Ductile Fe-based bulk metallic glasses at room temperature

In this work, Fe₇₀Ni₁₀P₁₃C₇ and Fe₆₀Ni₂₀P₁₃C₇ bulk metallic glasses (BMGs) with high ductility are synthesised through the appropriate adjustment of flux conditions. Microstructure analyses indicate that the ductility of these two Fe-based BMGs is primarily determined by Fe–Ni and Ni–Ni metal–metal bonds, which enable the glasses to undergo a strain of more than 50% in the absence of fracture, as well as extensive bending ductility, similar to metals such as Fe and Ni. This study aims to find a solution to the brittleness problem in Fe-based BMGs, and the results have implications for understanding the deformation mechanism in Fe-based BMGs.     

Deformation at ambient and high temperature of in situ Laves phases-ferrite composites

The mechanical behavior of a Fe₈₀Zr₁₀Cr₁₀ alloy has been studied at ambient and high temperature. This Fe₈₀Zr₁₀Cr₁₀ alloy, whoose microstructure is formed by alternate lamellae of Laves phase and ferrite, constitutes a very simple example of an in situ CMA phase composite. The role of the Laves phase type was investigated in a previous study while the present work focuses on the influence of the microstructure length scale owing to a series of alloys cast at different cooling rates that display microstructures with Laves phase lamellae width ranging from ～50 nm to ～150 nm. Room temperature compression tests have revealed a very high strength (up to 2 GPa) combined with a very high ductility (up to 35%). Both strength and ductility increase with reduction of the lamella width. High temperature compression tests have shown that a high strength (900 MPa) is maintained up to 873 K. Microstructural study of the deformed samples suggests that the confinement of dislocations in the ferrite lamellae is responsible for strengthening at both ambient and high temperature. The microstructure scale in addition to CMA phase structural features stands then as a key parameter for optimization of mechanical properties of CMA in situ composites.     

Enhancement of high temperature strength and room temperature ductility of iron aluminides by alloying

Abstract

The chemical ordering in intermetallics results in reduced atomic mobility and therefore increased resistance to plastic deformation at elevated temperatures. This intrinsic source of high temperature strength leads to the inherent brittleness of polycrystalline ordered intermetallics at room temperature. The requirements for optimum high temperature strength and ductility at ambient temperature are often incompatible. Iron aluminides possess high strength up to 873 K. There is an anomalous (positive) temperature dependence of yield and flow strengths. Iron aluminides have yet to achieve satisfactory elevated load bearing capability. Alloy additions have the potential for improving elevated temperature strength and room temperature ductility; whichever is more critical for the application. Elements such as Cr, Ti, Mn, Co, and Mo produce higher flow stress due to solid solution strengthening. Elements such as Zr, Ta, Nb, Re, and Hf go into solution partly, reprecipitate, effectively pin dislocations and thereby cause strengthening. Mo, Zr, and Hf produce good tensile strength at elevated temperatures but ductility decreases. Element B strengthens by grain boundary cohesion. The improvement in room temperature ductility can be achieved through modification of the crystal structure by changes in stoichiometry, macroalloying, microalloying, and control of the environment. B, TiB₂, and Cr are notable for enhancing ductility. The paper is an overview of the present status of iron aluminides in this respect.     

Effects of silicon and nitrogen on hot ductility of low carbon steels

The effects of N on the hot ductility of low carbon steels have been studied with particular emphasis on the relation with Si. The ductility of Si, Al-killed steels is largely reduced by slow strain rate (10_?3–10_?4S_?1) deformation at temperatures from low temperature γ to γ/α duplex phase region (from 750 to 950 °C in this case), accompanied by ductile intergranular fracture of austenite. The cause of the loss of ductility is found to be dynamic precipitation of hexagonal close packed (hep) (Si, Al)N both on the γ grain boundaries and within the grains, and the phenomenon is largely enhanced by either increasing Si or N content. Similar phenomena, i.e. precipitation hardening-like behaviour and dependencies both of deformation conditions and of Si and N contents, are also observed in Al-free Si-killed steels. The cause of this ductility loss should be ascribed to dynamic precipitation of some kind of silicon nitride, although the precipitation has not been detected directly in all the specimens examined.     

Effect of specimen size in determining ductile to brittle transition temperature

Ductile to brittle transition temperature (DBTT) for 9Cr–1Mo steel has been determined from Charpy impact testing for full size and subsized specimens. DBTT was obtained at various percentage of upper shelf energy (USE). Assuming that most of the energy is spent in crack initiation, notch root volumes of subsized specimens (V_NS) were normalised with full size specimen (V_NF), and a power law relationship between DBTT and notch root volume has been established. From finite element method, it is observed that the sum of von Mises stress (σ_eq) and hydrostatic stress (σ_h) reaches ～2400 MPa (fracture stress, σ_f*) as the specimen dimension decreases at a temperature corresponding to 33% USE. This corresponds to ～68 J of full size specimen used in the determination of nil ductility transition temperature.     

Toughening and creep in multiphase intermetallics through microstructural control

The lack of engineering ductility in intermetallics has limited their structural applications, in spite of their attractive specific properties at high temperatures. Over the last decade, research in intermetallics has been stimulated by the discovery of remarkable ductilisation mechanisms in these materials. It has however often been the case that the process of ductilisation or toughening has also led to a decrease in high temperature properties, especially creep. In this paper we describe approaches to the ductilisation of two different classes of intermetallic alloys through alloying to introduce beneficial, second phase effects. The Ti₂AlNb based intermetallics in the Ti-Al-Nb system can be ductilised by stabilising thebcc phase of titanium into the structure. The principles of microstructural and compositional optimization developed to achieve adequate plasticity, while retaining creep properties of these alloys, are described. An entirely different approach has been successful in imparting plasticity to intermetallics based on Fe₃Al. The addition of carbon to form the Fe₃AlC_0.5 phase imparts ductility, while enhancing both tensile and creep strength.     

Understanding the low temperature end of the hot ductility trough in steels

Abstract

The low temperature end of the hot ductility trough has been examined for steels which have been solution treated at ～1300°C before tensile testing in the temperature range of 1000–600°C. Failure in the trough in this region is intergranular ductile and occurs by strain intensification in the thin film of ferrite surrounding the prior austenite grain. The strain causes voiding to occur at the inclusions situated at the boundaries, the cavities gradually linking up to give failure. In steels which are solution treated before tensile testing, the depth of the trough is shown to be controlled by the volume fraction of the second phase particles, their size and the separation between the particles. Recovery in ductility on the low temperature side of the trough is solely dependent on being able to produce a sufficiently large quantity of ferrite to prevent strain concentration (～40%). Often this has to await the test temperature falling below the AR ₃ in which case wide troughs are formed. However, if conditions are right, very narrow troughs can be produced in which the ferrite that is formed is deformation induced. The width of the trough at the low temperature end of the trough is shown to decrease with increase in strain rate, refinement of the austenite grain size, increase in cooling rate from the solution treatment temperature, decrease in the volume fraction of sulphides situated at the austenite grain boundaries and reduction in the Mn and C contents. The depth of the trough decreases in a similar manner with all these variables except for C and Mn, where for the former there is no effect and for the latter, increasing the Mn level reduces the depth. Narrow troughs on this side of the trough are dependent on being able to form deformation induced ferrite in sufficiently large amounts so as to improve the ductility at temperatures above the AR ₃. A model is proposed to account for most of these observations.     

Possible ways of enhancing high temperature ductility in CuO doped cubic zirconia (8Y–CSZ)

Abstract

The effect of initial density and rapid prestraining on superplastic ductility of 1 wt-%CuO doped cubic zirconia (8Y–CSZ) was investigated. To obtain a range of initial densities, the tensile test specimens were slip cast to net shape and pressureless sintered over a range of temperatures in air. The specimens were then superplastically tested at a temperature of 1500 K and at a constant strain rate of 1×10^-4 s^-1. The results showed that specimens with low initial densities had lower flow stresses and higher superplastic elongations to failure than higher density specimens. The reasons for the ductility change were discussed with reference to the presence of porosity and grain growth. For the prestraining test, a specimen with an initial density of 95% was prestrained to 30% at a temperature of 1550 K and at a prestrain rate ? · ₁ of 1×10^-3 s^-1, followed by elongation to failure at a slower test strain rate ? · ₂ of 1×10^-4 s^-1. It was seen that prestraining at the above test conditions considerably improved superplastic ductility. The reasons for this ductility enhancement were explained in terms of suppression of grain growth.     

Relative importance of transformation temperatures and sulphur content on hot ductility of steels

Abstract

Low (0·3%) and high manganese (1·4%) plain C – Mn steels with varying sulphur levels have had their hot ductility determined over the temperature range 700 – 1000°C, both after 'solution treatment' at 1330°C and directly after casting. It has been established that the width, depth and position of the hot ductility curves after solution treatment is more related to the transformation behaviour than either the sulphur in solution or the sulphide volume fraction or distribution. The growth of deformation induced ferrite at the austenite boundaries seems to be mainly diffusion controlled, and the higher is the transformation temperature for the γ – α phase change, the faster is the growth. Large amounts of ferrite can then form, giving good ductility. Thus, high transformation temperatures Ae ₃ or Ar ₃ are required to produce narrow ductility troughs. It is believed that any detrimental influence of the sulphides on these 'solution treated' steels is swamped by the rapid increase in ferrite volume fraction. For the as cast state, as more sulphides are able to precipitate at the interdendritic boundaries and austenite grain boundaries than in the solution treated condition, increasing the sulphur level causes a small deterioration in ductility at the high temperature end of the trough. In the present work, only narrow troughs have been found. This is in contrast to previous work on as cast C – Mn – Nb – Al steels, which exhibited wide troughs in the ductility curves, where it was shown that higher total sulphur levels lead to considerably worse ductility and that sulphur can be as detrimental to the ductility as niobium. It is recommended that, to avoid transverse cracking during continuous casting, in addition to keeping the sulphur level low, the carbon and manganese should also be as low as possible.     

Ductility of a Zr-based bulk-metallic glass with different specimen's geometries

The ductility of a Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk-metallic glass with different specimen's geometries in quasistatic compression experiments was investigated. The length (l) to diameter (d) ratio, l / d, of specimens significantly affects the demonstration of the ductility. The specimens with the l / d larger than 0.75 exhibit a poor ductility, while those with the l / d equal to and smaller than 0.75 show an excellent ductility. The maximum elongation before the failure was observed to be up to about 80%. This difference in ductility is a result of the geometrical constraints. The present study demonstrates that the bulk-metallic glass has the outstanding intrinsic ductility.     

Mechanical behaviour of Zr doped Ni3Al alloy: effects of heat treatment,environment, and temperature

Abstract

The mechanical behaviour of polycrystalline Ni₇₆Al₂₃Zr alloy was studied as a function of heat treatment, environment, and temperature. It was found that the tensile ductility was very sensitive to temperature, the alloy showing low ductility at temperatures from 700 to 1000°C both in air and vacuum. Environmental embrittlement could be alleviated for those specimens with elongated grains. The ductile transgranular fracture was explained by stress concentration at the intersection of slip bands and grain boundaries. It was also found that an oxide layer, formed during tensile testing at elevated temperature, affected the environmental embrittlement of Ni₃Al(Zr) alloy. An adherent Al rich oxide film was effective in protecting the underlying alloy from oxygen penetration.     

Influence of P on hot ductility of high C,Al, and Nb containing steels

Abstract

Hot ductility curves for high carbon Nb and Nb free steels have been determined immediately after casting at two P levels, ~0.01% and ~0.045%. High strain rates of 0.1-0.55 s^-1 were generally used but some limited low strain rate testing at 7 × 10^-3 s ^-1 was carried out on Nb containing steels. Nb containing steels showed, as expected, worse ductility than the Nb free steel but high P level was detrimental to ductility for both steels and ductility in general was very poor. Failure was intergranular with the presence of films of a P rich phase at the boundaries in the case of the Nb free steels and in addition to this, in Nb containing steel there was a Nb rich phase. The films were thicker and more continuous in the higher P steels. It is suggested that the P rich films are probably the low melting point phase Fe₃P or Fe₃(Mn)P, which can remain liquid down to temperatures as low as 950°C. Some back diffusion of P into the grain interior is possible if the strain rate is reduced and/or at high testing temperatures during the 5 min hold prior to testing. This allowed some improvement in ductility to occur in the lower P containing steels by reducing the amount of the low melting point phase at the boundaries.     

Carbon composition and temperature dependence of relative change in length during isothermal decomposition of Fe–C austenite

Abstract

The carbon composition and temperature dependence of the relative change in length during isothermal decompos ition of Fe–C austenite has been modelled. Decomposition of austenite above and below the A₁ temperature has been considered, as well as a two-step procedure where a specimen decomposes into ferrite and carbon enriched austenite at a temperature above A₁ and is subsequently subjected to transformation below A₁ leading to ferrite and cementite. Analytical expressions have been given for the relative change in length as a function of carbon composition, isothermal decomposition temperature, and degree of transformation. Predicted changes in length have been compared with experimental results.     

Fabrication and mechanical properties of Cu-coatedwoven carbon fibers reinforced aluminum alloy composite

Cu-coatedwoven carbon fibers/aluminum alloy composite (C_f/Al) was prepared by spark plasma sintering. Microstructure and mechanical properties of the composite were investigated. Microstructure observation indicates that the interface reaction is evidently inhibited by Cu coating. Woven carbon fibers are adhered to the matrix alloy by anchor locking effect of matrix alloy immersing into the interstices between carbon fibers. Under the quasi-static and dynamic compressive conditions, the composite exhibits excellent ductility even when the strain reaches 0.8. Adding carbon fibers into ZL205A alloy has no obvious influence on compressive flow stress of the composite. The compressive true stress–true strain curves show that the composite is a strain rate insensitive material. During the tensile tests, the elongation of the composite shows a sharp increase from 4.5% to 13.5% due to the adding of woven carbon fibers. Meanwhile, the tensile strength of the composite is increased slightly from 168 MPa to 202 MPa compared to that of ZL205A alloy. The good ductility of the composite is ascribed to the cracks deflection, fibers pulling out, debonding and breakage mechanisms.     

Influence of carbon on aging response and tensile properties of eutectoid beta titanium alloy Ti–13Cr

Abstract

The influence of carbon addition on the aging response of quenched Ti–13Cr (wt-%) has been investigated using hardness tests, tensile tests, optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It has been found that carbon refines beta grain size, leads to fine and homogenous alpha precipitation and reduces grain boundary alpha. The carbon addition accelerates the rate at which hardening occurs during aging and increases the peak hardness of the aged specimens. A significant improvement in room temperature tensile strength and ductility has also been achieved in the carbon containing alloy after aging at 500°C. The effects of carbon on the aging response appear to be attributed to dislocations introduced by carbides during quenching, elastic strain created in the matrix by carbides and gettering effect of Ti₂C carbides. The influence of each of those mechanisms has been demonstrated through experiments and the factors giving rise to the improvements in properties are also discussed in terms of the microstructural observations.     

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 Al and C on structure and mechanical properties of Fe–Al–C alloys

Abstract

Effect of aluminium and carbon content on the microstructure and mechanical properties of Fe–Al–C alloys has been investigated. Alloys were prepared by combination of air induction melting with flux cover (AIMFC) and electroslag remelting (ESR). The ESR ingots were hot forged and hot rolled at 1373 K. As rolled alloys were examined using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to understand the microstructure of these alloys. The ternary Fe–Al–C alloys containing 10·5 and 13 wt-%Al showed the presence of three phases: FeAl with disordered bcc structure, Fe₃Al with ordered DO₃ structure and Fe₃AlC_0·5 precipitates with L′1₂ structure. Addition of high concentration of carbon to these alloys resulted in excellent hot workability and superior tensile at room temperature as well as tensile and creep properties at 873 K. An increase in Al content from 9 to 13 wt-% in Fe–Al–C alloys containing the same levels of carbon has no significant influence on strength and creep properties at 873 K, however resulted in significant improvement in room temperature strength accompanied by a reduction in room temperature ductility.     

Isochronal aging behaviour of AI-5.8%Mg alloy during deformation

The mechanical properties of an Al-5.8%Mg alloy quenched and aged for 24 hat 393 and 433 K have been investigated in the temperature range 293-573 K to assess the effect of aging temperature on deformation in the presence of precipitation. The results indicate that at a constant strain rate the ductility increases with test temperature, reaches a maximum value and then decreases to a minimum value, followed by a further increase at higher temperatures. The temperatures corresponding to the minimum ductility, the elevated 0.2% yield strength and the ultimate tensile strength were found to be dependent on the starting material conditions. The variations in the yield and ultimate tensile strength, parabolic stress-strain relationship and work-hardening index were recorded as functions of experimental variables. Electron microscopy revealed that the loss in ductility of the alloy can be attributed to the precipitation of MnAl₆, Mg₂Al₃ and MgsAl₃ or MgsAl_B, whose size, quantity and morphology depend on the experimental conditions. An attempt has been made to correlate strength, ductility and structural changes at elevated temperatures.     

Studies on high temperature deformation behaviour of 3Y-TZP ceramics

The present work reports the results on the deformation behaviour of ZrO₂-3 mol% Y₂O₃ (3Y-TZP) ceramics which were prepared by pressureless sintering at 1400°C. Dense, cylindrical samples were subjected to uniaxial compression tests under a constant stress of 15 MPa in the temperature range of 1200–1400°C. The ceramics exhibit considerable ductility, attaining over 60% true strain without any edge cracking. Microstructural changes due to interaction of grain boundary viscous phase with the ultrafine and equiaxed grains were analyzed by transmission electron microscopy. Results show the grain boundary sliding accompanied by a diffusion accommodation process as the predominant deformation mechanism in these ceramics.     

Experimental correlation between ductility and J-integral in the transition region of 1045 steel

A study of the variation of fracture toughness j_IC and ductility, measured under both tensile loading and biaxial plane strain (bulge) loading, of AISI 1045 steel in the annealed condition, in the transition temperature range of ?60 to 25°C was carried out. This temperature range delineates the changes in behavior from linear elastic to elastic-plastic behavior for this steel. It was found that the variation of j_ic with temperature shows a transition at about ?20°C while the bulge ductility only has a marked transition below ?40°C. These trends are explained in terms of the effect of material properties, namely the flow stress, on crack blunting, while the bulge ductility is correlated with the total strain to cause significant crack growth to take place. In the elastic-plastic region, a linear relationship between j_IC and bulge ductility was found to occur.