Phase transformation reactions in rapidly solidified Al-6.5Fe-1.5V alloys

Phase transformation reactions, occurring during heating of as-atomised Al-6.5Fe-1.5V powders, extrusion of the powders, and heating of the as-extruded alloys produced from the powders, have been studied by DSC, XRD and TEM. The DSC studies of the as-atomised powders revealed several phase transformation reactions. The solid solution in zone A decomposed to form metastable phases at 360°C. These metastable phases further transformed to form equilibrium phases at 500°C. The microquasi-crystalline icosahedral (MI) phase particles present in zone A and zone B transformed to equilibrium phases at 500°C. The globular clusters of microquasi-crystalline icosahedral (GCMI) phase particles in zone C transformed polymorphously to icosahedral (I) phase particles at 450°C. These reactions were believed to occur during extrusion of the powders. During heating of the as-extruded alloys produced from coarse powder particles, I phase transformed polymorphously to hexagonal phase at 550°C. The hexagonal phase decomposed to monoclinic Al₄₅(V, Fe)₇ and Al₁₃Fe₄ phases upon heating for longer times.     

Powder metallurgy aluminium alloys: characteristics of an Al-Cr-Fe rapidly solidified alloy

An Al-4Cr-1 Fe alloy has been evolved utilizing the advantages of rapid solidification technology. The paper describes the formation of the as-atomized (inert gas) powder microstructure and its decomposition duringin situ heating. It was observed that the most typical powder microstructure had a cellular morphology with a fine intercellular network consisting of ironrich phases. Decomposition of the powder duringin situ heating commenced from the intercellular network, finally resulting in a matrix with a high volume fraction of chromium-rich globular-like precipitates. Consolidation was achieved through cold compaction and hot extrusion; the alloy being easily extrudable. The room-temperature mechanical properties of the alloy were also assessed. The 0.2% proof stress and the tensile strength were below the target limits for dispersion-strengthened alloys, but the elongation and fracture toughness values were very promising. Finally, the extruded microstructure was related to mechanical properties.     

Effects of Fe concentration on microstructure and corrosion of Mg-6Al-1Zn-xFe alloys for fracturing balls applications

Mg-6Al-1Zn-xFe (x = 0, 1, 3, 5 and 7 wt%) alloys were prepared by powder metallurgy and followed by hot extrusion. Majority of Fe element exists as insoluble particles in the alloys. The as-extruded alloys showed higher degradable rates but less stable mechanical properties than as-annealed alloys. Corrosion rate of all the alloys increased with increasing Fe concentration, reaching 2.4 mL cm⁻² h^-1. 0.2% yield strength of all the alloys was higher than 150 MPa. In short, Mg-6Al-1Zn-xFe alloys have an attractive combination of corrosion and mechanical properties, which holds a bright future for fracturing balls applications.     

Effect of extrusion conditions on mechanical properties of Al-20Si-3Cu-1 Mg alloy prepared from rapidly solidified powder

A study on the optimization of extrusion conditions for a prospective Al-20Si-3Cu-1 Mg alloy prepared from rapidly solidified powder was carried out by evaluating tensile properties at room and elevated temperatures. It was found that extrusion conditions influence the asextruded microstructure and mechanical properties of the alloy to a certain extent. The relationship between the as-extruded mechanical properties and such extrusion variables as temperature, reduction ratio and die shape, can be described by using temperature-compensated strain rate, so that the as-extruded properties can be tailored in a certain range by adjusting this process parameter. In addition, the comparison between the as-extruded and as-T6 tempered tensile properties at elevated temperatures has opened the question as to the necessity of applying the heat treatment to the alloy — a normal practice subsequent to the extrusion. The experimental results suggest that for the material used at temperatures at and above 200 °C, the T6 temper treatment can be eliminated.     

Effects of samarium content on microstructure and mechanical properties of Mg–0.5Zn–0.5Zr alloy

Effects of samarium (Sm) content (0, 2.0, 3.5, 5.0, 6.5 wt%) on microstructure and mechanical properties of Mg–0.5Zn–0.5 Zr alloy under as-cast and as-extruded states were thoroughly investigated. Results indicate that grains of the as-cast alloys are gradually refined as Sm content increases. The dominant intermetallic phase changes from Mg₃Sm to Mg₄₁Sm₅ till Sm content exceeds 5.0 wt%. The dynamically precipitated intermetallic phase during hot-extrusion in all Sm-containing alloys is Mg₃Sm. The intermetallic particles induced by Sm addition could act as heterogeneous nucleation sites for dynamic recrystallization during hot extrusion. They promoted dynamic recrystallization via the particle stimulated nucleation mechanism, and resulted in weakening the basal texture in the as-extruded alloys. Sm addition can significantly enhance the strength of the as-extruded Mg–0.5Zn–0.5 Zr alloy at room temperature, with the optimal dosage of 3.5 wt%. The optimal yield strength (YS) and ultimate tensile strength (UTS) are 368 MPa and 383 MPa, which were enhanced by approximately 23.1% and 20.8% compared with the Sm-free alloy, respectively. Based on microstructural analysis, the dominant strengthening mechanisms are revealed to be grain boundary strengthening and dispersion strengthening.     

Cu-10Cr-0.4Zr形变原位复合材料的组织演变特征

制备了Cu-10Cr和Cu-10Cr-0.4Zr合金,并经冷变形形成了原位复合材料。研究Zr添加剂对合金铸态组织和复合材料的纤维形貌的影响,以及随着形变率的提高β-Cr纤维演变特征。研究表明,在Cu-10Cr合金中添加的0.4%(质量分数)Zr,Cr析出相的直径由15~80μm细化到10~20μm;对Cu-10Cr-0.4Zr合金能谱分析表明,在Cu-10Cr-0.4Zr铸态组织中存在Cu5Zr相的形成和析出;随着形变率增大,β-Cr相之间的间距不断减小,其宽厚比也进一步增大,纤维相发生比较明显的弯曲和扭折,特别是当形变率η=6.2时,纤维相的厚度能够达到250~350nm,纤维相变形和分布也趋于均匀;当η=6.2时,Cu-10Cr-0.4Zr形变复合材料的抗拉强度达到1089 MPa,采用改进的Hall-Petch公式计算其值为1037 MPa,理论计算数值与观测结果基本一致。     

Mechanical response and structural development during the hot extrusion of a rapidly solidified Al-20Si-7.5Ni-3Cu-1Mg alloy powder

The consolidation of an air-atomized Al-20Si-7.5Ni-3Cu-1 Mg alloy powder was performed utilizing hot extrusion, to determine its extrudability and understand its structural development in relation to process parameters. One of the main features exhibited by the material in this process was a high degree of softening over a peak extrusion pressure, which has been explained by the simultaneous onset of dynamic recovery and recrystallization during deformation. The peak extrusion pressure was shown to be strongly dependent upon the temperature applied, and this dependence has been described with temperature compensated strain rate. It was also observed that the process parameters had a fairly narrow range applicable to the extrusion of the powdered alloy and a significant influence on the deformation behaviour of the powder particles. The combination of heating and deformation, primarily used to convert the loose powder particles into an engineering material, resulted in the decomposition of the meta-stable aluminium matrix and transformations of constituent phases, initially formed in the rapidly solidified powder. Additionally, it was found that the extrusion temperature had an effect on the lattice size and perfection of the as-extruded matrix in the material. Three intermetallic dispersoids containing nickel were detected in the consolidated material, independent of extrusion temperature, and their formation was promoted by hot deformation. The silicon crystal phase in the extruded material was reshaped, and its size was insensitive to the extrusion temperature, which is thought to be caused by a high volume fraction of the coexistent dispersoids. The dispersions of the silicon crystals and intermetallic compounds with various sizes in the matrix substantially modified the deformation mode of the alloy. Evidence of dynamic recrystallization was found, which co-operated with dynamic recovery during deformation, giving rise to a duplex microstructure in the extruded material.     

THE INFLUENCE OF HYDROGEN ON THE FATIGUE BEHAVIOUR OF THE BETA-TITANIUM ALLOY Ti-3Al-8V-6Cr-4Mo-4Zr

In order to characterize the influence of hydrogen on the mechanical properties of β-titanium alloys, monotonic tensile and strain-controlled fatigue tests were performed on samples of the metastable alloy Ti-3Al-8V-6Cr-4Mo-4Zr in uncharged (0.5 at.% hydrogen) and hydrogen-charged (3-4 at.% hydrogen) conditions. The hydrogen was introduced into the material during the last 8 h of an ageing treatment (28 h at 482°C) from the gas phase, whereas the reference (uncharged) specimens were annealed completely in vacuum. The results of the mechanical tests indicate that hydrogen slightly increases the strength of the alloy in monotonic as well as in cyclic loading. Under tensile loading the fracture strain decreases as a result of hydrogen. Under cyclic loading both charged and uncharged conditions show initial softening followed by a saturation state. The cyclic lifetime at a constant total strain amplitude, however, is not reduced by the hydrogen charging. The effect of hydrogen on the mechanical behaviour can be interpreted and understood on the basis of microstructural observations that reveal a hydrogen-induced change in the precipitation state. This indirect influence of hydrogen on the microstructure, which leads to a reduction of the mean size of the α-precipitates, in combination with a slight decrease on the volume fraction of the α-phase, seems to dominate over any direct intrinsic hydrogen effect     

Microstructures and Tensile Properties of Isothermally-forgedTi-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr Alloys

Microstructures and room-temperature tensile properties of isothermally-forged γ-base (γ + α2)alloys in Ti-Al-Nb-Cr-V system with different heat treatments were investigated. The results show that the microstructures of Ti-47Al-2Nb-1Cr-1V and Ti-47Al-2V-1Cr (at. pct) alloys are mainly determined by heat treating temperature in the (cr + 7) tWo-phase field, and the joint additions of Nb, Cr and V in the Ti-47Al alloy afFect Ta significantIy. The microstructure of Ti-47Al alloy with additions of Nb, Cr and V (1~2 at. pct) can be dupIex or nearly-lamellar by a suitable heat treatment after isothermal forging at 1000℃ for over 50% plastic strains.Therefore its tensile properties can be improved at room temperature.     

The microstructure of Al-8 wt% Fe-based alloys prepared by rapid quenching from the liquid state

The results of a transmission electron microscope study of the microstructure of splat-quenched Al-8% Fe are described in detail. Two distinct structures, zone A and zone B, are examined in as-quenched samples, and each characterized in terms of the dispersions and types of phases present. The decomposition behaviours of zone A and zone B during annealing at temperatures between 573 and 823 K (300 and 550°C) are investigated and the associated phase transformations determined. The effect on the as-quenched, and annealed, microstructures of adding either 3% Mn or 1% Zr to the alloy is described. The observed microstructures and phase transformations are correlated with micro-hardness measurements.     

Influence of powder metallurgical processing on production and properties of rapidly solidified Al–5Cr–2Zr,Al–6·43Cr–1·67Zr,and Al–4Cr–1Fe extrudates

Abstract

Rapidly solidified Al–5Cr–2Zr, Al–6·43Cr–1·67Zr, and Al–4Cr–1Fe alloy powders were processed using cold compaction and hot extrusion. It was found that the iron containing alloy was more easily extruded than the zirconium containing alloys, and this was attributed to phase transformations occurring during deformation of the latter. By obtaining extrudates in the form of rectangular bars, mechanical properties could be studied for both the transverse and the longitudinal directions. Compared with the longitudinal direction, no significant decrease of strength was detected in the transverse direction for any of the alloys, whereas significant decreases of ductility were recorded, especially for Al–6·43Cr–1·67Zr alloy. Fracture was observed to occur along primary powder particle boundaries. The relationship between microstructure and mechanical properties was also investigated. Compared with the addition of iron, it was found that additions of zirconium are more beneficial in that they promote formation of fine intermetallic phases. In addition, between the Al–Cr–Zr alloys, a reduction of chromium content yields a more homogenous and fine microstructure, which combined with the beneficial effects of increased additions of zirconium results in superior properties.

MST/1118     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Fabrication,properties and structure of a high temperature light alloy composite

Paniculate alumina reinforced Al-4Cr-1 Fe alloys were fabricated from rapidly solidified aluminium alloy powder and commercially purchased alumina powder by traditional powder metallurgical techniques involving powder mixing and cold compaction followed by hot extrusion. The tensile tests at ambient temperature indicated a considerable improvement in the mechanical strength at the expense of ductility and modulus. Poor values of modulus were explained by the presence of porosity in the composites. The high temperature mechanical properties of the matrix, tested at 350 °C after prolonged exposure to the test temperature under static air conditions, were intrinsically poor. Additions of the filler material, alumina particles, up to a weight fraction of 15% did not improve the high temperature performance of the matrix substantially. Possible causes for this are discussed and alternatives proposed.     

Effect of Si content on microstructure and mechanical properties of Al–Si–Mg alloys

Microstructure and mechanical properties of as-cast and as-extruded Al–Si–Mg alloys with different Si content are investigated by tensile test, microstructure observation. High density of Si particles in the Al alloys can induce dynamic recrystallization during hot extrusion and it becomes more matured with an increase in the density of Si particles. The tensile strength of as-cast and as-extruded alloys can be improved with the increase of Si content and hot extrusion make the elongation of alloys increase dramatically. Considerable grain refining effect caused by recrystallization occurred during hot extrusion of S2 (equivalently commercial A356 alloy) and S3 (near eutectic alloy) alloys plays an important role in the improvement of elongation. A good combination of strength and elongation for the as-extruded S3 alloy indicates that near eutectic Al–Si alloys can be hot-extruded to produce aluminum profiles with high performance.     

固溶温度对Ti-4Al-6Mo-2V-5Cr-2Zr钛合金的组织和拉伸性能的影响

研究了固溶温度对一种亚稳β钛合金(Ti-4Al-6Mo-2V-5Cr-2Zr)的锻态组织和室温拉伸性能的影响。结果表明,固溶温度低于相变点时大量的α相在β基体中析出并聚集在滑移带附近,随着固溶温度接近相变点α相的数量减少且部分滑移带消失。固溶温度高于相变点时显微组织为单一的β相且滑移带完全消失,随着固溶温度继续升高β晶粒聚集且长大。这种合金经750℃×1 h固溶处理后达到良好的强度塑性匹配,气抗拉强度、屈服强度和伸长率分别为957 MPa、887 MPa和11.7%。     

The structure and properties of a cold-rolled and annealed Al-0.8 wt % Zr alloy

The effect of annealing at 400? C on the microstructure of a cold-worked Al-0.8 wt % Zr alloy is reported. It is shown that the initially high dislocation density in the cold-rolled material is progressively reduced, although the grains and subgrains were exceptionally resistant to coarsening. Precipitation of the metastable cubic Al₃Zr phase occurred, both discontinuously in the form of fan shaped precipitates and also on the grain boundaries and within the grains as small, nearly spherical particles. The mechanical properties of the alloy at 400? C are consistent with a major dislocation contribution to the overall deformation process, in contrast with most other fine grained materials which are superplastic.     

Behavior of Zr-rich phase in a cast NiAl-33.5Cr-0.5Zr alloy during heat treatment

There are many Zr particles in as-cast NiAl-33.5Cr-0.5Zr (at. pct) alloy, which usually exist at the edge of eutectic of beta -NiAl and cx-Cr. After air and furnace cooling solution treatments, far 1400 degreesC, 2 h and 1450 degreesC, 1 h, pure Zr phase remains in the furnace cooling (F.C.) state alloys and Ni2AlZr phase forms in the air cooling (A.C.) state alloys. During solution treatment at 1450 degreesC, bulk and "fish bone" shape Zr-rich phases form respectively in F.C. and A.C. state alloys. A 'river' shape Ni2AlZr phase forms after 1450 C for 1h F.C. and 850 degreesC for 12 h, F.C.. The alloy has less pure Zr and Ni2AlZr phase after 1400 degreesC with both air and furnace cooling followed by 850 C and 950 C for 12 h, F.C. aging treatments, respectively. Additionally, there is a ternary eutectic of NiAlZr and a phase enriched Zr and Cr forms at the edge of the eutectic of beta -NiAl and alpha -Cr in the alloy treated at 1400 degreesC, 2 h, F.C. and 950 degreesC, 12 h, F.C.     

Characterization of phase development in non-isothermally annealed mould-cast and heat-treated Al–Mn–Sc–Zr alloys

The effect of Mn addition on microstructure and mechanical properties during isochronal annealing in the temperature range of 20 °C–570 °C of the mould-cast and heat-treated Al–Sc–Zr alloys with a various content of Mn and Zr was studied. The electrical resistometry together with the microhardness (HV0.3) measurements were compared to microstructure development. The microstructure development was examined by scanning electron microscopy, transmission electron microscopy and electron diffraction. Relative resistivity changes and the microhardness of the mould-cast and heat-treated Al–Mn–Sc–Zr alloys exhibit similar dependence on annealing temperature. Precipitation of the Al₃Sc particles is responsible for the peak microhardness in all these alloys. The microhardness decrease is slightly delayed during the isochronal annealing and during the high temperature heat treatment in the mould-cast alloy with the higher Zr-content due to a higher oversaturation of Zr. The decomposition sequence of the oversaturated solid solution of the studied Al–Mn–Sc–Zr alloys is compatible with the recently published decomposition sequence of the Al–Sc–Zr system and also with the formation of Mn,Fe-containing particles. It seems very probable that the addition of Mn does not influence the decomposition of solid solution of the ternary Al–Sc–Zr system.     

Structural development during the extrusion of rapidly solidified Al-20Si-5Fe-3Cu-1Mg alloy

An investigation concerning the changes of powder structure and microstructure during the extrusion of an important Al-Si-Fe-Cu-Mg alloy prepared from rapidly solidified powder has been carried out. The fragmentation of needle-shaped intermetallics in the alloy has been regarded as one of the main features of the process, which happens concurrently with the interparticle bonding and the shaping of the porous billets. The as-extruded microstructure is found to be mainly composed of the dynamically recovered -Al matrix with numerous microcells, which are retained because of the inhibiting effect exerted by massive, fine second-phase particles on cell wall motion. Some recrystallized grains are also observed but their growth is effectively prevented. The refined intermetallics together with massive silicon particles and precipitates dispersed in the matrix can be expected to improve the thermal stability and high-temperature strength of the alloy to a great extent.     

Influence of rapid solidification route on microstructure and properties of a thermally stable Al-Cr-Zr alloy

An Al-5% Cr-2% Zr (wt%) alloy for elevated temperature service has been rapidly solidified (RS) by gas atomization and planar flow casting (PFC). A comparative study of the atomized powder and the planar flow cast ribbon has been performed during consolidation by extrusion. Particular attention has been given to the microstructural changes during fabrication and the influence of RS particulate type on extrusion performance, microstructure and mechanical properties. Microstructurally it has been shown that the more homogeneous PFC ribbon has advantages over the atomized powder and can improve mechanical properties. However, optimization of the thermomechanical processing of the PFC ribbon is necessary to achieve significant performance benefits in comparison to the atomized powder.