Low Tensile Ductility Research Articles

Tensile behavior of friction-stir-welded A356-T6/Al 6061-T651 bi-alloy plate

Abnormally low tensile ductility has often been reported for the friction-stir-welded (FSWed) dissimilar metals. The mechanism(s) for such a low tensile ductility has, however, not been established. In the present study, the tensile behavior of FSWed A356-T6/Al 6061-T651 bi-alloy plate was studied to understand the underlying mechanism for the reduced tensile ductility with the friction stir welding of dissimilar metals based on thorough micrographic and fractographic observations. The present study also demonstrated that the tensile ductility of the friction-stir-welded A356-T6/Al 6061-T651 bi-alloy specimen was substantially lower than that of the weighted mean value of the uni-alloy counterparts, including A356-T6 and Al 6061-T651 alloys. Interestingly, a relatively large number of acicular shaped Si particles were observed locally in the FSWed bi-alloy specimens compared to the dominantly globular shaped particles in the FSWed uni-alloy counterpart. Moreover, these acicular shaped Si particles were found to be mostly aligned parallel to the tool-rotating direction. Such agglomerated areas of the preferentially oriented, acicular Si particles in the present study appeared to serve as initiation sites for the tensile fracture and eventually caused low tensile ductility.

High tensile ductility in a nanostructured metal.

Nanocrystalline metals--with grain sizes of less than 100 nm--have strengths exceeding those of coarse-grained and even alloyed metals, and are thus expected to have many applications. For example, pure nanocrystalline Cu (refs 1-7) has a yield strength in excess of 400 MPa, which is six times higher than that of coarse-grained Cu. But nanocrystalline materials often exhibit low tensile ductility at room temperature, which limits their practical utility. The elongation to failure is typically less than a few per cent; the regime of uniform deformation is even smaller. Here we describe a thermomechanical treatment of Cu that results in a bimodal grain size distribution, with micrometre-sized grains embedded inside a matrix of nanocrystalline and ultrafine (<300 nm) grains. The matrix grains impart high strength, as expected from an extrapolation of the Hall-Petch relationship. Meanwhile, the inhomogeneous microstructure induces strain hardening mechanisms that stabilize the tensile deformation, leading to a high tensile ductility--65% elongation to failure, and 30% uniform elongation. We expect that these results will have implications in the development of tough nanostructured metals for forming operations and high-performance structural applications including microelectromechanical and biomedical systems.

Optimization of mechanical properties of a Ni–Mo–Cr alloy by structural modifications induced by changes in heat treatment

The influence of modifications of heat treatment applied to a commercial Ni–25 wt.%Mo–8 wt.%Cr age-hardenable alloy (HAYNES® 242™) on microstructure and mechanical properties were investigated. The standard heat treatment (aging at 650°C for 72 h), producing coherent long-range ordered domains with the Ni 2(Mo, Cr) stoichiometry, was modified by short duration thermal exposures at temperatures at between 760 and 980°C (preceding the alloy's aging). The intermediate heat treatment (IHT) affected the size of the long-range order domains and produced precipitations of a μ phase on the grain boundaries. The size of the ordered domains depended on the temperature of the IHT. The largest domains occurred after the IHT at 760°C. With the increase in the IHT temperature, the domain size decreased. The yield strength of the alloy increased with increasing domain size. The amount of grain boundary particles, and not the size of the ordered domains, controlled the ductility of the material subjected to the IHT: The more grain boundary precipitates there were, the lower tensile ductility. Optimization of the tensile properties (strength and ductility) of the examined alloy occurred when the conventional aging was preceded by the IHT at 760°C.

Erosion of in-situ TiC particle reinforced Al–5Cu composite

Erosion behavior of in-situ 15 vol% TiC particle reinforced Al–5Cu based composite and the monolithic Al–5Cu alloy was studied at oblique and normal impacts. The erosion rate of the composite was higher than that of the monolithic alloy. There existed low-angle peaks in erosion rate vs. impact angle curves for the two materials, which indicated ductile erosion behavior although the composite had a low tensile ductility. Theoretical model for ductile materials cannot predict the erosion rate of the composite. The presence of TiC particles in the composite increased, rather than decreased the erosion rate even though the particles obviously increased the hardness of the composite. Erosion process involved the displacement of the target material and the removal of the displaced material. By comparison to the monolithic alloy, the composite exhibited higher resistance to the displacement of the target material due to higher hardness, while lower resistance to the removal of the displaced material due to poor ductility. The combined effect from these two aspects resulted in a higher erosion rate of the composite than that of the monolithic alloy.

Recent advances in B2 iron aluminide alloys: deformation, fracture and alloy design

This paper reviews the deformation, fracture and alloy design of B2 iron aluminides based on FeAl. Moisture-induced environmental embrittlement is shown to be a leading cause of low tensile ductility and brittle cleavage fracture of Ferich FeAl alloys at ambient temperatures. With increasing Al concentration, two other factors, namely intrinsic grain-boundary weakness and quenched-in vacancies become important in limiting the tensile ductility of FeAl alloys. FeAl alloys show a yield-strength anomaly at intermediate temperatures. Recent work indicates that the anomaly is a result of hardening by thermal vacancies at elevated temperatures. The understanding of the deformation and fracture behavior has led to the development of FeAl-base alloys and composites with improved metallurgical and mechanical properties for structural applications. These FeAl-based alloys can be prepared by melting and casting or by powder processing. The unique combination of the excellent oxidation and carburization/sulfidation resistance coupled with relatively low material density and good mechanical properties at room and elevated temperatures has sparked industrial interest in FeAl alloys and composites for a number of applications.

The effect of increased magnesium on the fatigue behaviour of 5XXX aluminum: Chernenkoff, R.A., Krause, A.R. and Davies, R.G. Conference: Recent Metallurgical Advances in Light Metals Industries, Vancouver, British Columbia, Canada (20–24 Aug. 1995) 197–206

To study the effect of deformation mode used for grain refinement on the low-cycle-fatigue (LCF) behaviour of ultrafine-grained (UFG) materials, a commercially pure Al was subjected to cryorolling, and room temperature symmetric and asymmetric rolling. The LCF test was conducted under fully reversed total strain amplitude control. Despite its lower tensile ductility, the cryorolled Al had significantly higher fatigue ductility. This has been attributed to a higher density of shear bands in the cryorolled Al.

Fatigue and fracture behavior of a fine-grained lamellar TiAl alloy

The fatigue and fracture resistance of a TiAl alloy, Ti-47Al-2Nb-2Cr, with 0.2 at. pct boron addition was studied by performing tensile, fracture toughness, and fatigue crack growth tests. The material was heat treated to exhibit a fine-grained, fully lamellar microstructure with approximately 150-µm grain size and 1-µm lamellae spacing. Conventional tensile tests were conducted as a function of temperature to define the brittle-to-ductile transition temperature (BDTT), while fracture and fatigue tests were performed at 25 °C and 815 °C. Fracture toughness tests were performed inside a scanning electron microscope (SEM) equipped with a high-temperature loading stage, as well as using ASTM standard techniques. Fatigue crack growth of large and small cracks was studied in air using conventional methods and by testing inside the SEM. Fatigue and fracture mechanisms in the fine-grained, fully lamellar microstructure were identified and correlated with the corresponding properties. The results showed that the lamellar TiAl alloy exhibited moderate fracture toughness and fatigue crack growth resistance, despite low tensile ductility. The sources of ductility, fracture toughness, and fatigue resistance were identified and related to pertinent microstructural variables.

Effect of particle size and matrix aging condition on toughness of particle reinforced aluminium based metal matrix composites

Room temperature fracture toughness and tensile tests were carried out on metal matrix composites based on the aluminium alloy 7075, and also on monolithic 7075. The particulate reinforcements used were SiC in three nominal sizes: 5, 13, and 60 μm. Three aging conditions were studied: peak aged, underaged, and overaged conditions of equivalent matrix micro hardness values.The addition of 5 and 13 μm particles increased the 0·2% proof stress and tensile strength, and reduced the ductility, compared with the monolithic material. Composites containing 60 μm particles had lower 0·2% proof stress and tensile strength, and very low tensile ductility. In all cases the toughness of the composites was lower than that of the unreinforced material. However, in contrast to the tensile ductility, the material containing 60 μm particles was the toughest of the composites. The failure mechanism is believed to be one of particle fracture and/or decohesion at low applied stress intensities, followed by ductile failure of the matrix material. A model to predict toughness from tensile ductility and nominal interparticle spacing is proposed which is consistent with the observed experimental results.MST/3217

The influence of C and Si on the flow behavior of NiAl single crystals

Alloys based on the intermetallic compound NiAl are considered potential replacements for Ni and Co-based superalloys in high temperature structural applications due to their excellent oxidation resistance, low densities, high thermal conductivities, and increased melting points. Unfortunately, NiAl exhibits low tensile ductility at room temperature and low strengths at elevated temperatures which have combined to hinder its development. Recent efforts, have revealed that NiAl in the presence of sufficient solute levels, is subject to the phenomenon of strain aging which manifests itself as: sharp yield points, abnormally low strain rate sensitivities (SRS), plateaus or peaks in yield stress and work hardening rate as a function of temperature, flow stress transients upon an upward change in strain rate, reduced tensile elongations at elevated temperatures, and serrated stress-strain curves. Though recent efforts via either alloying or the removal of interstitial impurities, have resulted in consistent room-temperature tensile elongations exceeding 5% and the elimination of serrated flow, the effects of particular substitutional and interstitial elements and the mechanisms by which they might enhance or hinder the mechanical properties remain unknown. Consequently, the purpose of the present paper is to provide a preliminary assessment of the influence of common substitutional and interstitial impurities on the deformation behavior of NiAl. To accomplish this goal a series of NiAl single crystal alloys containing various interstitial solutes were prepared and their mechanical properties were evaluated between 77 and 1100 K. Because Si is a common impurity in conventional purity single crystals grown by the Bridgman method, Si concentrations were also varied in order to determine the influence of this element.

Tensile Properties and Structure of Several Superalloys after Long- Term Exposure to LiF and Vacuum at 1173 K

The use of the solid- to- liquid phase transformation of LiF to store thermal energy is under consideration for a space- based solar dynamic system. Although advantageous in terms of its energy density, the melting point of this salt (1121K) is beyond the commonly accepted upper- use temperature of 1100 K for chromium- bearing superalloys in vacuum. However, one commercially available nickel- base superalloy (Hastelloy B- 2) is chromium free; unfortunately, because of its high molybdenum content, this alloy can form phases that cause high- temperature embrittlement. To test the suitability of Hastelloy B- 2, it has been exposed to molten LiF, its vapor and vacuum at 1173 K for 2500, 5000, and 10 h. For control, the chromium- containing cobalt- base Haynes alloy 188 and nickel- base Haynes alloy 230 were also exposed to LiF and vacuum at 1173 K for 5000 h. Neither LiF nor vacuum exposures had any significant effect on Hastelloy B- 2 in terms of microstructural surface damage or weight change. Measurement of the post exposure tensile properties of Hastelloy B- 2, nevertheless, revealed low tensile ductility at 1050 K. Such embrittlement and low strength at elevated temperatures appear to preclude the use of Hastelloy B- 2 as a containment material for LiF. Little evidence of significant attack by LiF was seen in either of the chromium- containing superalloys; however, considerable weight loss and near- surface microstructural damage occurred in both alloys exposed to vacuum. Although measurement of the post exposure room-temperature tensile properties of Haynes alloys 188 and 230 revealed no significant loss of strength or ductility, the severe degree of microstructural damage found in unshielded alloys exposed to vacuum indicates that chromium-bearing superalloys might also be unsuitable for prolonged containment of LiF in space above 1100 K. Keywords

Tensile behavior of a sintered steel at elevated temperatures

The tensile behavior of a sintered steel is perfectly described by the empirical Ramberg-Osgood equation, and also can be modeled by the Bodner-Partom formulation. Tensile properties of the sintered steel strongly depend on temperature. At 300 C the Young`s modulus is 24% lower and the yield strength is 44% lower than those at ambient temperature respectively. The strain hardening behavior of the sintered steel differs from that of dense metals. The sintered steel has higher strain hardening at elevated temperatures than at room temperature; while the wrought metal behaves normally, strain hardening reduces with increasing temperature. The tensile fracture mode of the sintered steel remains the same as dimple coalescence from 23 to 300 C. The low tensile ductility of the sintered steel is attributed to micro-scale necking in the ligament between pores.

The effect of impurity content on thermally activated deformation in NiAl single crystals

NiAl-base materials are attractive candidates for high temperature structural applications due to their low density, excellent oxidation resistance and high thermal conductivity. However, in single crystal and polycrystalline form, NiAl suffers from low tensile ductility and toughness. The mechanisms responsible for the limited ductility of NiAl single crystals are still in question. For this reason, thermally activated deformation in NiAl single crystals has been studied with the use of strain-rate change experiments. The preliminary results suggest that the intrinsic mobility of a , and to a lesser extent a dislocations, is limited below the DBTT in NiAl. To examine the influence of interstitial impurities and the possibility that strain-aging embrittlement is the mechanism responsible for the limited ductility of NiAl, further experiments on high purity NiAl crystals have been conducted for comparison to the results on commercial purity crystals.

High strain rate superplasticity of AlN particulate reinforced aluminium alloy composites

Ceramic whisker or particulate reinforced aluminium alloy composites have a great potential for automobile engineering components, aerospace structures, semi-conductor packaging and so on, because of the composites ability to exhibit a high specific elastic modulus and specific tensile strength, excellent wear resistance and heat resistance, low thermal expansion and good dimensional stability. A serious problem involving practical application of ceramic whisker or particulate reinforced aluminium alloy composites is due to the low tensile ductility, fracture toughness at room temperature and, also, their hardness qualities that make it difficult to deform by conventional forming processing and machining by ordinary tools. It has been found, however, that aluminium alloy composites reinforced by SiC or Si[sub 3]N[sub 4] whiskers or particulates produce superplasticity at a high strain rate of about 0.1s[sup [minus]1]. Superplastic deformation mechanisms of the ceramic whisker or particulate reinforced aluminium alloy composites are fine grain boundary sliding, interfacial sliding at a liquid phase and dynamic recrystallization. An AlN particulate reinforced aluminium alloy composite exhibits a high elastic modulus and a high thermal conductivity, and their thermal expansion is similar to silicon in that the AlN particulate reinforced aluminum alloy composite is expected to apply to semi-conductor packaging in the aerospacemore » structure. In addition, if the composite could produce superplasticity at high strain rates, the market of aerospace application for superplastic composites could be expanded. The purpose of this study is to make clear if an AlN particulate reinforced aluminium alloy composite can produce superplasticity at high strain rate and the superplastic characteristics.« less

Some observations on the ambient temperature deformation behavior of fully lamellar Ti46Al2Nb2Cr alloy

Titanium aluminide TiAl-based alloys have received considerable interest in recent years because of their potential as high temperature structural materials. Within a certain composition range these alloys can be heat-treated to exhibit two-phase microstructures, often referred to as fully lamellar, nearly lamellar or duplex depending on the relative components of lamellar colonies and equiaxed gamma grains. The fully and nearly fully lamellar microstructures have been found to exhibit relatively good fracture toughness at both ambient and elevated temperatures but poor tensile ductility even at temperatures as high as 800 C. On the other hand, duplex microstructures exhibit relatively low fracture toughness but moderate tensile ductility at ambient temperatures. Reported herein are some observations on the ambient temperature deformation behavior of a TiAl-based alloy in a fully lamellar microstructural condition.

Ordering and mechanical strength in l12 cubic titanium trialuminides

There have been many studies of the structure and mechanical properties of titanium trialuminides over the last few years, in particular those alloys modified to the L1[sub 2] cubic structure by substitution of 6 to 10 pct of the aluminum by a suitable transition metal. Such alloys are of interest as high-temperature materials in view of their reasonable strength and good oxidation resistance. Unfortunately, these alloys show low tensile ductility and toughness. Attempts are being made to improve these properties. The present report examines earlier studies determining the state of order of these intermetallics and subsequently attempts to relate the ordered state and other microstructural characteristics, such as the appearance of second-phase precipitates, to strength or ductility.

Effect of post-forging heat treatment on the microstructure and room temperature tensile properties in Ti-25 Al-10Nb-3V-1Mo (super α2)

The room temperature tensile and fracture behaviour of super α2 have been assessed as a function of a range of thermomechanical treatments. It has been shown that the best properties are obtained in samples which have been forged in the β phase field using cool dies, followed by aging at 800 °C for 2 h. These property measurements have been correlated with the microstructural changes caused by the various heat treatments. It has been found that the highest strength is associated with a two-phase structure of transformed β phase containing about 17% primary α2 and that improved fracture behaviour is associated with a transformed fine Windmanstätten structure. It is concluded that the room temperature tensile ductility improved by aging at 800 °C for 2 h, which reduces the dislocation density and the stress concentration caused by the heavy forging deformation within the α2 and B2 grains. Grain boundary α2 films and the high Nb interface α2 phases at primary α2 and B2 matrix interfaces form after high temperature solution treatments. Also, these two kinds of α2 phase result in low room temperature tensile ductility. Grain boundary strengthening plays a dominant role in super α2, and transgranular fracture is the main failure feature of this alloy.

In situ observation of microcrack growth in 8090-T 4 alloy

Aluminum-lithium alloys have become of recent interest as potentially important aerospace structural materials because of their low density, high elastic modulus and strength. However, some Al-Li alloys show very low tensile ductility and fracture toughness, particularly at peak-aged conditions. Vasudevan and Starke et al. recognized that Al-Li alloys were highly sensitive to intergranular fracture. Noble emphasized the effect of strain localization at coarse slip bands. Although much research has been published, the problem still has not been satisfactorily resolved. In contrast, the 8090-T[sub 4] alloy exhibits good comprehensive properties. In this paper, in situ observations in a TEM were carried out in the 8090-T[sub 4] alloy in order to study the deformation characteristics near the crack tip as well as microcrack growth. The results can be used to define the microstructure/property relationship in Al-Li alloys. Some results on fracture mechanisms at peak-and over-aged conditions will be reported later, so that the relationship between fracture mechanism, tensile ductility and the resistance to crack growth can be elucidated.

Rate and environmental effects on fracture of a two-phase TiAl-alloy

The influence of strain rate and environment on the fracture behavior of a two-phase TiAl-alloy, Ti-47Al-2.6Nb-2(Cr + V), heat-treated to a nearly fully lamellar microstructure has been studied by performing conventional tensile, compression, and fracture toughness tests in air, argon, and vacuum at 25 °C and 800 °C. Both tensile and compression tests were conducted at strain rates of 1 × 10−3 and 1 × 10−5 s−1, and fracture toughness tests were performed under displacement rates of 0.25 to 2.5 mm/min. In addition,in situ fracture toughness tests were conducted at slow rates both in vacuum and in air. The results indicated that both strain rate and environment affected the tensile stress-strain behavior and ductility and the fracture resistance of the TiAl-alloy at 800 °C. In contrast, neither the tensile ductility nor the fracture toughness was significantly affected by the environment at ambient temperature. For compression in air, the stress-strain behavior was insensitive to both strain rate and test temperature within the conditions tested. Studies of fracture surfaces revealed that low tensile ductility in this alloy at ambient temperature is associated with the tendency to delaminate alongγ/γ andγ/α 2 interfaces.

Environmental embrittlement and grain-boundary fracture in Ni 3Al

Recent studies have demonstrated that many ordered intermetallics exhibit environmental embrittlement when tested in air at ambient temperatures. Brittle grain-boundary fracture and poor ductility have been observed in a number of binary ordered intermetallics with an L1{sub 2} ordered crystal structure such as Ni{sub 3}Al, Ni{sub 3}Si, Ni{sub 3}Ge, and Ni{sub 3}Ga. The grain boundaries in the alloys have been considered to be intrinsically brittle, as evidenced by detailed Auger analyses and as suggested by atomic-simulation calculations. In this paper the study of environmental embrittlement at room temperature is extended to include binary Ni{sub 3}Al. The present results also show that moisture-induced hydrogen embrittlement contributes substantially to brittle grain-boundary fracture and low tensile ductility in Ni{sub 3}Al containing 24 and 23.5at.% Al, prepared by repeated cold forging and annealing of cast-alloy ingots.

Deformation behavior of a Ni−30Al−20Fe−0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature range of 300 to 1300 K under initial tensile strain rates varying between about 10−6 and 2×10−3 s−1 and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy for deformation in this region was about 310±30 kJ mol−1 for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10−4, where σ is the applied stress andE is the Young’s modulus.