Cryogenic mechanical behavior of 5000- and 6000-series aluminum alloys: Issues on application to offshore plants

The mechanical behavior of aluminum alloys was investigated in terms of four aspects: temperature, strain rate, material type, and fracture shape. The candidate materials were 5000- and 6000-series alloys. The material characteristics were investigated and summarized as a function of low temperature (110–293 K) and quasi-static strain rate (10⁻⁴ and 10⁻² s⁻¹). The results confirmed that the strength and ductility of aluminum alloys improved with a decrease in the temperature. The aluminum alloys showed a strain rate effect only in terms of the ductility of the 5000-series alloys. In addition, fractography analyses were performed on the fracture specimens to explain the material behavior at cryogenic temperatures.     

Cryogenic reliability of composite insulation panels for liquefied natural gas (LNG) ships

The major carrier of liquefied natural gas (LNG) is LNG ships, whose containment system is composed of dual barriers and composite insulation panels. The LNG containment system should have cryogenic reliability and high thermal insulation performance for safe and efficient transportation of LNG. The secondary barrier composed of adhesive bonded aluminum strips should keep tightness for 15 days, when the welded stainless primary barrier fails. However, cracks are generated in the composite insulation panels due to the local stress concentration and the brittleness of insulation materials at the cryogenic temperature of −163 °C. If cracks generated in the insulation panel propagate into the secondary barrier, LNG leakage problem might occur, which is a remaining concern in ship building industries.In this study, crack retardation capability in the composite insulation panel was investigated with glass fabric reinforcement. Finite element analysis was conducted to estimate the thermal stress at the cryogenic temperature and a new experimental method was developed to investigate the failure of secondary barrier of composite insulation panel. From the experimental results, it was found that the glass fabric reinforcement was effective to retard the crack propagation into the aluminum secondary barrier from the polyurethane insulation foam at the cryogenic temperature.     

Strain-rate effects on the mechanical behavior of the AISI 300 series of austenitic stainless steel under cryogenic environments

A series of uni-axial tensile tests were carried out under various low temperatures and strain-rate ranges for AISI 300 austenitic stainless steel. The strain-rate dependencies of the materials under investigation were evaluated at temperatures ranging from ambient to cryogenic. Non-linear mechanical behavior such as phase transformation, discontinuous yielding and micro-damage of four kinds of commercial stainless steel-based material were quantitatively investigated by measuring transformation induced plasticity (TRIP) and threshold strain for 2nd hardening. In this study, the main properties of each material were analyzed and compared based on the conditions of strain-rates and temperature. Test results showed that all the test materials were strongly dependent on temperature and strain rate. It is expected that the findings in this study could be used for the cryogenic design and further research of structure materials under cryogenic environments.     

铝合金超塑变形研究进展

综述了铝合金材料超塑变形的研究现状和进展情况.着重介绍了高应变速率下铝合金超塑性的基本特征,探讨了铝合金超塑变形机理,介绍了铝合金超塑性的应用情况.超塑铝合金是性能优良,具有广泛用途的新型材料,随着高应变速率条件下铝合金超塑变形研究的深入,将不断提高铝合金生产的经济效益和实用性.     

镁合金在大变形和高应变率下塑性变形研究进展

介绍了强应变塑性大变形下镁合金研究现状.重点综述了在较高应变率及冲击载荷作用下关于镁合金变形的研究情况,同时也比较详细地综述了在不同温度、不同载荷作用下镁合金塑性变形特征及其物理机制.最后简要介绍了几个描述材料在较高应变率和冲击载荷作用下变形行为的数学表示式,并就镁合金作为结构材料的研究说明了作者的一些看法.     

Fatigue crack growth threshold at cryogenic temperatures: A review

The cryogenic temperature near-threshold fatigue crack growth behavior of several structural materials: aluminum alloys, copper, steels, nickel alloys and titanium alloys, was reviewed. It was observed that the resistance to near-threshold fatigue crack propagation in the alloy systems investigated generally improved with decreasing temperature. Environmental effects were not responsible for the influence of temperature on near-threshold crack growth rate behavior. Furthermore, crack closure alone could not account for this temperature effect. The dislocation dynamics model appears to offer a possible rationale to explain the improved near-threshold crack growth performance typically exhibited at cryogenic temperatures by the variety of materials examined herein. Crack closure, on the other hand, rationalized the influence of load ratio on low temperature near-threshold crack propagation behavior.     

Flow stress and ductility of AA7075-T6 aluminum alloy at low deformation temperatures

The present investigation has been conducted in order to develop a rational approach able to describe the changes in flow stress of AA7075-T6 aluminum alloy with deformation temperature and strain rate, when this material is deformed at temperatures in the range of 123-298 K at strain rates in the range of 4 × 10⁻⁴ to 5 × 10⁻² s⁻¹. The constitutive formulation that has been advanced to accomplish these objectives represents a simplified form of the mechanical threshold stress (flow stress at 0 K) model developed at Los Alamos National Laboratory (Los Alamos, New Mexico, USA). Thus, it is assumed that the current flow stress of the material arises from both athermal and thermal barriers to dislocation motion. In the present case, the effect of three thermal barriers has been considered: solid solution, precipitation hardening and work-hardening. The first two effects do not evolve during plastic deformation, whereas the last one is considered as an evolutionary component of the flow stress. Such an evolution is described by means of the hardening law earlier advanced by Estrin and Mecking (1984) [20]. The law is implemented in differential form and is integrated numerically in order to update the changes in strain rate that occur during tensile tests carried out both at constant and variable crosshead speed. The extrapolation of the hardening components from 0 K to finite temperatures is accomplished by means of the model earlier advanced by Kocks (1976) [19]. The results illustrate that the constitutive formulation developed in this way is able to describe quite accurately both the flow stress and work-hardening rate of the material, as well as temperature and strain rate history effects that are present when deformation conditions change in the course of plastic deformation. The evaluation of the ductility of the alloy indicates that the changes in this property are mainly determined by deformation temperature rather by strain rate. When deformation temperature decreases from 298 to 123 K, ductility also decreases from ∼35 to 24%. However, despite these relatively small variations, significant changes in the fracture morphology could be observed on the fracture surfaces of the examined specimens, with the predominance of a mixed ductile-brittle mechanism at lower temperatures.     

Prediction of mechanical properties of extra deep drawn steel in blue brittle region using Artificial Neural Network

Blue brittle region also known as dynamic strain ageing (DSA) regime is very important in the materials because in this region material properties behave in very unpredictable ways. In this work, Artificial Neural Network (ANN) models are developed for the prediction of mechanical properties such as yield strength (YS), ultimate tensile strength (UTS), % elongation, strength coefficient (K) and strain hardening exponent (n) for the extra deep drawn (EDD) quality steel in blue brittle region. To calculate the mechanical properties at elevated temperatures, experiments were conducted at the interval of 25 °C from room temperature till 700 °C in three rolling directions. Based on the experimental results, the blue brittle region for EDD steel is identified between 350 °C and 450 °C and ANN model is trained in all the three rolling directions. Trained up ANN model is tested with the experimental results at two different temperatures with in blue brittle region. Experimental and modeling errors in the prediction of mechanical properties are found within the permissible range.     

Role of initial texture on the plastic anisotropy of Mg-3Al-1Zn alloy at various temperatures

The effect of initial texture on the anisotropic properties of AZ31 Mg alloys was investigated on the basis of microstructure evolution after compression tests and Lankford parameter (r-value) experiments at various temperatures. Two kinds of sheets were used: one is the cast-and-rolled sheet, and the other is the strip-cast sheet. Compression tests were conducted up to a strain of 0.3 at temperatures of 200-400 °C, and tensile tests were performed to obtain the r-value at various temperatures (25-450 °C). The results showed that, at all test temperatures, the average r-value of the RD plane were greater than those of the ND and TD planes in the cast-and-rolled material. When comparing the average r-value of the RD plane, the cast-and-rolled material revealed much higher values than those of strip-cast material. It was observed that the cross-sectional shape of RD compressive specimens (the compression axis was parallel to the rolling direction) of cast-and-rolled materials changed from an initial circular shape to an ellipsoidal shape due to the plastic anisotropy. Compression processes of specimens were simulated using a finite element method where the Hill's anisotropic yield criterion was adopted. The simulated results were in a good agreement with experimental data.     

Development of carbon—Low alloy steel grades for low temperature applications

Low alloy steels are processed to fulfill the requirements of low temperature applications. Besides the chemical composition, the steel should receive a suitable heat treatment to ensure the targeted mechanical properties at low temperature. In other words, the steels are designed to delay the ductile to brittle transition temperature to resist dynamic loading at subzero temperatures. Steel alloys processed for liquefied gas pipeline fittings are examples for applications that need deep subzero impact transition temperature (ITT).The main purpose of the present work was to find a suitable heat treatment sequence for alloys LC2 and LC2-1. Further, it aimed to correlate the impact toughness with the microstructure and the fracture surface at different sub-zero temperatures.The steels under investigation are carbon-low alloy grades alloyed with Ni, Cr and Mo. LC2 steel alloy has been successfully processed and then modified to LC2-1 alloy by addition of Cr and Mo. Oil quenching from 900 °C followed by tempering at 595 °C was used for toughness improvements. Hardness, tensile and impact tests at room temperature have been carried out. Further impact tests at subzero temperatures were conducted to characterize alloys behavior. Metallographic as well as SEM fractographic coupled with XRD qualitative analysis are also carried out.Non-homogenous martensite-ferrite cast structure in LC2 was altered to homogeneous tempered martensite structure using quenching-tempering treatment, which is leading to shift the ITT down to −73 °C. Addition of Cr and Mo creates a very fine martensitic structure in LC2-1 alloy. Quenching-tempering of LC2-1 accelerates ITT to −30 °C. It is expected that the steel was subjected to temper embrittlement as a result of phosphorus segregation on the grain boundary due to Cr and Mo alloying, as it was concluded in reference no. [6].     

Effect of Ni content on the tensile properties and strain-induced martensite transformation for 304 stainless steel

The effect of Ni content (8.3-12 wt.%) on the tensile properties and strain hardening behavior was studied on type 304 stainless steels (STS) used for the membrane of LNG storage tanks. The tensile test temperature was varied from 25 °C to −196 °C. At room temperature, the hardening and ductility indices (tensile strength, strain hardening exponent and elongation) increased with decreasing Ni content. For the 8.3-9.0 wt.% Ni STS, a lower yield point was observed at temperatures below −60 °C. It was due to the dynamic strain softening and/or transformation-induced plasticity (TRIP) that accompanied the rapid increase in the amount of strain-induced martensite (α′) at low strains. Neither dynamic strain softening nor TRIP was observed for the 12 wt.% Ni STS because only the ?-martensite transformation was produced at the low strains.     

Anisotropic and tensile flow behaviors of Mg alloy AZ31B thin sheet in H24 condition at elevated temperatures

In this work, a series of experiments was performed to explore the effects of anisotropy, strain rate, and temperature on microstructure change and associated mechanical response of a rolled AZ31B-H24 Mg alloy sheet under tension. Tensile tests were carried out on specimens in the 0, 45, and 90° to the rolling direction, using initial strain rates in the range of 4 × 10⁻³ to 1 × 10⁻¹ s⁻¹ at temperatures of 250 and 370 °C. Results showed that variations in flow behavior under tension could be related to the changes in microstructure resulting from applied tensile conditions. Resultant microstructures, such as degree of dynamic recrystallization, grain growth, and shape of the grain, were associated with temperature, strain rate, and tensile loading direction. The initial texture influenced the variations in changes in microstructure and mechanical properties upon testing in different directions. The specimens upon testing in the 45° to the rolling direction yielded higher m-value, lower strength, and greater elongation to failure under all test conditions.     

Thermal conductivity of Tecamax® SRP from millikelvin temperatures to room temperature

Tecamax® SRP (self-reinforced polyphenylene) is a new commercially available amorphous polymer which is suitable for use at cryogenic temperatures. It has a high tensile strength (210 MPa at room temperature), resulting from the molecular structure of the polymer rather than by the addition of reinforcing materials. We have measured the thermal conductivity between 60 mK and 280 K. We find that the conductivity below 10 K is similar to, but lower than, most amorphous materials, and the material offers a good combination of low conductivity at low temperatures and high tensile strength. Our results suggest that the material may in fact have a small crystalline component, which may be a partial explanation for the low conductivity. Above 10 K, the temperature dependence of the conductivity is different from most amorphous materials. We are unaware of previous measurements of the thermal conductivity of this material, even at room temperature.     

Fatigue behaviour and crack growth rate of cryorolled Al 7075 alloy

The effects of cryorolling (CR) on high cycle fatigue (HCF) and fatigue crack growth rate behaviour of Al 7075 alloy have been investigated in the present work. The Al 7075 alloy was rolled for different thickness reductions (40% and 70%) at cryogenic (liquid nitrogen) temperature and its tensile strength, fatigue life, and fatigue crack growth mechanism were studied by using tensile testing, constant amplitude stress controlled fatigue testing, and fatigue crack growth rate testing using load shedding (decreasing ΔK) technique. The microstructural characterization of the alloy was carried out by using Field emission scanning electron microscopy (FESEM). The cryorolled Al alloy after 70% thickness reduction exhibits ultrafine grain (ufg) structure as observed from its FESEM micrographs. The cryorolled Al 7075 alloys showed improved mechanical properties (Y.S, U.T.S, Impact energy and Fracture toughness are 430 Mpa, 530 Mpa, 21 J, 24 Mpa m^1/2 for 40CR alloy) as compared to the bulk 7075 Al alloy. It is due to suppression of dynamic recovery and accumulation of higher dislocations density in the cryorolled Al alloys. The cryorolled Al alloy investigated under HCF regime of intermediate to low plastic strain amplitudes has shown the significant enhancement in fatigue strength as compared to the coarse grained (CG) bulk alloy due to effective grain refinement. Fatigue crack growth (FCGR) resistance of the ufg Al alloy has been found be higher, especially at higher values of applied stress intensity factor ΔK The reasons behind such crack growth retardation is due to diffused crack branching mechanism, interaction between a propagating crack and the increased amount of grain boundaries (GB), and steps developed on the crack plane during crack-precipitate interaction at the GB due to ultrafine grain formation.     

Split Hopkinson pressure bar multiple reloading and modeling of a 316 L stainless steel metallic hollow sphere structure

The high strain rate (600 s⁻¹) compression deformation of a 316 L metallic hollow sphere (MHS) structure (density: 500 kg m⁻³; average outer hollow sphere diameter: 2 mm and wall thickness: 45 μm) was determined both numerically and experimentally. The experimental compressive stress–strain behavior at high strain rates until about large strains was obtained with multiple reloading tests using a large-diameter compression type aluminum Split Hopkinson Pressure Bar (SHPB) test apparatus. The multiple reloading of MHS samples in SHPB was analyzed with a 3D finite element model using the commercial explicit finite element code LS-DYNA. The tested MHS samples showed increased crushing stress values, when the strain rate increased from quasi-static (0.8 × 10⁻⁴ s⁻¹) to high strain rate (600 s⁻¹). Experimentally and numerically deformed sections of MHS samples tested showed very similar crushing characteristics; plastic hinge formation, the indentation of the spheres at the contact regions and sphere wall buckling at intermediate strains. The extent of micro-inertial effects was further predicted with the strain rate insensitive cell wall material model and with the strain rate sensitive behavior of MHS structure similar to that of the cell wall material. Based on the predictions, the strain rate sensitivity of the studied 316 L MHS sample was attributed to the strain rate sensitivity of the cell wall material and the micro-inertia.     

Investigating the transverse behavior of Glass–Epoxy composites under intermediate strain rates

In this study, the strain rate effects on transverse tensile and compressive properties of unidirectional Glass fiber reinforced polymeric composites are investigated. To demonstrate strain rate effects, the tensile and compressive composite specimens with identical configuration are fabricated and tested to failure in the transverse direction at quasi-static strain rate of approximately 0.001 s⁻¹ and intermediate strain rates of 1–100 s⁻¹. The tensile and compressive tests are performed using a servo-hydraulic testing apparatus equipped with strain rate increasing mechanisms. For performing the practical tests, a jig and a fixture and other test supplies are designed and manufactured. The performance of the test jig is evaluated and showed that it is adequate for composites testing under tension and compression loads. The effects of strain rate on mechanical properties (maximum strength, modulus, and strain to failure) are considered. The characteristic results for the transverse properties indicate that damage evolution is strain-rate-dependent for the examined material. Also, a strain-rate-dependent empirical material model associated with different regression constants is proposed based on the experimental results obtained to characterize the rate dependent behavior of Glass/Epoxy composite material.     

Tool wear in cryogenic turning of Ti-6Al-4V alloy

Though titanium alloys are being increasingly sought in a wide variety of engineering and biomedical applications, their manufacturability, especially machining and grinding imposes lot of constraints. Rapid tool wear encountered in machining of titanium alloys is a challenge that needs to be overcome. Cryogenic machining with liquid nitrogen as coolant is being investigated by researchers to reduce the cutting zone temperatures and enhance the tool life. The effects of cryogenic cooling have been studied on growth and nature tool wear in the present investigation while turning Ti-6Al-4V alloy bars with microcrystalline uncoated carbide inserts under dry, wet and cryogenic cooling environments in the cutting velocity range of 70-100 m/min. Cryogenic cooling by liquid nitrogen jets enabled substantial improvement in tool life through reduction in adhesion-dissolution-diffusion tool wear through control of machining temperature desirably at the cutting zone.     

Effect of Ca and Sr additions on high temperature and corrosion properties of Mg-4Al-2Sn based alloys

This study examined the effects of Ca and Sr addition on the creep and corrosion properties of Mg-Al-Sn based alloys with the aim of developing new Mg-Al-Sn-x alloys for automotive powertrain applications. The materials were cast using the squeeze casting process to obtain a dense microstructure without pores. Creep tests were carried out at a constant temperature between 150 °C and 200 °C and a constant applied stress between 50 and 80 MPa until the minimum creep rate had been reached. Potentiodynamic and immersion tests were carried out to evaluate corrosion properties of the alloys. The creep and corrosion resistance were improved by adding Ca and Sr.     

Effect of strain rate on tensile behavior of carbon fiber reinforced aluminum laminates

In this paper, the tensile behavior of carbon fiber reinforced aluminum laminates (CRALL) has been determined at a strain rate range from 0.001 s^− 1 to 1200 s^− 1. Experimental results show that CRALL composite is a strain rate sensitive material, and the tensile strength and failure strain both increased with increasing strain rate. A linear strain hardening model has been combined with Weibull distribution function to establish a constitutive equation for CRALL at different strain rates. The analysis of the model shows that the Weibull scale parameter, σ₀, increased with increasing strain rate, but Weibull shape parameter, β, can be regarded as a constant.     

Considerations on the selection of alloys for use in pressure cells at low temperatures

Issues surrounding the choice of metals subject to large tensile stresses in a cryogenic environment are discussed. Problems with the use of materials under such conditions often arise from their loss of ductility and toughness. The focus of the present article is therefore mainly on these properties; their dependency on temperature, crystal structure, grain size and purity level; and their application to the quantitative prediction of fracture behaviour. It also discusses the selection of alloys (on the basis of their toughness, ductility and freedom from flaws) that can be used safely under ambient and cryogenic conditions. A recent proposal to employ a particular titanium alloy for the above purpose is considered, in light of the tendency of metals with a bcc crystal structure (including β-phase titanium alloys) to become brittle at low temperatures. Other materials, which are known to be (or are likely to be) satisfactory under such conditions are examined. For example, certain platinum alloys may possess very high cryogenic tensile strengths and ductilities, and low magnetic susceptibilities. Nanocrystalline metals have the potential to provide these characteristics, as well as low specific heats and high thermal conductivities. Such properties could make nanocrystalline materials very useful in (for example) work at microkelvin temperatures, or more generally where very accurate cryogenic temperature control is needed. Sources of cryogenic mechanical property information on a wide range of materials are also presented.