trade-off without the need for higher additions of costly alloying elements such as Ni, Al, Mn, and Cu.     

Effect of Zr on microstructure and mechanical properties of the Al–Cu–Er alloy

ABSTRACT

The microstructure and mechanical properties of the Al–4Cu–2.7Er–0.3Zr alloy were investigated. The precipitates of the L1₂ structured phase with sizes 37?±?12?nm were formed in lines and homogenously distributed inside the aluminium matrix after annealing at 605°C for 1?h. The as-rolled Al–4Cu–2.7Er–0.3Zr alloy developed an increased hardness after 1?h annealing at 100–550°C and 0.5–6?h annealing at 150–250°C due to precipitation of the Al₃(Er,Zr) phase. Addition of Zirconium improved the tensile properties relative to those of the Zr-free alloy by approximately 20?MPa: yield strength?=?273–296?MPa and ultimate tensile strength?=?296–328?MPa in the alloys annealed at 100–150°C.     

New materials produced by mechanical alloying

The application of mechanical alloying (MA) to alloys based on Fe, Cu, Al, Ti, Co, Ni, Mg, and Nb is reviewed. Enhancement in physical and mechanical behavior, beyond ingot metallurgy and rapid solidification levels, can be achieved by MA, and should lead to commercialization of a number of MA alloys.Conducted under the joint Moscow-Moscow program on Synthesis of Advanced Materials (SAM).     

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.     

Microstructure and mechanical properties of as-cast Mg-4Zn-0.5Zr-0.2Cu-0.2Ce alloy

In this study, an approach is proposed to improve the microstructure and mechanical properties of Mg-4Zn-0.5Zr alloy by combining trace Cu and rare earth Ce addition. The results showed that Cu and Ce additions led to obvious grain refinement and the formation of Mg-Zn-Cu and Mg-Zn-Ce phases. The Mg-Zn-Ce phase was identified to have an orthorhombic structure. The length of the [0001]_α rods in the Cu-containing alloys remarkably decreased. The yield strength increased slightly after Cu and Ce co-addition, which was attributed to grain refinement and precipitation strengthening. The coarse Mg-Zn-Ce phase distributed at the grain boundaries would reduce the ductility by promoting crack propagation during tensile processes.     

Effect of Minor Alloying on Crystallization Behavior and Thermal Properties of Zr_(64.5)Ni_(15.5)Al_(11.5)Cu_(8.5) Bulk Amorphous Alloy

Minor alloying plays an important role in the synthesis and improvement of thermal stability of bulk metallic glasses(BMGs).The present study was conducted to investigate the effect of minor additions of Y,Ti and Nb on the crystallization behavior and the thermal properties of Zr64.5Ni15.5Al11.5Cu8.5 alloy.Thermal parameters and the activation energies for crystallization were calculated for four(Zr0.645Ni0.155Al0.115-Cu0.085)100-xMx(M=Y,Ti and Nb,while x=0,2 at.) alloys.The present alloys have wide supercooled liquid region of ≥87 K.Maximum activation energy was found to be greater than 300 kJ/mol for the base alloy.Four crystalline phases were identified in the samples annealed at 823 K for 20 min.Reduced glass transition temperature(Trg) and other thermal parameters such as γ,δ and β were improved by Y and Ti addition.Nb addition resists crystallization below annealing temperature 713 K,however,its effect on thermal properties is not very promising.     

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     

Microstructure and precipitation in Al–Li–Cu–Mg–(Mn,Zr) alloys

Abstract

Hot rolled Al–6Li–1Cu–1Mg–0·2Mn (at.-%) (Al–1·6Li–2·2Cu–0·9Mg–0·4Mn, wt-%) and Al–6Li–1Cu–1Mg–0·03Zr (at.-%) (Al–1·6Li–2·3Cu–1Mg–0·1Zr, wt-%) alloys developed for age forming were studied by tensile testing, electron backscatter diffraction (EBSD), three-dimensional atom probe (3DAP), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). For both alloys, DSC analysis shows that ageing at 150°C leads initially to formation of zones/clusters, which are later gradually replaced by S phase. On ageing at 190°C, S phase formation is completed within 12 h. The precipitates identified by 3DAP and TEM can be classified into (a) Li rich clusters containing Cu and Mg, (b) a plate shaped metastable precipitate (similar to GPB2 zones/S″), (c) S phase and (d) δ′ spherical particles rich in Li. The Zr containing alloy also contains β′ (Al₃Zr) precipitates and composite β′/δ′ particles. The β′ precipitates reduce recrystallisation and grain growth leading to fine grains and subgrains.     

Microstructural evolution and strengthening mechanisms in Ti–Nb–Zr–Ta,Ti–Mo–Zr–Fe and Ti–15Mo biocompatible alloys

The microstructural evolution and the strengthening mechanisms in the two quaternary alloys, TNZT (Ti–34Nb–9Zr–8Ta) and TMZF (Ti–13Mo–7Zr–3Fe), and one binary alloy, Ti–15Mo, have been investigated. In the homogenized condition both the quaternary alloys exhibited a microstructure consisting primarily of a β Ti matrix with grain boundary α precipitates and a low volume fraction of primary α precipitates while the binary alloy showed single-phase microstructure with large β grains. On ageing the homogenized alloys at 600 °C for 4 h, all the alloys exhibited the precipitation of refined scale secondary α precipitates distributed homogeneously in the β matrix. However, after ageing while the hardness of TMZF marginally increased, that of the TNZT and Ti–15Mo alloys decreased. Furthermore, the modulus of TNZT decreased while other two alloys showed opposite trends. TEM studies indicate that there is initially a B2 ordering in TNZT that is destroyed after ageing causing a reduction in both hardness and modulus of this alloy. Also in Ti–15Mo, dissolution of ω precipitates on ageing causes the hardness to reduce, while the precipitation of secondary α causes an increase in the modulus. Using these examples, the important influence of thermal processing on the property–microstructure relationships in orthopaedic alloys for implant applications will be highlighted.     

Tensile and fatigue properties of a cast aluminum alloy with Ti, Zr and V additions

The development of aluminum alloys for automotive powertrain applications is in high demand due to the required weight reduction and fuel efficiency. The aim of this study was to evaluate the microstructure and mechanical properties of a newly developed Al-7%Si-1%Cu-0.5%Mg cast alloy with further additions of Ti, Zr and V. The microstructure of the alloys consisted of Al dendrites surrounded by Al-Si eutectic structures with Mg/Cu/Fe-containing Si particles, and contained nano-sized trialuminide precipitates in the Ti/Zr/V added alloys. The alloys had a significantly (60-87%) higher yield strength but lower ductility than A356-T6 and 319-T6 alloys. With the addition of Ti/Zr/V both monotonic and cyclic yield strengths increased, but ductility and hardening capacity decreased due to reduced dislocation storage capacity caused by stronger interactions between dislocations and trialuminide precipitates. The Zr/V-modified alloy had a longer fatigue life, and all the alloys exhibited cyclic stabilization at low strain amplitudes and cyclic hardening at higher strain amplitudes. With increasing strain amplitude, the extent of cyclic hardening increased. Both cyclic yield strength and cyclic strain hardening exponent were higher than the corresponding monotonic yield strength and strain hardening exponent, indicating that a stronger cyclic hardening ability of the alloys developed. Fatigue cracks were observed to initiate at near-surface defects, and crack propagation was mainly characterized by the formation of fatigue striations together with secondary cracks.