Tensile deformation behaviour of two aluminium alloys at elevated temperatures

Abstract

The tensile deformation behaviour of two recently developed aluminium alloys in the temperature range 200–550°C is characterized in this paper. The aluminium alloys studied here are an automotive stamping grade Al–Mg–Mn alloy and an Al–Li–Cu alloy. Tensile properties at elevated temperatures were determined under different temperature-strain rate combinations. An analysis of deformation and fracture behaviour at elevated temperatures is also presented. The Al–Mg–Mn alloy and the Al–Li–Cu alloy exhibited extended ductility or mild superplasticity at elevated temperatures. Metallographic and fractographic studies revealed appreciable grain growth and cavitation at elevated temperatures. The fracture elongation of Al–Mg–Mn alloy decreased beyond 430°C. Pronounced apparent strain hardening was observed in the case of the Al–Li–Cu alloy in the temperature range 525–550°C at a very low strain rate. This could be due to dynamic grain growth and/or dislocation structure evolution.     

Electrical and tensile properties of Cu-ThO2, Au-ThO2, Pt-ThO2 and Au-Al2O3, Pt-Al2O3 alloys

Dispersion-strengthened alloys of Pt, Au and Cu containing ThO₂ and Al₂O₃ were prepared by precipitating the elements from a solution containing a suspension of the oxide phase. The precipitate deposited on the oxide particles of 0.05μm average diameter and produced dispersions of good homogeneity on compaction. Alloys containing less than 2 vol% oxide phase had sufficient ductility to permit fabrication of wire. Tensile strength and elongation, hardness, and electrical resistivity were measured as a function of temperature up to 1000°C. The dispersion-strengthening caused a relatively small increase in the resistivity of the alloys compared to the resistivity of the metals. The alloys are useful where high electrical or thermal conductivity combined with superior tensile strength, hardness and oxidation resistance at elevated temperatures, are desirable.     

Localized strain and heat generation during plastic deformation in nanocrystalline Ni and Ni–Fe

Room temperature tensile testing was performed on a coarse-grained polycrystalline Ni (32 μm), a nanocrystalline Ni (23 nm) and two nanocrystalline Ni–Fe (16 nm) electrodeposits at two strain rates of 10^?1 and 10^?2/s. Strain localizations and local temperature increases were simultaneously recorded during tensile testing. For all materials, higher loads or higher strain rate generally resulted in higher peak temperature with the highest temperatures recorded in the fracture regions. The maximum temperature for the nanocrystalline materials was just over 80 °C, which is significantly below the reported temperatures for the onset of thermally activated grain growth. Therefore, the previously reported grain growth observed on similar materials after tensile deformation is likely not thermally activated but a stress-induced phenomenon. Despite the wide grain range from 16 nm to 32 μm, all samples exhibited similar strain localization behavior. Local strain variations initiated in the early stage of macroscopic uniform deformation, subsequent necking and fracture took place in the region of initial strain localization. While the coarse-grained polycrystalline Ni exhibited little strain rate sensitivity, gradually increased strain rate sensitivity was observed for the 23 nm Ni and the two 16 nm Ni–Fe samples, suggesting that both dislocation-mediated and grain-boundary-controlled mechanisms were operative in the deformation of the nanocrystalline Ni and Ni–Fe samples.     

Influence of grain size on hot ductility of plain C–Mn steels

Abstract

The influence of grain size on the hot ductility of 0·19 and 0·65wt-%C steels of the C–Mn type has been determined. For the low-carbon steel, a gram Size increase from 70 to 180 μm had only a small influence on hot ductility, as measured by tensile reduction in area values. However, increasing the grain size to 290 μm raised the temperature at which ductility started to fall by 50°C. In the finer grained steels it is believed that the ductility trough starts at the Ar₃ temperature when films of ferrite form round the stronger austenite grains. Ductility soon recovers as the temperature is lowered because of a thickening of the ferrite and a consequent reduction of strain concentration at the boundaries, so that only a narrow trough is observed. In coarser grained steels it is considered that deformation induced ferrite can have a pronounced influence on hot ductility over a wide range of temperatures leading to a wide ductility trough. Refining the grain size had an even greater influence on the hot ductility of the 0·65wt-%C steel. Intergranular tensile fracture at coarse grain size was by grain boundary sliding in the austenite resulting in a very wide ductility trough. Refining the grain size prevented intergranular failure occurring in the γ down to the lowest temperature examined: 700°C. Although the main influence of grain size is in controlling the width of the trough, the depth also increased with an increase in grain size.

MST/420     

Intermetallic reinforced Cr alloys for high-temperature use

Abstract

A new family of Cr(Ta)-Cr₂Ta intermetallic alloys based on Cr-(6-10)Ta (at%) is currently under development for structural use in oxidizing environments in the 1,000–1,300°C temperature range. These alloys show excellent strength and creep resistance and good oxidation resistance at high temperatures in air. Oxidation resistance comparable to commercial reactive element doped chromia-forming alloys and creep resistance comparable to single-crystal superalloys have been demonstrated. To date, only modest room-temperature fracture toughness (in the 11–12 MPa m^½ range) has been achieved. Preliminary results of a promising approach to improve room-temperature fracture toughness via ductilization of Cr with MgO additions are discussed.     

Mechanical properties of the Ni3(Si,Ti) alloys doped with carbon and beryllium

The Ni₃(Si, Ti) alloys doped with small amounts of carbon and beryllium were tensile tested in two environments, vacuum and air, over a wide range of test temperatures. The yield stresses of the carbon-doped alloys were almost identical to the undoped alloys while those of the beryllium-doped alloys were slightly higher than the undoped Ni₃(Si, Ti) alloys. The doping with carbon enhanced the elongation and ultimate tensile strength (UTS) whereas doping with beryllium reduced the elongation over the entire temperature range tested. The fracture patterns were primarily associated with the ductility behaviour. As the elongation (or UTS) increased, the fracture pattern changed from the intergranular to the transgranular fracture patterns. No environmental embrittlement of the ductility of the carbon-doped alloys was found at ambient temperatures but it was evident at elevated temperatures. Ductilities were reduced at high temperatures when the carbon-doped alloys were tensile tested in air. At high temperatures the environmental embrittlement observed is suggested to be due to the penetration of (free) oxygen into the grain boundaries causing the ductility loss in the carbondoped alloys.     

Effect of solution heat treatments on superalloys Part 1 – alloy 718

Abstract

The hot ductility as measured by Gleeble testing of Alloy 718 at four different solution heat treatments (954°C/15 h, 954°C/1 h, 982°C/1 h and 1050°C/3 h+954°C/1 h) has been investigated. It is concluded that constitutional liquation of NbC assisted by δ phase takes place and deteriorates the ductility. Parameters established by analysing the ductility dependence on temperature indicate a reduced weldability of the material in the coarse grain size state (ASTM 3) while indicating an increased weldability when containing a large amount of δ phase due to a grain boundary pinning effect. The accumulation of trace elements during grain growth at the highest temperature is believed to be the cause for the observed reduced on-cooling ductility.     

The development of Fe-Al intermetallics

The intermetallics based on aluminides have long been known for their excellent resistance to high-temperature oxidation. However, for use in structural components the poor ductility at ambient temperatures has always been felt as a stumbling block. Interest in these materials has been revived recently, after achieving some success in improving the ductility at ambient temperatures and creep at elevated temperatures in titanium aluminides. For the iron aluminides, too, similar methodologies have been attempted, namely alloying with elements such as titanium, boron, molybdenum, chromium, silicon and manganese, as well as grain refinement for improving high-temperature creep and room-temperature ductility. Raising the creep resistance close to 600 °C and improving the ambient-temperature ductility to around 6% have been the major immediate aims. Attempts are also being made to improve the high-temperature fatigue and creep properties in these materials, particularly by pushing the stability temperature of ordered D0₃ upwards. It is now visualized that once the above properties are achieved, the iron aluminides, particularly the types based on Fe₃Al, could offer themselves as excellent candidate materials For structural purposes. Their attractiveness also stems to a large extent from their low cost, as they contain only abundantly occurring materials. The present work examines two routes for introducing ductility in the Fe₃Al-based materials: one by ternary-Quarternary additions and the other by grain refinement. Structural studies have been made on materials obtained through conventional casting as well as through rapid solidification with minor alloy additions. The results confirm that Fe₃Al-based alloys, even when air-melted, are amenable to a high degree of hot working and could be made to display improved ductility at room temperature by a careful control of the chemistry. Rapidly solidified ribbons also show reasonably good bonding during high-temperature compaction. Ordering in these alloys is not suppressed even by rapid solidification.     

Effect of annealing on tensile properties of electrodeposited nanocrystalline Ni with broad grain size distribution

Abstract

The microstructures and tensile properties of electrodeposited nanocrystalline Ni (nc-Ni) with a broad grain size distribution after annealing at 150, 200 and 300°C for 500 s were investigated. The as deposited broad grain size distribution nc-Ni sample exhibited a moderate strength σ_UTS of ～1107 MPa but a markedly enhanced ductility ?_TEF of ～10%, compared with electrodeposited nc-Ni with a narrow grain size distribution. Annealing below 200°C increased the strength but caused a considerably reduction in tensile elongation. This behaviour is attributed to the grain boundary relaxation and the increased order of grain boundaries after annealing, which can make the grain boundary activities, such as the grain boundary sliding and grain rotations, more difficult. Further annealing at 300°C decreased both the yield strength and tensile elongation significantly due to significant grain growth.     

Grain boundary crystallography in two Fe-C-P alloys

Abstract

Misorientation, grain growth and brittle fracture were investigated in two iron - carbon alloys containing 0.06 wt-% phosphorus (0.06P) and 0.12 wt-% phosphorus (0.12P) after selected heat treatment schedules. A 'fracture surface serial sectioning' technique was devised and combined with misorientation measurements to reconstruct specimens after fracture. Anomalous grain growth occurred in the 0.06P specimen only, after 1000°C annealing. This was attributed to the inhomogeneous distribution of phosphorus at the interfaces. No evidence was found for the direct influence of misorientation angle distributions or coincidence site lattice distributions on anomalous grain growth. The proportion of Σ3s increased greatly after annealing at 1000°C, attributed to the twinning that developed in the austenite range. There was strong evidence that Σ3s were in general more resistant to brittle fracture than were random boundaries. It is suggested that alloys of this type could be 'grain boundary engineered' to improve fracture resistance.     

Optimization of mechanical property,antibacterial property and corrosion resistance of Ti-Cu alloy for dental implant

Ti-Cu alloys with different Cu contents (3, 5 and 7 wt%) were fabricated and studied as novel antibacterial biomaterials for dental application. The Ti-Cu alloys were annealing treated at different temperatures (740 °C, 830 °C and 910 °C) in order to obtain three typical microstructures, α-Ti + Ti₂Cu, α-Ti + transformed β-Ti, and transformed β-Ti. Mechanical, antibacterial and biocorrosion properties of Ti-Cu alloys with different microstructures were well analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), tensile test, electrochemical test and antibacterial test. The results indicated that the Ti-Cu alloys with microstructure of α-Ti + Ti₂Cu showed the best ductility compared with other Ti-Cu alloys with microstructures of α-Ti + transformed β-Ti and complete transformed β-Ti, and meanwhile, increase of the Cu content significantly contributed to the decreased ductility due to the increasing amount of Ti₂Cu, which brought both solid solution strengthening and precipitation strengthening. Finally, the Ti-5Cu alloy with microstructure of α-Ti + Ti₂Cu exhibited excellent ductility, antibacterial property and corrosion resistance, providing a great potential in clinical application for dental implants.     

Tensile properties and fracture behaviour of an ultrafine grained Ti–47Al–2Cr (at.%) alloy at room and elevated temperatures

An ultrafine grained (UFG) Ti–47Al–2Cr (at.%) alloy has been synthesized using a combination of high energy mechanical milling and hot isostatic pressing (HIP) of a Ti/Al/Cr composite powder compact. The material produced has been tensile tested at room temperature, 700 and 800 °C, respectively, and the microstructure of the as-HIPed material and the microstructure and fracture surfaces of the tensile tested specimens have been examined using X-ray diffractometry, optical microscopy, scanning electron microscopy and transmission electron microscopy. The alloy shows no ductility during tensile testing at room temperature and 700 °C, respectively, but very high ductility (elongation to fracture 70–100%) when tensile tested 800 °C, indicating that its brittle to ductile transition temperature (BDTT) falls within the temperature range of 700–800 °C. The retaining of ultrafine fine equiaxed grain morphology after the large amount of plastic deformation of the specimens tensile tested at 800 °C and the clear morphology of individual grains in the fractured surface indicate that grain boundary sliding is the predominant deformation mechanism of plastic deformation of the UFG TiAl based alloy at 800 °C. Cavitation occurs at locations fairly uniformly distributed throughout the gauge length sections of the specimens tensile tested at 800 °C, again supporting the postulation that grain boundary sliding is the dominant mechanism of the plastic deformation of the UFG TiAl alloys at temperatures above their BDTT. The high ductility of the UFG alloy at 800 °C and its fairly low BDTT indicates that the material a highly favourable precursor for secondary thermomechanical processing.     

Deformation behavior and mechanisms of a nanocrystalline multi-phase aluminum alloy

A nanocrystalline (nc) Al–Fe–Cr–Ti alloy containing 30 vol.% nc intermetallic particles has been used to investigate deformation behavior and mechanisms of nc multi-phase alloys. High compressive strengths at room and elevated temperatures have been demonstrated. However, tensile fracture strengths below 300 °C are lower than the corresponding maximum strengths in compression. Creep flow of the nc fcc-Al grains is suppressed even though rapid dynamic recovery has occurred. It is argued that the compressive strength at ambient temperature is controlled by propagation of dislocations into nc fcc-Al grains, whereas the compressive strength at elevated temperature is determined by dislocation propagation as well as dynamic recovery. The low tensile fracture strengths and lack of ductility at temperatures below 300 °C are attributed to the limited dislocation storage capacity of nanoscale grains. Since the deformation of the nc Al-alloy is controlled by dislocation propagation into nc fcc-Al grains, the smaller the grain size, the higher the strength. This new microstructural design methodology coupled with ductility-improving approaches could present opportunities for exploiting nc materials in structural applications at both ambient and elevated temperatures.     

Effect of yttrium on elastic strains and fracture of Cr2O3 scales on Ni-30Cr alloys

It has long been known that the addition of so-called reactive elements, such as Y, to alloys forming chromia scales improves the protectiveness of those scales by reducing the rate of scale growth and by reducing the tendency of the scales to spall during thermal cycling. It has been suggested that reduced spallation might arise from better scale adherence or from improved resistance to fracture.

In this study Ni-30Cr and Ni-30Cr-0.5Y alloy samples were oxidized at 1000°C in pure O₂ for various times, then were either furnace cooled to room temperature, or thermally cycled between 1000°C and different lower temperatures. Scale fracture events, which are detected by acoustic emission, were collected throughout the experiment. An in situ X-ray diffraction technique was used to measure the elastic strains in the oxide scales at the isothermal scale growth temperature and at several temperatures during cooling.

These measurements found higher elastic strains in the Cr₂O₃ scales, during both isothermal oxidation and cooling, but for fewer cracking events, on the yttrium-containing alloy than on Ni-30Cr. We infer that the addition of yttrium increased the adherence of the scale to the substrate, since the scale was able to withstand higher elastic stresses.     

Effects of grain-boundary GeO2 particles on intergranular fracture of Cu-GeO2 polycrystals

The Cu-GeO₂ alloy polycrystals containing plastically hard GeO₂ particles are tensile tested to study intergranular fracture behavior of the alloys at intermediate temperatures. The effects of large GeO₂ grain-boundary particles on the intergranular fracture are discussed using the specimens containing the particles of the fixed size (3 m in diameter) and different area-fractions. The ductility of the Cu-GeO₂ alloy polycrystals is larger than that of Cu polycrystals. The grain-boundary GeO₂ particles improve the ductility by suppressing grain-boundary sliding. The grain-boundary voids to cause theintergranular fracture preferentially nucleate between the grain-boundary GeO₂ particles. The ductility of the Cu-GeO₂ alloys increases with increasing the area fraction of the grain-boundary GeO₂ particles. The area-fraction dependence of the ductility is explained by considering the amount of GBS as a criterion of the intergranular fracture.     

Influence of grain size and precipitation on hot ductility of microalloyed steels

Abstract

The influence of grain size on the hot ducility of microalloyed steels (C–Mn–Al, C–Mn–V–Al, and C–Mn–Nb–Al) has been determined by heating them above their solution temperatures and cooling to the test temperature of 850°C. The C–Mn–Al steel showed excellent hot ductility which was independent of grain size. Dynamic recrystallization readily occurred and there was no evidence for AlN precipitation. Marked dynamic precipitation occurred during the tensile test for vanadium- and niobium-containing steels but this did not vary significantly with reheating temperature, provided complete dissolution of the precipitates had occurred. Isolating the influence of grain size from that of precipitation in these steels showed that a change in grain size from 150 to 300 μm reduced the reduction of area values by 15–20%. Precipitate distribution was also varied by heating to temperatures in the range 850–1330°C and tensile testing at 850°C. When present before testing at the γ grain boundaries in the form of a fine grain-refining precipitate, AlN reduced the hot ductility in the C–Mn–Al steel and delayed the onset of dynamic recrystallization. Coarser precipitates produced by raising the reheating temperature allowing dynamic recrystallization to occur gave improved ductility. For the niobium- and vanadium-containing steels, precipitate distributions which were in a coarse randomly precipitated form gave the best hot ductility. These occurred with the niobium-containing steel when heated to 1100°C and more generally in the vanadium-containing steel throughout a wide temperature range. The worst precipitate distribution occurred in the niobium containing steel when the NbCN was taken into solution before testing and reprecipitated in a fine form at the γ grain boundaries and within the matrix during the test.

MST/490     

Role of processing temperature on microstructure,mechanical properties and corrosion behavior of fine grained AZ31 magnesium alloy produced by groove pressing

AZ31 magnesium alloy sheets were processed at 250 °C and 300 °C by groove pressing, a severe plastic deformation technique to achieve grain refinement. The influence of processing temperature on the evolution of microstructure, mechanical properties and corrosion behavior was studied. Groove pressing significantly reduced the grain size of the alloy from 46 μm to 6.5 μm at 250 °C processing temperature. With the higher processing temperature (300 °C), grain growth (11.4 μm) was observed for the alloy. Number of twins appeared in the groove pressed samples. Higher hardness and tensile strength were measured for the groove pressed samples processed at 250 °C without significant loss in the ductility as reflected from the % of elongation due to the grain refinement. Corrosion performance of the samples assessed by potentiodynamic polarization studies indicated increased corrosion resistance for both of the grove pressed samples. However, sample at 300 °C exhibited better corrosion resistance compared with the sample processed at 250 °C. This can be understood by considering the effect of higher processing temperature on reducing the crystal imperfections which alters the corrosion behavior.     

Microstructure and mechanical properties of directionally solidified high-Nb containing Ti–Al alloys

Two high-Nb containing Ti–Al alloys, Ti–16Al–8Nb and Ti–16Al–8Nb–1Sn were fabricated using directional solidification. Their microstructures and mechanical properties at both room and high temperatures were studied. Results showed that the addition of 1% Sn promoted the formation of laths and contributed remarkably to the enhancement in room-temperature strength and high temperature ductility of Ti–Al alloy. The alloys exhibited the feature of quasi-cleavage fracture at room temperature and they experienced significant plastic deformation at high temperatures.     

Room and high temperature tensile tests on the AA6061/10vol.%Al2O3p and AA7005/20vol.%Al2O3p composites

Aluminium alloy based Metal Matrix Composites (MMCs), reinforced with ceramic particles such as Al₂O₃ or SiC, have a number of advantages over conventional aluminium alloys, primarily enhanced stiffness and increased wear resistance. In order to improve the fields of application, fundamental understanding of the relationship between microstructural features and mechanical properties is however required. In this work, the tensile behaviour of two composites based on 6061 and 7005 aluminium alloys, reinforced with Al₂O₃ particles, at room temperature, at 100°C and at 150°C was studied. The ductility of the composites was found to be much lower than that of the unreinforced alloys, while a significative increase of the elastic modulus and tensile strength was found. Both materials showed a slight decrease of the tensile strength with temperature, while elongation increased. Large particles and clusters of the reinforcement were found to be locations prone to failure in the composite, due to the high stress concentrations, mainly at room temperature. With increasing temperature, the fracture surfaces showed a dimpled appearance with a large amount of plastic deformation of the matrix, indicating that void nucleation, growth and coalescence is the main fracture mechanism.     

Influence of minor elements additions on microstructure and properties of 93W-4·9Ni-2·1Fe alloys

Effects of elements rhenium and chromium additions on properties and microstructure of 93W-4·9Ni-2·1Fe alloys were investigated. Optical microscope (OM), scanning electron microscope (SEM) and EDAX energy spectrometer were used to characterize the microstructure and compositions of the alloys, respectively. The tensile strength and elongation of alloys were evaluated using the quasi-static tensile testing machine, and the relative densities of the alloys were evaluated using the Archimedes water immersion method. The experimental results indicated that when elements Re and Cr were in the range of 0–1·0 wt.%, relative density, elongation, tensile strength of 93W-4·9Ni-2·1Fe alloys varied from 99·4%, 26·4%, 997·2 MPa without Re additions to 99·5%, 8·6%, 1161·2 MPa with 1·0 wt.% Re addition, respectively. Rhenium generated solid-solution strengthening, grain refinement, reducing ductile tearing and increasing transcrystalline fracture, which resulted in the ductility reduction and the strength increase of the heavy alloys. With the increase of Cr content from 0–1·0 wt.%, the tensile strength, relative density and elongation of 93W-Ni-Fe alloy reduced from 997·2 MPa, 99·3%, 15% to 844·4 MPa, 95·2%, 5·7%, respectively. Element Cr formed interphases with elements W, Ni, Fe and O and gathered along the interface of the alloys, which induced interfacial cohesion and resulted in lower mechanical properties of 93W-Ni-Fe alloys.