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.     

Precipitate coarsening and its effects on the hot deformation behavior of the recently developedγ’-strengthened superalloys

Recently,theγ’-strengthened superalloys are of great interests in high temperature applications due to their excellent high temperature strength which is derived from theγ’strengthening phase.For theseγ’-strengthened superalloys,the changes in morphology,size and distribution ofγ’precipitates due to coarsening during thermal exposure have a significant impact on the properties of alloys.This article briefly summarizes the recent advances on the coarsening behavior of gamma prime precipitates in the recently-developedγ’-strengthened superalloys and its effects on the hot deformation behavior of superalloys,drawing specific examples on Allvac^■718 Plus TM and Ni3 Al-based intermetallic superalloys.It is found that the particle size plays an important role in morphological evolution ofγ’precipitates.For instance,the morphology ofγ’precipitates evolves from cuboidal to strip-like or other complex structures in Ni3 Al-based intermetallic alloys,while theγ’precipitates in Allvac^■718 Plus alloy always present nearspherical morphology due to the relatively small initial particle size.The Lifshitz-Slyozof-Wagner(LSW)theory and its modifications,as well as Trans-Interface Diffusion Controlled(TIDC)theory have been applied to describing the coarsening kinetics ofγ’precipitates.Additionally,the hot deformation behavior ofγ’-strengthened superalloy is found to be greatly influenced by the coarsening ofγ’precipitates.     

Influence of fine-grained structure and superplastic deformation on the strength of aluminium alloys

Samples of 1560 (Al-Mg-Mn) and 1960 (Al-Zn-Mg-Cu) alloys have been used to investigate the nature of the effect of grain size and superplastic treatment on the strength of aluminium alloys. The observed increase in the work needed for crack formation with the transition from coarse-grained (CG) to fine-grained (FG) structure is connected to a greater homogeneity of the plastic deformation in the material volume. This leads to a reduction in local stress concentrations at the sites of preferential crack initiation. The easier crack growth in FG alloys is mainly caused by a reduction in the energy for plastic deformation at the head of a long crack and also for the formation of free fracture surfaces.Deceased.     

Effect of phosphorus segregation and Ni2Cr formation on hydrogen embrittlement in 70Ni–30Cr alloys

Abstract

The effects of aging at 773 K on hydrogen embrittlement in Ni–30Cr (wt-%) alloys having two levels of P have been investigated by considering the grain-boundary segregation of impurity atoms and the Ni₂ Cr ordered-phase formation. Aging at 773K suppressed intergraular fracture and reduced the susceptibility to hydrogen embrittlement in the low-P alloy. Such behaviour can be explained in terms of the grain-boundary strengthening caused by the segregation of C atoms. During aging at 773 K, the Ni₂Cr ordered phase formed and the deformation mode changed from wavy slips to coplanar slip with paired dislocations, and then to coplanar slip with microtwins. In the low-P alloy, this change of deformation mode induced step-like cracks which may have occurred by the separation of either the {111} slip planes or the microtwin interfaces. In the high-P alloy, aging for short times caused C segregation to the grain boundaries which suppressed intergranular fracture. However, aging for longer times induced drastic intergranular hydrogen embrittlement because of the grain-boundary segregation of P atoms, which offset the effect of the boundary strengthening caused by C atoms.

MST/177     

Twinning behavior and deformation mechanisms of extruded AZ31 Mg alloy

Complicated deformation mechanisms such as twinning, non-basal slips and grain boundary sliding besides the basal slip occur at room temperature for Mg and its alloys. The roles of these mechanisms should be understood from the viewpoint of the homogeneous deformation of a polycrystalline and the relaxation of stress concentrations at the grain boundaries. In the present paper, twinning behavior of AZ31 Mg alloy extrusions with different texture intensity and grain sizes was investigated. Twinning played a vital role in a coarse-grained Mg, not in a fine-grained Mg. Thus, the deformation behavior was drastically changed by grain refinement. Based on the results, the deformation mechanisms are discussed from the viewpoint of the deformation and relaxation mechanisms.     

Intergranular fracture of Al–Li–Cu–Mg alloy resulting from non-equilibrium eutectic melting during solution treatment

Abstract

This investigation has examined intergranular fracture during heat treatment and deformation of an Al–Li–Cu–Mg alloy and of an Al–Li–Cu alloy. When solution treatment of the Al–Li–Cu–Mg alloy was initiated by rapid heating to temperatures ≥ 545°C, non-equilibrium eutectic melting of a grain boundary precipitate phase occurred and the liquid spread along grain boundaries as a thin film. On quenching, intergranular cracks were observed at grain boundaries into which a liquid film had penetrated during solution treatment. For less rapid heating rates, non-equilibrium eutectic melting did not occur and no intergranular cracks were observed after quenching. No evidence of non-equilibrium eutectic melting was observed in the Al–Li–Cu alloy irrespective of the rate of heating to 550°C. During tensile testing of as quenched and quenched and aged specimens of the two alloys, intergranular fracture occurred in most specimens, whether or not non-equilibrium eutectic melting had taken place during solution treatment, indicating that at least one additional mechanism of intergranular fracture was initiated by deformation.

MST/947     

Superplasticity of fine-grained magnesium alloys for biomedical applications: A comprehensive review

The superplastic behavior of medical magnesium alloys is reviewed in this overview article. Firstly, the basics of superplasticity and superplastic forming via grain boundary sliding (GBS) as the main deformation mechanism are discussed. Subsequently, the biomedical Mg alloys and their properties are tabulated. Afterwards, the superplasticity of biocompatible Mg-Al, Mg-Zn, Mg-Li, and Mg-RE (rare earth) alloys is critically discussed, where the influence of grain size, hot deformation temperature, and strain rate on the tensile ductility (elongation to failure) is assessed. Moreover, the thermomechanical processing routes (e.g. by dynamic recrystallization (DRX)) and severe plastic deformation (SPD) methods for grain refinement and superplasticity in each alloying system are introduced. The importance of thermal stability (thermostability) of the microstructure against the grain coarsening (grain growth) is emphasized, where the addition of alloying elements for the formation of thermally stable pinning particles and segregation of solutes at grain boundaries are found to be major controlling factors. It is revealed that superplasticity at very high temperatures can be achieved in the presence of stable rare-earth intermetallics. On the other hand, the high-strain-rate superplasticity and low-temperature superplasticity in Mg alloys with great potential for industrial applications are summarized. In this regard, it is shown that the ultrafine-grained (UFG) duplex Mg-Li alloys might show remarkable superplasticity at low temperatures. Finally, the future prospects and distinct research suggestions are summarized. Accordingly, this paper presents the opportunities that superplastic Mg alloys can offer for the biomedical industries.     

The control of grain boundary segregation and segregation-induced brittleness in iron by the application of a magnetic field

Recycling of iron and steel becomes an universally important issue from the viewpoint of energy and resource saving. Impurity elements like Sn and Cu tend to accumulate in steels by repeated recycling and remarkably degrade mechanical properties of recycled iron alloys due to segregation-induced intergranular embrittlement. The goal of this work is to study the potential of magnetic annealing for the control of grain boundary segregation and intergranular embrittlement in iron alloy. This paper reports several important findings regarding the effect of magnetic annealing on segregation-induced brittleness in iron-tin alloy. Of particular importance is the observations that the concentration of tin at grain boundaries in iron is decreased by magnetic annealing and fracture toughness of iron-tin alloy is drastically improved to the level as high as pure iron.     

Influence of vanadium on grain boundary segregation of phosphorus in iron and iron-carbon alloys

The influence of vanadium on grain boundary segregation of phosphorus has been studied in iron and iron-carbon alloys by means of fracture experiments in a scanning Auger microprobe. The emphasis here is to study the effects of vanadium on the interaction processes operative under circumstances when structure in the interior of the grain (in the present case carbide formation) and grain boundary segregation form simultaneously. It is emphasized that to predict and analyse the behaviour of an alloy, it is important to consider atomic interactions both at the grain boundaries and in the grain interior and that between the constituents and the grain boundaries. The study suggests that the principal determining factor in the scavenging or retardation of migration of phosphorus to the grain boundaries is whether vanadium is present in the combined form (say, carbide) or is available in solid solution form. When vanadium is present in solid solution form, grain boundary segregation of phosphorus is low because of the chemical interaction of vanadium and phosphorus. However, as carbon is increasingly introduced in the alloy, vanadium now preferentially reacts with carbon in view of higher interaction for carbon as compared to phosphorus. A consequence of this is the increase in the grain boundary concentration of phosphorus. In such a situation the presence of excess carbon in addition to what is stoichiometrically required to precipitate the entire vanadium as vanadium carbides, serves as a palliative with regard to the reduction in the intergranular concentration of phosphorus. This palliative behaviour is explained in terms of the sitecompetition model. An effort is also made to examine the behaviour of segregating elements in terms of whole range of probable interactions (both at the grain boundaries and in the grain interior) and chemical interaction energies.     

Nanoscale deformation of multiaxially forged ultrafine-grained Mg-2Zn-2Gd alloy with high strength-high ductility combination and comparison with the coarse-grained counterpart

Cold processing of magnesium(Mg) alloys is a challenge because Mg has a hexagonal close-packed(HCP)lattice with limited slip systems, which makes it difficult to plastically deform at low temperature. To address this challenge, a combination of annealing of as-cast alloy and multi-axial forging was adopted to obtain isotropic ultrafine-grained(UFG) structure in a lean Mg-2Zn-2Gd alloy with high strength(yield strength: ~227 MPa)-high ductility(% elongation: ~30%) combination. This combination of strength and ductility is excellent for the lean alloy, enabling an understanding of deformation processes in a formable high strength Mg-rare earth alloy. The nanoscale deformation behavior was studied via nanoindentation and electron microscopy, and the behavior was compared with its low strength(yield strength: ~46 MPa)-low ductility(% elongation: ~7%) coarse-grained(CG) counterpart. In the UFG alloy, extensive dislocation slip was an active deformation mechanism, while in the CG alloy, mechanical twinning occurred.The differences in the deformation mechanisms of UFG and CG alloys were reflected in the discrete burst in the load-displacement plots. The deformation of Mg-2Zn-2Gd alloys was significantly influenced by the grain structure, such that there was change in the deformation mechanism from dislocation slip(non-basal slip) to nanoscale twins in the CG structure. The high plasticity of UFG Mg alloy involved high dislocation activity and change in activation volume.     

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

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

Effects of Sn segregation and precipitates on creep response of Mg‐Sn alloys

Mg‐Sn alloys are promising for the development of new cheap creep resistant magnesium alloys. In the present paper, the creep behaviours of Mg‐Sn and Mg‐Sn‐Ca alloys were examined at the constant temperature and different stresses. The measurements of stress exponents indicate that the dislocation climbing is the dominant mechanism during the creep of Mg‐3Sn or Mg‐3Sn‐2Ca alloys. The poor creep resistance of the binary Mg‐3Sn alloy is caused by the easy movement of dislocation and the segregation of Sn at the boundaries. Both T4 and T6 heat treatments improve the creep resistance of Mg‐3Sn alloy due to the alleviation of Sn segregation at grain boundaries and the precipitation of Mg₂Sn particles, respectively. Ca is an effective alloying element to increase the creep resistance of Mg‐Sn alloys. The Ca addition leads to the formation of thermal stable phases Mg₂Ca and CaMgSn in Mg‐3Sn‐Ca alloys. These two phases effectively hinder the movement of dislocations and the sliding of grain boundaries. On the other hand, the addition of Ca alleviates the segregation of Sn by the interaction of Ca with Mg and Sn to form the phase CaMgSn.     

Quantitative metallographic assessment of the electromagnetic casting influence on the microstructure of 7075 Al alloy

This study presents an attempt to obtain the better quality of an aluminum super-high strength alloy by application of electromagnetic field during the casting process. The conventional continuous casting process of aluminum alloys causes many defects, such as surface imperfections, grain boundary segregation, non-uniform grain size, and porosity. The better ingot surface along with the homogeneous fine-grained microstructure, and hence the better mechanical properties of the ingot, can be achieved by applying the electromagnetic casting process. The microstructure characterization, accompanied by quantitative metallographic assessment, reveals that it is possible to avoid or decrease many defects of as cast ingots during electromagnetic casting process. In this article, the microstructure of the samples of as cast 7075 aluminum alloy, obtained with and without electromagnetic field influence, was analyzed by optical microscope and the variation of key alloying elements content, i.e., Zn and Mg, through the ingot cross section was examined by chemical analysis. Besides, the microstructural parameters such as dendrite arm spacing, interdendritic space width, as well as eutecticum and intermetallic phases volume fraction, were measured using linear method. The electromagnetic field influence on the microstructure of the as cast 7075 Al alloy was evaluated based on measured quantitative metallographic data.     

Effects of Sr and Sn on microstructure and corrosion resistance of Mg–Zr–Ca magnesium alloy for biomedical applications

Magnesium based alloy is a biodegradable metal that has significant potential advantages as an implant material. Element alloying is one of the effective methods to modify the performance of the magnesium alloy. In the paper, Sr and Sn as alloy elements were simultaneously added into the Mg–Zr–Ca alloy to improve the corrosion resistance. The differences of Mg–Zr–Ca alloy and Mg–Zr–Ca–Sr–Sn alloy were compared. The X-ray diffractometer (XRD) and the scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) were used to analysis the phases and the microstructure of the alloys. The results indicated that the addition of Sn could form Mg₂Sn mainly within the grain interior; Sr phase was mainly detected along the grain boundary. Immersion tests and electrochemical measurements showed that the corrosion resistance was improved obviously with simultaneous addition of Sr and Sn in Mg–Zr–Ca alloy. It suggested that bio-magnesium based alloy can use Sr and Sn as effective alloy elements to modify its performance.     

通过离子掺杂剂界面偏析来提高氧化物弥散强化合金的机械性能

氧化物弥散强化钨合金(ODS-W)因具有显著提升的抗辐照能力、高温强度以及抗蠕变性能而展现出巨大的高温应用潜力.然而,分散在合金中的氧化物第二相颗粒容易在钨晶界处团聚并长大(甚至到微米尺寸),这大大抑制了它们对钨合金的强化效果.目前,如何有效细化和分散钨晶界处的氧化物第二相颗粒一直是人们面临的巨大挑战.在本文中,我们通...     

Surface alloying of Mg alloys after surface nanocrystallization

Surface nanocrystallization using a surface mechanical attrition treatment effectively activates the surface of magnesium alloys due to the increase in grain boundary diffusion channels. As a result, the temperature of subsequent surface alloying treatment of pure Mg and AZ91 alloy can be reduced from 430 degrees C to 380 degrees C. Thus, it is possible to combine the surface alloying process with the solution treatment for this type of alloy. After surface alloying, the hardness of the alloyed layer is 3 to 4 times higher than that of the substrate and this may significantly improve the wear resistance of magnesium alloys.     

Lüders elongation in AI-Mg alloy AA5182

Abstract

Aluminium-magnesium alloys such as AA5182 exhibit a yield point elongation, or Lüders strain, when deformed in the annealed O temper, and the extent of the Lüders strain is a function of processing history and the deformation conditions. The yield point effect is usually attributed to Mg solute pinning, but is has been appreciated for some time that factors such as grain size, and cooling rate from the annealing temperature, affect the Lüders strain, and it has not been clear how this behaviour is consistent with solute pinning. In this paper it is shown that the presence, or absence of Lüders effect (Lüdering) is related to the grain boundary structure, and processing which results in a high concentration of grain boundary dislocations removes the yield point effect, whereas fabrication routes which produce defect free bondaries result in a Lüders strain. The extent of the Lüders strain, and its dependence on grain size and deformation conditions, is reasonably consistent with some recent models of propagative plastic instabilities.     

The mechanism of elevated temperature intergranular cracking in heat-resistant alloys

Reheat or stress relief cracking phenomena have been reassessed in 2.25Cr1.5W heat-resistant alloys. During rupture test, time to intergranular failure increases with decreasing temperature and tensile stress and is shorter in the alloy containing a higher bulk content of phosphorus. Also the time to intergranular failure can be expressed by t = t₀·σ⁻ⁿ·exp(Q/RT) where t₀ is the proportional constant, n the stress exponent and Q the activation enthalpy. Matrix softening is accelerated under tensile stress and an active carbide growth occurs at grain boundaries oriented normal to the tensile stress direction. Because impurities segregate actively to dimples frequently observed at reheat intergranular fracture surfaces, the dimples are not micro-ductile fracture areas but the grain boundary carbide interfaces. The segregation concentration of the impurities is much higher at the grain boundary carbide interfaces than the carbide-free grain boundaries. The phosphorus segregation at the carbide interfaces of the alloy containing the higher bulk content of phosphorus is mainly replaced by the segregation of nitrogen, tin and tellurium in the alloy containing a lower bulk content of phosphorus. The elevated temperature intergranular cracking under tensile stress occurs finally due to the carbide-free grain boundary cracking following the decohesion of the grain boundary carbide interfaces.     

The effect of selected alloying element additions on properties of Mg-based alloy as bioimplants: A literature review

This review investigates the current application limitations of Mg and Mg alloys. The key issues hindering the application of biodegradable Mg alloys as implants are their fast degradation rate and biological consideration. We have discussed the effect of some selected alloying element additions on the properties of the Mg-based alloy, especially the nutrient elements in human (Zn, Mn, Ca, Sr). Different grain sizes, phase constituents and distributions consequently influence the mechanical properties of the Mg alloys. Solution strengthening and precipitation strengthening are enhanced by the addition of alloying elements, generally improving the mechanical properties. Besides, the hot working process can also improve the mechanical properties. Combination of different processing steps is suggested to be adopted in the fabrication of Mg-based alloys. Corrosion properties of these Mg-based alloys have been measured in vitro and in vivo. The degradation mechanism is also discussed in terms of corrosion types, rates, by-products and response of the surrounding tissues. Moreover, the clinical response and requirements of degradable implants are presented, especially for the nutrient elements (Ca, Mn, Zn, Sr). This review provides information related to different Mg alloying elements and presents the promising candidates for an ideal implant.     

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.