Grain size dependence of tensile behavior in nanocrystalline Ni–Fe alloys

The tensile behaviors of FCC Ni–Fe alloys were investigated within three grain size regimes: >100 nm, 15–100 nm, and <15 nm. The results show that the nanocrystalline metals demonstrated large strain hardening rates, which increase with decreasing the grain size. With the similar grain size, lowing the stacking-fault energy (SFE) by addition of alloying element increases the yield strength and strain hardening ability. The “low” tensile elongation of nanocrystalline metals is due to the basic tradeoff between the strength and tensile elongation, i.e. nanostructured metals are not inherently brittle. Both the tensile results and fracture surface observations suggest that the tensile ductility increases with increasing the grain size. Furthermore, within the large grain size regime, the fracture surface exhibited the real void structure; while the fracture surface showed the concave and convex features when the grain size is less than the critical value.     

Enhancing tensile properties of ultrafine-grained medium-carbon steel utilizing fine carbides

The aim of the present study is to evaluate the influence of nano-sized carbides upon tensile behavior in UFG medium-carbon steels and to develop a material with improved tensile properties. UFG medium-carbon steels with fine carbides were successfully fabricated by multi-pass caliber rolling at 773 K. Alloying chromium and molybdenum resulted in thinner pearlitic lamellae, which were transformed into finer particles after severe plastic deformation. The UFG steel containing the alloying elements exhibited superior tensile properties, which was attributed to the enhanced strain hardening rate by the imbedded finer particles. Subsequent annealing induced growth of grains and particles, which also recovered elongation at the expense of strength. All UFG steels investigated here showed a yield-point phenomenon due to the decreased hardening rate and lack of mobile dislocations and their sources. The deteriorating effect of particle growth overwhelmed the improving effect of grain growth after annealing of the UFG medium-carbon steel, leading to a reduced strain hardening rate. This resulted in a positive correlation between a grain size and Lüders elongation in the investigated UFG steels.     

Tensile deformation behavior of spray-deposited FVS0812 heat-resistant aluminum alloy sheet at elevated temperatures

The tensile deformation behavior of spray deposited FVS0812 heat-resistant aluminum alloy sheet was studied by uniaxial tension tests at temperatures ranging from 250 °C to 450 °C and strain rates from 0.001 to 0.1 s^− 1. The associated fracture surfaces were examined by scanning electron microscopy (SEM). The results show that the degree of work-hardening increases with decreasing temperature, and exhibits a small decrease with increasing strain rate; the strain rate sensitivity exponent increases with increasing temperature. The flow stress increases with increasing strain rate but decreases with increasing temperature. The total elongations to fracture increase not only with increasing temperature, but also with increasing strain rate, which is in marked contrast with the normal inverse dependence of elongation on the strain rate exhibited by conventional aluminum alloy sheets. The SEM fracture analysis indicates that the dependence of elongation on the strain rate may be due to the presence of a transition from plastic instability at lower strain rates to stable deformation at higher strain rates for fine-grained materials produced by spray deposition.     

High-rate deformation and spall fracture of hadfield steel under action of high-current nanosecond relativistic electron beam

Features of the plastic deformation and dynamic spall fracture of Hadfield steel under conditions of shock wave loading at a straining rate of ∼10⁶ s⁻¹ have been studied. The shock load (∼30 GPa, ∼0.2 μs) was produced by pulses of a SINUS-7 electron accelerator, which generated relativistic electron bunches with an electron energy of up to 1.35 MeV, a duration of 45 ns, and a peak power on the target of 3.4 × 10¹⁰ W/cm². It is established that the spalling proceeds via mixed viscous-brittle intergranular fracture, unlike the cases of quasi-static tensile and impact loading, where viscous transgranular fracture is typical. It is shown that the intergranular character of the spall fracture is caused by the localization of plastic deformation at grain boundaries containing precipitated carbide inclusions.     

Achieving high strain rate superplasticity in cast 7075Al alloy via friction stir processing

Cast 7075Al alloys under as-cast and homogenized conditions were subjected to single-pass friction stir processing (FSP). FSP converted the coarse as-cast structure to the fine-grained structure with a grain size of 2.5–3.2 μm. A pre-homogenization prior to FSP was beneficial to the generation of a more uniform microstructure in the FSP sample with smaller particles and grains. Both FSP samples exhibited high strain rate superplasticity at 1 × 10⁻² s⁻¹ and 450 °C. Cavitation developed at the particles and the grain triple junctions. The superplasticity of the FSP sample was significantly improved by the pre-homogenization prior to FSP, with a maximum superplasticity of 890% being observed, due to reduced particle size. The analyses of the superplastic data and scanning electronic microscopic (SEM) examinations indicated that grain boundary sliding is the main deformation mechanism for the FSP 7075Al.     

Microstructure and mechanical properties of V–Ti–N microalloyed steel used for fracture splitting connecting rod

A new kind of V–Ti–N high strength microalloyed medium carbon steel has been developed, which is used for fracture splitting connecting rod. In this article, the characteristics of this carbon steel and its production process were studied. The microstructure, precipitated phases and their effects on mechanical properties were investigated by optical microscope, SEM, and TEM. The results showed that the steel was constituted of ferrite and pearlite. By reducing the finish rolling temperature and accelerating the cooling rate after rolling, microstructure with fine grain ferrite and narrow lamellar space pearlite could be obtained in V–Ti–N microalloyed medium carbon, and a large number of precipitated phases distributed over ferrite. These led the tensile strength to be more than 1000 MPa, yield strength (YS) more than 750 MPa. The impact fractograph showed typically brittle fracture characteristic.     

Tensile Properties of Electrodeposited Nanocrystalline FCC Metals

Bulk nanocrystalline Ni and Ni-15wt%Fe alloy were fabricated via electrodeposition techniques. The nominal grain size of nickel samples was varied from 15 to 200 nm by employing different deposition parameters. The grain size was further reduced to 9 nm by alloying nickel with iron. The tensile properties were evaluated at room temperature using dog-bone shaped samples. The results of this study confirm that strength and strain hardening rate increase with decreasing grain size. The fracture behavior was found to depend on the grain size, presence of large and small defects, and the stress state. The tensile elongation and reduction in area varied significantly among the samples and did not correlate with the fracture behavior. Three categories of behavior were identified. In Type I the samples showed completely ductile fracture but very low tensile elongation. In Type II the samples showed a relatively brittle behavior but impressive tensile elongation. In Type III the samples showed ductile behavior with reasonable tensile elongation. In this article, the tensile elongation and the fracture mode of nanocrystalline face centered cubic (FCC) metals are discussed in terms of deformation behavior and presence of defects.     

Hot tensile deformation behaviors and constitutive model of 42CrMo steel

The hot tensile deformation behaviors of 42CrMo steel are studied by uniaxial tensile tests with the temperature range of 850–1100 °C and strain rate range of 0.1–0.0001 s⁻¹. The effects of hot forming process parameters (strain rate and deformation temperature) on the elongation to fracture, strain rate sensitivity and fracture characteristics are analyzed. The constitutive equation is established to predict the peak stress under elevated temperatures. It is found that the flow stress firstly increases to a peak value and then decreases, showing a dynamic flow softening. This is mainly attributed to the dynamic recrystallization and material damage during the hot tensile deformation. The deformation temperature corresponding to the maximum elongation to fracture increases with the increase of strain rate within the studied strain rate range. Under the strain rate range of 0.1–0.001 s⁻¹, the localized necking causes the final fracture of specimens. While when the strain rate is 0.0001 s⁻¹, the gage segment of specimens maintains the uniform macroscopic deformation. The damage degree induced by cavities becomes more and more serious with the increase of the deformation temperature. Additionally, the peak stresses predicted by the proposed model well agree with the measured results.     

Hydrogen redistribution under stress-induced diffusion and corresponding fracture behaviour of a structural steel

ABSTRACT

Hydrogen redistribution under stress-induced hydrogen diffusion and corresponding fracture behaviour of a 960?MPa grade martensitic steel were studied. Slow strain rate tensile (SSRT) tests after hydrogen pre-charging were performed and the fracture surface was observed and analysed. The strain rate ranged from 10^?6 to 10^?4?s^?1. In the pre-charged sample with a certain hydrogen content of 0.62?ppm, hydrogen distribution was homogeneous before the SSRT test. After tensile testing, brittle fracture features appeared in the centre of the fracture surface, while ductile features appeared in the surrounding area. Brittle region size increased with the strain rate slowing down in the range from 10^?4 to 5?×?10^?6?s^?1, while it stabilised at the strain rate slower than 5?×?10^?6?s^?1. Relationship between the strain rate and the brittle region size was established and discussed based on the present data of hydrogen content in the material.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

2.47 GPa grade ultra-strong 15Co-12Ni secondary hardening steel with superior ductility and fracture toughness

This study investigated the effect of multi-step heat treatment on the microstructure, mechanical properties and fracture behavior of thick 15 Co-12 Ni secondary hardening steel. As-quenched sample was found to have elongated prior austenite grain(PAG) and coarse lenticular martensitic structure. On the other hand, heat-treated sample was observed to have fine lenticular martensitic structure due to fine PAG size and a lot of nano-sized carbides. Also, after heat treatment, nano-scale reverted austenite film was formed at the martensite interfaces. The heat-treated sample showed 2.47 GPa superior tensile strength and superior elongation of about 12 %. The high strength was mainly due to fine block size and high number density of nano-sized carbides. The average value of plane strain fracture toughness(KIC) was 29.3 MPa m1/2, which indicated a good fracture toughness even with the high tensile strength. The tensile fracture surface was observed to have ductile fracture mode(cup-and-cone) and the formation of about ~1 μm ultra-fine dimples. In addition to this, nano-sized carbides were observed within the dimples.The findings suggested that the nano-sized carbide had a positive effect not only on the strength but also on the ductility of the alloy. The fractured surface after toughness test, also showed ductile fracture mode with a lot of dimples. Based on the above results, correlation among microstructural evolution,deformation and fracture mechanisms along the heat-treatment was also discussed.     

The effect of heat treatments on the creep–rupture properties of a wrought Ni–Cr heat-resistant alloy at 973 K

The effect of heat treatments on the creep–rupture properties was investigated on a wrought Ni–Cr heat-resistant alloy at 973 K. Short-time aging (aging for 3.6 ks (1 h) at 973 K) was made on the solution-treated specimens with different grain sizes. The fine-grained specimen (the grain diameter, d = 45.2 μm) produced by short-time solution treatment exhibited almost the same rupture life and superior creep ductility as those of the medium-grained specimen (d = 108 μm) produced by normal solution treatment. The fine-grained specimen and medium-grained specimen showed the longer rupture life compared with the specimen with recommended aging. The principal strengthening of specimens was attributed to the precipitation hardening by γ′ phase particles. The fine-grained specimen had the highest hardness, and the increase of the hardness was observed in both the fine-grained and the medium-grained specimens during creep at 973 K. However, coarse-grained specimen (d = 286 μm) with high-temperature long-time solution treatment exhibited significantly short rupture life owing to insufficient precipitation hardening after the short-time aging and during creep. Ductile intergranular fracture with dimples occurred in the fine-grained specimen, while brittle intergranular fracture was observed in the medium-grained specimen and in the specimen with recommended aging. Both transgranular fracture and brittle intergranular fracture were observed in the coarse-grained specimen. A simple heat treatment composed of short-time solution treatment and short-time aging is applicable to high-temperature components of wrought Ni–Cr alloys.     

Tensile properties and fracture behaviour of carbon fibre filament materials

Monotonic tensile properties and fracture behaviour of carbon fibre filament materials, namely single/mono- and multi-filaments (two and four filaments) as well as virgin carbon tows have been evaluated and discussed. Micro composite or single fibre approach is used in this study, which facilitated the evaluation of tensile properties and nature of fracture of carbon filament materials in a relatively short time with a large number of inexpensive trials. Tensile tests have been conducted on these filament materials at ambient temperature and laboratory air atmosphere. Load–elongation and the corresponding stress–strain plots thus obtained have been analysed to understand the tensile behaviour. The peak tensile strength of single carbon filament is found to be 3.8 GPa, and the value of the resilience obtained is 19 MJ/m³. The peak tensile strength was found to increase moderately with further increase in number of filaments. However, the value of resilience was found to increase with increase in the number of fibres, which is attributed to the controlled failure of multi-filaments. On the other hand, the tensile strength of virgin carbon tow without matrix was found to be 1.13 GPa, and the value of the fracture energy was determined to be 9.9 MJ/m³, which is nearly one fourth or even less than the corresponding values of the mono- and multi-filaments. The data obtained in the case of the virgin carbon tows were further analysed to evaluate the Weibull statistical parameters.     

Fabrication of dense β-calcium orthophosphate with submicrometer-sized grains and its high-temperature superplastic deformation

High-density β-calcium orthophosphate (β-Ca₃(PO₄)₂, also called β-tricalcium phosphate: β-TCP) ceramics with submicrometer-sized grains were fabricated using a pulse-current pressure firing route. The maximum relative density of the β-TCP compacts was 98.7% at 1050 °C and this was accompanied by a translucent appearance. The mean grain size of the β-TCP compacts increased slightly with temperature to reach 0.78 μm at 1000 °C. However, upon further increasing the firing temperature to 1050 °C the mean grain size increased significantly to 1.6 μm. The extent of plastic deformation during tensile testing was examined at temperatures between 900 and 1100 °C using a strain rate in the range 9.26 × 10⁻⁵ to 4.44 × 10⁻⁴ s⁻¹. The maximum tensile strain achieved was 145% for a test temperature of 1000 °C and strain rate of 1.48 × 10⁻⁴ s⁻¹ and this was attributed to the relatively high density and small grain size.     

Effects of microstructure changes on the superplasticity of 2205 duplex stainless steel

The high temperature deformation behavior of 2205 duplex stainless steel under different conditions had been studied by tensile tests. The whole tensile test was conducted at a constant temperature 950 °C with an initial strain rate 1.5 × 10⁻³/s. Some tests were interrupted purposely and then the samples were quenched using water. Elongations of the fractured specimens were calculated. Microstructure changes just before and during the deformation were observed. Phase ratio of σ precipitate was analyzed. The results showed that the superplasticity of 2205 duplex stainless steel was directly affected by the microstructure before the deformation. The recrystallization phenomenon was distinct along with the homogenizing time and the grains became equiaxed and stable. Meanwhile, the quantity of σ phase increased when prolonged the homogenizing time. After homogenized for 7 min before the tensile test, the σ phase ratio was about 4.8% and the grain size was about 998 nm, the maximum elongation value 1260% was obtained. During the deformation progress, dynamic recrystallization was observed and quantity of σ phase increased with the increasing of deformation strain. The σ phase restricted the grain growth and kept the equiaxed duplex structure stable with a grain size of about 1 μm.     

Lowering the temperature for high strain rate superplasticity in an Al–Mg–Zn–Cu alloy via cooled friction stir processing

An Al–Zn–Mg–Cu alloy was friction stir processed over two kinds of backing anvils, at two different cooling rates. A finer grain size, 0.3 vs 0.5 μm, was obtained by processing at the highest cooling rate. Both materials showed superplastic behavior with a maximum elongation to fracture of about 510%. Grain boundary sliding was the operative deformation mechanism. Furthermore, the finer grain size material showed high strain rate superplasticity, at 10⁻² s⁻¹, at lower temperatures, as low as 250 °C.     

Effect of finishing rolling temperature on fire resistance and dynamic strain aging behavior of a structural steel

The influence of finishing rolling temperature (FRT) on dynamic strain aging (DSA) behavior and high-temperature resistance of a fire resistant steel microalloyed with Mo and Nb was investigated by means of tensile tests performed at temperatures ranging from 25 to 600 °C and strain rates of 10⁻⁴ to 10⁻¹ s⁻¹. In these steels, DSA manifestations are less intense than those observed for carbon steels and they take place at higher temperatures. The precipitation behavior of the steels was also considered. Hardness of samples heat treated at 100–600 °C displayed a maximum at 400 °C. Samples treated at this temperature and tensile tested at 600 °C did not show a higher yield stress than the untreated specimens. Results obtained indicated that DSA in the fire resistant steel might have a contribution for its fire resistance. The empirical activation energies related to the appearance of serrations on the stress–strain curves and to the maxima on the variation of tensile strength with temperature suggested that the high-temperature strengthening associated with DSA in this steel is the dynamic interaction of interstitial-substitutional solute dipoles and dislocations. The steel with lower FRT is more susceptible to DSA because of its higher amount of carbon in solid solution and showed better results in terms of high-temperature resistance.     

Influence of heat treatment on the strength and fracture behaviour of Fe-12Cr-6Al ferritic stainless steel

The changes in the tensile properties and fracture mode brought about by heat treatment of Fe-12Cr-6Al ferritic stainless steel have been studied. A favourable combination of high strength and good ductility is obtained by heating the material at 1370 K for 2 h followed by a water quench. The high-temperature treatment results in carbide dissolution as well as an increase in the grain size. The mechanism of strengthening has been evaluated from the apparent activation energy (28 kJ mol^–1) and is identified to be the unpinning of dislocations from the atmosphere of carbon atoms. As the heat-treatment temperature is increased, the fracture behaviour changes from ductile to brittle mode and this is related to the changes in grain size and friction stress.     

Hot deformation behavior of an 8% Cr cold roller steel

An 8% Cr cold roller steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s⁻¹. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the microstructure evolution and the mechanism of flow instability. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 8% Cr steel; the efficiency of power dissipation decreased with increasing Z value; flow instability was observed at higher Z-value conditions and manifested as flow localization near the grain boundary. The hot deformation equation and the dependences of critical stress for dynamic recrystallization and dynamic recrystallization grain size on Z value were obtained. The suggested processing window is in the temperature range 1050–1200 °C and strain rate range 0.1–1 s⁻¹ in the hot processing of 8% Cr steel.     

Tensile deformation of a Mg–2.54Nd–0.26Zn–0.32Zr alloy at elevated temperature

The engineering stress versus engineering strain curves for a Mg–2.54Nd–0.26Zn–0.32Zr cast alloy were measured by Gleeble-1500D thermo-simulation machine in the temperature range of room temperature to 400 °C at initial strain rates of 10⁻⁴–10⁻² s⁻¹. The effects of strain rate on stress, elongation to facture, and section shrinkage were analyzed. The fractograph morphologies were investigated by using SEM. It was found that strain rate has little effect on engineering stress for the Mg–2.54Nd–0.26Zn–0.32Zr alloy when tested at below 250 °C. When tested at above 250 °C, low strain rate resulted in decreased engineering stress, increased elongation to fracture, and section shrinkage. The fracture mode is cleavage fracture with elongated dimple below 250 °C and changes to typical ductile failure when tested above 250 °C.     

Atom probe tomographic observation of hydrogen trapping at carbides/ferrite interfaces for a high strength steel

A three-dimensional atom probe (3DAP) technique has been used to characterize the hydrogen distribution on carbides for a high strength AISI 4140 steel. Direct evidence of H atoms trapped at the carbide/ferrite interfaces has been revealed by 3DAP mapping. Hydrogen is mainly trapped on carbide/ferrite interfaces along the grain boundaries. Slow strain rate tensile (SSRT) testing shows that the AISI 4140 steel is highly sensitive to hydrogen embrittlement. The corresponding fractographic morphologies of hydrogen charged specimen exhibit brittle fracture feature. Combined with these results, it is proposed that the hydrogen trapping sites present in the grain boundaries are responsible for the hydrogen-induced intergranular fracture of AISI 4140. The direct observation of hydrogen distribution contributes to a better understanding of the mechanism of hydrogen embrittlement.