High Temperature Creep behaviour of a Si_3N_4 Whisker Reinforced Al-Fe-V-Si Composite

The creep behaviour of β-Si3N4 whisker reinforced Al-8.5Fe-1.3V-1.7Si composite has been investigated at the temperature 773 and 823 K. The results are characterized by high stress exponent and high apparent creep activation energy The creep data can be interpreted based on the incorporation of a threshold Stress and a load transfer coefficient into the power-law creep equation. A good correlation between the normalized creep rate and normalized effective stress is available which demonstrates that the creep behaviour of both the alloy and the composite is controlled by the matrix lattice self-diffusion in AI. EXamination on microstructure shows that edge dislocations exist at the interfaces between two adjacent whiskers and the intedeces emit edge dislocations in parallel paired-columns.     

Undercooling and supersaturation of alloying elements in rapidly solidified Al-8.5% Fe-1.2% V-1.7% Si alloy

Dispersion-strengthened Al-8.5% Fe-1.2% V-1.7% Si alloy was produced by inert gas atomization and atomized melt deposition processes. Differential scanning calorimetry was used to estimate the extent of undercooling in the alloy powders as a function of powder size and in the atomized melt-deposited alloy as a function of process parameters. The estimated undercooling was found to be a strong function of powder size and processing conditions and varied from 380–200 °C. Alloy powders of diameter greater than 180 jam did not experience any undercooling during solidification. X-ray diffraction analysis was performed to study the dependence of supersaturation of alloying elements and metastable phase formation on the extent of undercooling. When the undercooled alloy was heated to about 400 dgC, formation of Al₁₂(Fe, V)₃Si phase with b c c crystal structure from the supersaturated matrix was observed.     

多层喷射共沉积制备 SiCP/Al-8.5Fe-1.3V-1.7Si复合材料

采用多层喷射沉积工艺制备SiC_P/Al-8.5Fe-1.3V-1.7Si复合材料, 研究了雾化及沉积工艺参数对沉积坯状态及SiC颗粒捕获的影响。结果表明, 液流直径大、雾化气体压力小、喷射高度小会导致沉积坯组织恶化, 反之则造成收得率低、致密度低。雾化器扫描不均匀则会造成沉积坯形状不均匀, 而且会由于热量集中导致显微组织恶化。SiC颗粒输送压力的提高有利于SiC颗粒的捕获以及颗粒的均匀分布。多层喷射沉积制备SiC_P/Al-8.5Fe-1.3V-1.7Si的优化工艺参数为: 液流直径3.6 mm, 雾化气体压力0.8 MPa, 喷射高度200 mm, SiC 颗粒输送压力0.5 MPa。 沉积坯存在两种SiC-Al界面: 晶态Si界面层与非晶态SiO₂界面层。     

Threshold creep behaviour of discontinuous aluminium and aluminium alloy matrix composites: An overview

Several sets of creep data for aluminium and aluminium alloy matrix composites reinforced by silicon carbide particulates, silicon carbide whiskers or alumina short fibres are analysed. It is shown that for this class of discontinuous composites the threshold creep behaviour is inherent. Applying the concept of threshold stress, the true stress exponent of minimum creep strain rate of approximately 5 follows from the analysis even when the matrix solid solution alloy exhibits Alloy Class creep behaviour, for which the value of 3 for the true stress exponent is typical. The creep strain rate in the discontinuous aluminium and aluminium alloy matrix composites is shown to be matrix lattice diffusion controlled. The usually observed high values of the apparent stress exponent of creep strain rate and the high values of the apparent activation energy of creep are then rationalized in terms of the threshold creep behaviour. However, the origin of the threshold stress decreasing with increasing temperature but not proportional to the shear modulus in creep of discontinuous aluminium and aluminium alloy matrix composites is still awaiting identification. The creep-strengthening effect of silicon carbide particulates, silicon carbide whiskers and alumina short fibres is shown to be significant, although the particulates, whiskers and short fibres do not represent effective obstacles to dislocation motion.     

Microstructure and mechanical properties of RSP/M Al-Fe-V-Si and Al-Fe-Ce alloys

Two aluminium alloys with nominal compositions of Al-8Fe-4Ce and Al-8Fe-1V-2Si (all compositions in wt%) were rapidly solidified by ultrasonic gas atomization. The atomized powders with an average particle size (d₅₀) of 30 m were vacuum hot pressed and subsequently hot extruded. The P/M extrusion exhibited similar microstructure and elevated temperature tensile properties. The tensile and stress rupture samples of both the alloys exhibited ductile dimple failure. However, the Al-Fe-V-Si extrusion samples exhibited significantly better creep and stress rupture properties. The Al-Fe-Ce alloy was found to be more susceptible to cavitation at elevated temperatures which resulted in poor stress rupture properties.     

Tensile behaviour of two DO3-ordered alloys: Fe3Si and Fe-20 at % Al-5 at % Si

The tensile behaviour, including fracture modes and deformation substructures, of two powder-produced DO₃-ordered alloys having compositions Fe-25 at % Si (Fe₃Si) and Fe-20 at % Al-5 at % Si, has been investigated from room temperature to 800° C. The brittle-to-ductile transition temperature for the Fe₃Si alloy occurred at a temperature between 500 and 550° C, while that of the Fe-20 at % Al-5 at % Si alloy was approximately room temperature. In both alloys fracture occurred by transgranular cleavage at room temperature, with the occurrence of an increasing proportion of intergranular cavitation with increasing temperature. At low strains plastic deformation occurred chiefly by movement of perfect superlattice dislocations which, with increasing strain, dissociated to produce next-nearest-neighbour antiphase boundary trails.     

Deformation behaviour of Fe-3Si steel

Abstract

The deformation behaviour of an Fe-3Si (wt-%) steel was studied in the temperature range 400-900°C over six orders of magnitude of strain rate. It was found that the Fe-3Si steel exhibits a threshold behaviour. A correlation between the deformation behaviour and the temperature dependence of the threshold stress was etermined. An analysis in terms of the threshold stress showed that two modes of deformation behaviour exist in the power law creep regime. At normalised strain rate ?kT/(D ₁ Eb) ranging from 2 × 10^-6 to 10^-3, the value of the true stress exponent n is equal to 7, and at lower values of ?kT/(D ₁ Eb) the value of n is ~ 5. The true activation energy for plastic deformation Q _c increases from 250 ± 15 to 290 ± 30 kJ mol^-1 with increasing temperature from 550 to 700°C, and remains virtually unchanged at high temperatures. The relationship between rate controlling mechanisms of plastic deformation and mechanisms of interaction between lattice dislocations and dispersoids is discussed.     

Toughening of titanium alloys by twinning and martensite transformation

Two kinds of titanium alloys, titanium alloy (Ti-10V-2Fe-3Al) and titanium alloy (Ti-5Al-2.5Sn) were used to investigate the toughening mechanisms with new approaches. The results show that Ti-5Al-2.5Sn alloy possesses good combination of strength and ductility as well as satisfied low-cycle fatigue life both at 293 K and 77 K. As for Ti-10V-2Fe-3Al alloy, the microstructure with metastable phase shows lower strength and ductility but higher threshold stress intensity factor (K _th) than solution treated and aged microstructure composed of and phases. The microstructures also show that twinning occurs in deformation of Ti-5Al-2.5Sn alloy at 77 K. Twinning seems to be helpful for improving the low-cycle fatigue life to a great extent at cryogenic temperature. It's also found that owing to stress-assisted martensite transformation in metastable Ti-10V-2Fe-3Al alloy, the fatigue crack propagation path shows a very tortuous way, which decrease the effective stress intensity factor (K _eff) at crack tip, and increase threshold stress intensity factor (K _th).     

Structure, properties and fracture of friction stir welds in a high-temperature Al-8.5Fe-1.3V–1.7Si alloy (AA-8009)

Friction stir welds produced in a rapidly-solidified, powder metallurgy Al-8.5Fe-1.3 V-1.7 Si (wt.%) alloy were characterized in order to investigate the effects of deformation during welding on the weld zone microstructure, hardness, tensile properties, and fracture behavior. A weld produced using a tool rotational speed of 1200 rpm and a traversing rate of 4.3 mm/s exhibited a repetitive pattern of dispersoid-depleted bands that were attributed to the intense deformation that occurred in the vicinity of the tool. The significant softening associated with these regions, and the presence of occasional, irregularly-shaped voids near the boundary between the base metal and the weld zone on the advancing side of the weld, promoted a weld tensile strength of 60–70% of the base metal. The application of a lower tool rotational speed of 428 rpm and a lower traversing rate of 1.9 mm/s promoted fewer bands and a more uniform dispersoid distribution throughout the weld zone, and an absence of defects along the weld zone/base metal interface. Tensile strength of these welds approached 90% of the base metal. Fracture of the transverse-weld oriented tensile specimens for both weld types consistently occurred near the boundary between the weld zone and the base metal on the advancing side of the weld zone, with tensile specimen ductilities appreciably lower than that of the base metal.     

Isothermal hot deformation behaviour of metastable β titanium alloy Ti-10V-2Fe-3AI

Abstract

Elevated temperature true stress-true strain curves have been determined for the isothermal deformation of the metastable β titanium alloy Ti-10V-2Fe-3Al in both the β and (α + β) phase fields using hot compression testing. The flow stress behaviour of the alloy, in two initial microstructural conditions, has been correlated with the microstructural changes taking place during hot deformation. The shape of the isothermal hot deformation stress-strain curve is strongly dependent upon the starting microstructure of the material. The operative restoration mechanism during deformation in the β field isdynamic recovery since β subgrains are readily observed in the as β forged condition. Dynamic recovery of the β phase is also considered to be the dominant restoration mechanism for both microstructural conditions during deformation at temperatures high in the (α + β) field.     

High Temperature Deformation of a β''''(NiAl) Precipitation Strengthened Ferritic Alloy

A β'(NiAl) precipitation strengthened ferritic Fe-19Cr-2Al-4Ni alloy was tested under low cycle fatigue and creep at elevated temperatures. The dislocation microstructures developed were examined by transmission electron microscopy (TEM). The deformation behavior of the alloy was interpreted in terms of dislocationprecipitate configurations and interactions.fatigue deformation.     

Investigation of the effects of microstructure on creep deformation in α2-based titanium aluminide intermetallics

The results of a systematic investigation of the effects of microstructure/substructure on the secondary creep behaviour of α2-based titanium aluminide alloys are presented. This includes a study of the effects of heat treatment on the steady-state creep behaviour of α2+β-processed Ti-24Al-11Nb and Ti-25Al-10Nb-3V-1Mo at 540, 650 and 760°C, and an investigation of the effects of creep deformation on dislocation substructures in Ti-24Al-11Nb. The parameters that control secondary creep deformation are identified for both alloys, and the results are compared with data obtained for conventional high-temperature near-α titanium alloys and β-forged Ti-24Al-10Nb-3V-1Mo. This revised version was published online in November 2006 with corrections to the Cover Date.     

Impression creep behaviour of magnesium alloy-based hybrid composites in the transverse direction

The creep behaviour of a creep-resistant AE42 magnesium alloy reinforced with Saffil short fibres and SiC particulates in various combinations has been investigated in the transverse direction, i.e., the plane containing random fibre orientation was perpendicular to the loading direction, in the temperature range of 175–300 °C at the stress levels ranging from 60 to 140 MPa using impression creep test technique. Normal creep behaviour, i.e., strain rate decreasing with strain and then reaching a steady state, is observed at 175 °C at all the stresses employed, and up to 80 MPa stress at 240 °C. A reverse creep behaviour, i.e., strain rate increasing with strain, then reaching a steady state and then decreasing, is observed above 80 MPa stress at 240 °C and at all the stress levels at 300 °C. This pattern remains the same for all the composites employed. The reverse creep behaviour is found to be associated with fibre breakage. The apparent stress exponent is found to be very high for all the composites. However, after taking the threshold stress into account, the true stress exponent is found to range between 4 and 7, which suggests viscous glide and dislocation climb being the dominant creep mechanisms. The apparent activation energy Q_c was not calculated due to insufficient data at any stress level either for normal or reverse creep behaviour. The creep resistance of the hybrid composites is found to be comparable to that of the composite reinforced with 20% Saffil short fibres alone at all the temperatures and stress levels investigated. The creep rate of the composites in the transverse direction is found to be higher than the creep rate in the longitudinal direction reported in a previous paper.     

The effect of hydrogen on the strength and superplastic deformation of beta-titanium alloys

The effect of hydrogen on mechanical properties and superplastic deformation of two commercial titanium alloys, Ti-4Al-7Mo-10V-2Fe-1Zr (Ti-471021) and Ti-10V-2Fe-3Al (Ti-1023), was studied in the range of hydrogen concentration up to 1.3 wt%. The elevated temperature hardness of Ti-471021 alloy increased with hydrogen concentration. The stress levels during the superplastic deformation in both alloys increased with increasing concentrations of hydrogen in the -phase region. X-ray diffraction results and examination of the microstructure with TEM revealed that no hydrides had formed up to hydrogen concentrations of 1.3 wt%. The increase in flow stress was mainly due to the solid solution strengthening by hydrogen during the superplastic deformation.     

Creep features of Al2O3-Al alloy composites

Creep features of two cast aluminium alloy composites reinforced by Al₂O₃ short fibres randomly oriented in the matrices have been studied at 300 °C and several stress levels. The presence of short-term negative creep in primary creep is an important feature for the composites, which resulted from randomly oriented fibres strongly resisting dislocation creep in the matrix. However, the negative creep magnitude depended on both the applied stress and the nature of the material. There was a critical stress for the presence of the short-term negative creep. When the applied stress had exceeded the critical value, the negative creep disappeared. Fibres traversing grain boundaries can reinforce and resist grain boundary sliding at elevated temperature. The effect of stress on creep rate for the composites is not so strong as that for unidirectional metallic matrix composites. During the creep, some intermetallic phases in the Al₂O₃/Al-5Si-3Cu-1 Mg composite were precipitated and most of them were segregated at grain boundaries, leading to a small increase of the creep rate.     

The high-temperature creep behaviour of an Al-1 wt% Cu solid-solution alloy

The creep behaviour of an Al-1 wt% Cu solid-solution alloy is investigated at a temperature of 813 K under stress range of 0.5–5 MPa. The creep characteristics of the alloy including the stress dependence of the steady-state creep rate (n=4.4), the shape of creep curve (normal primary stage), the transient creep after stress increase, and the value of the true activation energy for creep, suggest that some form of dislocation climb is the rate-controlling process at higher stresses above 1 MPa. However, at low stresses (< 1 MPa), the creep curves show no distinguished steady state, and the stress dependence of the minimum creep rate is as high as ～ 8. The creep behaviour of the alloy is discussed based on recent theories available for describing creep in solid-solution alloys.     

Influence of Mn addition on the ordering of DO3 Fe-28%Al intermetallics

The DO₃-type ordering in Fe-28Al and Fe-28Al-1.5Mn alloys are investigated by TEM and XRD. The results show that Mn addition into DO₃-ordered Fe₃Al alloy could decrease degree of ordering. Two major factors are considered to have effect of Mn on ordering behaviour of the alloy: reducing grain size and reducing antiphase domain size. The further investigation of deformed antiphase boundaries and slip lines in the alloy with Mn addition suggests that Mn could promote slip and cross slip in DO₃ Fe₃Al alloy during deformation.     

Processing of nanocrystalline FeAlX (X = Ni, Mn) intermetallics using a mechanical alloying and hot pressing techniques

Fe-40Al-40Ni-20 and Fe-40Al-40Mn-20 (all in at.%) intermetallics were mechanically alloyed for 40 h and followed by hot-pressing at 650°C under 450 MPa for 1 h. As resulted from the X-ray diffraction studies, the ordered B2 structure was formed in the Fe-40Al-40Ni-20 alloy while in the case of Fe-40Al-40Mn-20 alloy, the disordered Fe(Al) solid solution was observed. The chemically homogenous rounded particles of size of about 5 μm were identified using scanning analytical electron microscopy in alloys after 40 h of milling. TEM studies of milled powders revealed a nanostructure in both alloys with grain size of about 20 nm. The hot pressing process of milled powders allowed to obtain compacts with the density of about 87 and 89% of the theoretical one for Fe-40Al-40Mn-20 and Fe-40Al-40Ni-20 alloys, respectively. The micro-hardness measurements have shown that the alloy with the Ni addition possesses the hardness of about 1200 HV₂₀, whereas in the alloy with the Mn addition it is 1100 HV₂₀. The TEM investigations allowed to identify a nanocrystalline structure of compacts with a mean grain size below 50 nm, with B2 ordered structure in both alloys.     

Subzero tensile properties of 7010 aluminium alloy and Ti-6A1-4V and IMI550 titanium alloys in sheet form

Tensile properties are reported for Al-6Zn-2.5Mg-1.7Cu-0.12Zr (7010), Ti-6Al-4V and Ti-4Al-4Mo-2Sn-0.5Si (IMI550) alloy sheet, 1.7 mm thick tested at 293, 223, 173 and 77 K. The strength of these alloys increased and the reduction of area decreased with decreasing test temperature. The Young's Modulus (E), 0.1% proof stress (σ_0.1) and true tensile strength ($\text{σ}{}_{\mathrm{TS}}$) were related to temperature T in degrees absolute (in the range 293-173 K for E and 293-77 K for σ_0.1 and $\text{σ}$_TS) by     

Ageing Effect on Hardness and Microstructureof Al-Zn-Mg Alloys

1. IntroductionALZn-Mg alloys are among the strongest of theage hardenable Al base alloysll'2]. These alloys havewide applications in aerospace and transport industries because of good mechanical properties especiallyexcellent strength to weight ratio which results ingreater fuel saving. Mechanical properties are affectedby various phases caused by heat treatment. Amountof solute (Zn+Mg) content and Zn/Mg ratio play animportant role in the precipitation of differeds phases,i.e., G.P. zones…