Proof Stress Research Articles

Influence of equal channel angular pressing temperature on texture, microstructure and mechanical properties of extruded AX41 magnesium

Abstract The objective of this study is the investigation of the effect of temperature of equal channel angular pressing (ECAP) on the microstructure, texture and mechanical properties of AX41 magnesium alloy. ECAP processing was performed at temperatures of 220 °C and 250 °C up to 8 passes via route Bc. It was found that during ECAP processing at 250 °C a smaller number of passes was enough to achieve a homogeneous microstructure than at 220 °C. However, the final grain size obtained after 8 passes at 250 °C was only 4 μm which is larger than the value of 2.7 μm achieved at 220 °C. Moreover, a significant influence of the temperature of ECAP processing on the dislocation density and the crystallographic texture was observed. The proof stress of the sample processed by 8 passes at 250 °C was lower than that obtained at 220 °C. It can be attributed to the lower dislocation density, the larger grain size and the texture, which facilitates the basal slip.

Influence of Cu and Mg addition on age-related deterioration in strength and creep behavior of Zn-12Al die casting alloys

Abstract The influence of the chemical composition of Zn-12Al-yCu-xMg (y = 1, 5; x = 0.05–1.4 mass%) die casting alloys on age-related deterioration in mechanical strength and creep behavior was investigated. Tensile tests were performed after aging at room temperature for 1 day and at 100 °C for 1 week. Creep tests were carried out under tensile stresses of 25, 50 and 100 MPa at 100, 130 and 160 °C. Cu addition suppressed age-related deterioration of proof stress by suppressing the coarsening of the eutectic structure during the aging. Creep resistance was improved by the Mg addition, whereas the Cu addition has a deleterious effect on creep resistance. The results show that the age-related deterioration of Zn-12Al alloys can be improved without sacrificing creep resistance by the complex addition of Cu and Mg.

Effect of bimodal grain size distribution on fatigue properties of Ti-6Al-4V alloy with harmonic structure under four-point bending

Titanium alloy (Ti-6Al-4V) with a bimodal harmonic structure, which is defined as a coarse-grained structure surrounded by a network structure of fine grains, was fabricated using powder metallurgy to improve both the strength and ductility. The microstructure of the sintered compacts was characterized using electron backscattered diffraction (EBSD). The areal fraction of the fine-grained structure in the harmonic structure tended to increase with the milling time. Tensile tests and four-point bending fatigue tests at a stress ratio of 0.1 were performed in air at room temperature. The tensile strength, 0.2% proof stress and fatigue limit of Ti-6Al-4V alloy with harmonic structure tended to increase as the areal fraction of the fine-grained structure increased. In contrast, elongation decreased due to the formation of a high areal fraction of the fine-grained structure (79.0%), which resulted in a reduction of the fatigue life with a low cycle regime. Thus, titanium alloy with high strength, ductility and fatigue resistance can be formed by optimization of the milling conditions. Furthermore, the mechanism for fatigue fracture of the Ti-6Al-4V alloy with a harmonic structure is discussed with respect to fractography and crystallography. A fatigue crack was initiated from the α-facet of the coarse-grained structure in the harmonic structure.

Effect of Reversed Austenite on the Stress Corrosion Cracking of Modified 17-4PH Stainless Steel

A study has been conducted investigating the effect of reversed austenite on the stress corrosion cracking property of a chemically modified 17-4PH with specially designed two-step tempering in a simulated geothermal environment. Two specimens with very similar 0.2% proof stress and different contents of reversed austenite were used to study the effect of reversed austenite on stress corrosion cracking (SCC). Slow strain rate testing (SSRT) with hydrogen precharged specimens and under cathodic polarization confirmed the detrimental effect of reversed austenite on hydrogen embrittlement. SSRT under anodic polarization confirmed the involvement of active path corrosion in the SCC of modified 17-4PH in a simulated geothermal environment, and showed that the effect of reversed austenite on active path corrosion was also detrimental. However, SSRT was not able to resolve the most impacted SCC property, SCC initiation, or SCC propagation.

Development of a high temperature high strength Al alloy by addition of small amounts of Sc and Mg to 2219 alloy

The paper reports a significant improvement in tensile properties, in particular at 200°C, of commercial 2219 Al alloy by addition of small amounts of Sc (0.8wt%) and Mg (0.45wt%), and employing copper mould suction casting followed by natural ageing and cold rolling. Microstructural examination and measurement of hardness were performed in order to explain the effects of Sc and Mg at each processing step. It is found that the remarkable improvement of room temperature strength occurs due to fine grain size, Al3Sc and Al3(Sc,Zr) dispersoids, GP zones on {100} and {111} planes, and work hardening. On exposure at 200°C, the GP zones transform primarily to θ′ precipitates and a few Ω precipitates. Sc and Mg atoms segregate at the θ′/matrix interface, which suppress the coarsening of θ′ precipitates and make them stable at higher temperatures. Thus, the work reports extremely high 0.2% proof stress of 542MPa at room temperature, 378MPa at 200°C and 495MPa at room temperature after 200h exposure at 200°C accompanied by reasonable ductility. Theoretical yield strength is calculated on the basis of the observed microstructure and is found to be in good agreement with the experimentally obtained value.

Synthesis & Characterization of Al-Ti-Cr MMC as friction material for disc brakes application

A stir die casting coupled with chill casting technique was used to prepare the Al-Ti-Cr composite with variable percentage of SiC and B4C particles. The electrolytic grade of 300 mesh powder was added in the molten of commercially master alloy of Al-Ti-Cr by the simultaneously addition of SiC and B4C. The SiC and B4C in the composite are varied from 0 to 10wt%.The developed composite were characterized using a Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction analysis (XRD). F.T.I.R. in corroboration with X.R.D,which confirms the crystallization of corundum (α-Al2O3) as one of the alumina phases.The presence of hard phase in the composite not only improved the ultimate tensile strength, 0.2% proof stress and the hardness of AMC composite. The ductility showed the adverse effect with increase of the SiC and B4Cin the matrix. The results from microstructure confirms the presence of second phase particles at the grain boundaries of aluminium-rich phase as well as within the grain itself which was confirmed by EDAX as well as XRD analysis. These composite have also been subjected to wear and frictiontesting at different operating parameters. Result shows that with increase in wt% of SiC and B4C from 2-10%, the ultimate strength and elongation percent of the Al-Ti-Cr alloy are still higher than that of monolithic alloys. Micro-structural characterization by using SEM depicted that the particles tend to be more agglomerated along with Al2O3 content, confirmed with FTIR analysis. The AFM results revealed that the surface roughness value of the composite is reduced by a factor of ∼3 μm. The results from microstructure showed the presence of second phase particles at the grain boundaries of aluminium-rich phase as well as within the grain itself which was confirmed by FTIR, XRD, AFM and EDAX.

Fracture Behaviors of Miniature Size Specimens of Sn-5Sb Lead-Free Solder under Tensile and Fatigue Conditions

Tensile and low cycle fatigue properties of Sn-5Sb (mass%) solder were investigated with miniature size tensile specimens. The effects of temperature and strain rate on tensile properties and the effect of temperature on low cycle fatigue properties were examined. Moreover, fracture behaviors of the specimens were investigated. Tensile strength and 0.1% proof stress increased with increasing strain rate and decreased with increasing temperature. The effects of temperature and strain rate on elongation were negligible. Chisel point fracture was observed in all specimens after the tensile test. The low cycle fatigue life of Sn-5Sb obeyed the Manson-Coffin's equation and fatigue ductility exponent, α was approximately 0.6 in the temperature range from 25 oC to 150 oC. From the observation for fracture surfaces and side areas of the fractured parts, it was found that cracking such as phase boundary cracking is dominant under fatigue conditions in the miniature size Sn-5Sb alloy.

Measurement and Analysis of the Elastic-Plastic Deformation Behavior of an Ultra-thin Austenitic Stainless Steel Sheet Subjected to In-plane Reverse Loading

Abstract In order to clarify the deformation behavior of an ultra-thin austenitic stainless steel sheet (SUS301) used for manufacturing electronic parts a new testing devise is designed and built. The test material is 0.2 mm thick and has a 0.2 % proof stress of 1800 MPa. The testing apparatus is equipped with comb-type die couples to measure the stress-strain curves of the sample under tension-compression cyclic loading without buckling for a strain amplitude of ±0.017. It is found that the stresses are higher in tension than in compression in the rolling direction (RD) for a strain range of |e|≥ 0.002, while in the transverse direction (TD) the stresses are higher in compression than in tension, and that the test material showed significant difference in the cyclic loading behavior between the RD and TD.

Evaluation of Very High Cycle Fatigue Properties of β-Titanium Alloy by Using an Ultrasonic Tensile-Compressive Fatigue Testing Machine

β-titanium alloy has been developed recently because β-titanium alloy has better cold workability, proof stress, and tensile strength. In order to use β-titanium alloy for automobile parts subject to cyclic loading, very high cycle fatigue properties of β-titanium alloy should be investigated. In this study, very high cycle fatigue properties of β-titanium alloy Ti-22V-4Al were evaluated by using an ultrasonic fatigue testing method, which allows us to reduce a fatigue testing period to 1/100 − 1/1000 of that by using conventional testing methods. An S-N diagram and fracture morphology of Ti-22V-4Al in the very high cycle region were investigated. Fatigue failure was observed and subsurface fracture occurred in the very high cycle region.

Microstructure Change and Mechanical Properties in Binary Ti-Al Containing Ti3Al

Grain morphology, phase stability and mechanical properties in binary Ti-Al alloys containing 43-52 mo1% Al have been investigated. Isothermal forging was used to control the grain sizes of these alloys in the range of 5 to 350 μm. Grain morphology and volume fraction of α2 phase were observed by optical metallography and scanning electron microscopy. Compressive properties were evaluated at room temperature, 1070 K, and 1270 K in an argon atmosphere. Work hardening is significant at room temperature, but it hardly took place at 1070 K and 1270 K because of dynamical recrystallization. The grain morphologies were determined as functions of aluminum content and processing conditions. The transus curve of α and α+γ shifted more to the aluminum-rich side than was the case in McCullough’s phase diagram. Flow stress at room temperature depends strongly on the volume fraction of the α2 phase and the grain size, whereas flow stress at 1070 K is insensitive to the alloy composition or the grain size, and flow stress at 1270 K depends mainly on the grain size. The α2 phase in the alloys does not increase the proof stress at high temperatures. These observations indicate that improvement of both the proof stress at high temperature and the room temperature ductility should be achieved to obtain slightly Ti-rich TiAl base alloys.

Effect of silicon content on densification, mechanical and thermal properties of Al-xSi binary alloys fabricated using selective laser melting

The effect of the silicon content on the densification of Al-xSi binary alloys (x=0, 1, 4, 7, 10, 12, and 20 mass%) fabricated using selective laser melting (SLM) were systematically studied. By optimizing laser scanning parameters for each Al-xSi powder, almost fully dense SLM samples (more than 99.5% relative density) could be achieved for the Al-0Si (pure aluminum), and Al-4–20Si alloys. The Al-1Si SLM sample, on the other hand, contained many microcracks, considered to be solidification cracks. The Al-1Si SLM sample, in the solid-liquid coexisting state, was brittle and no healing of the cracks by the infiltration of the liquid phase occurred. Therefore, the cracks were easily generated due to thermally induced tensile stresses during solidification. The microstructures and the mechanical and thermal properties of the Al-xSi SLM samples fabricated using the optimal laser scanning parameters were examined. As the silicon content increased, the ultimate tensile strength and proof stress increased, whereas the elongation and thermal conductivity decreased. These mechanical properties were attributed to larger amounts of the crystalized phases of silicon (obstacles for the movement of dislocations) with increasing silicon content. The crystalized phases can also scatter conduction electrons, thereby decreasing the thermal conductivity.

Preparation of Mg-TM-Y (TM=Transition Metal) Alloys with Long Period Stacking Ordered Phase and their Superior Mechanical Properties

Mg-Ni-Y alloy with composition ratio of 1 : 2 (Ni : Y) consisted of Mg, and 18R-type long period stacking ordered (LPSO) phases, whereas composition ratio of 1 : 1 (Ni : Y) consisted of Mg,14H-type LPSO and Mg2Ni phases, respectively. After hot-rolling at 693K, strong basal texture parallel to the plane sheet was formed in the LPSO, and Mg phases. Tensile test was performed along rolling direction (R.D) from room temperature (R.T) to 573K. The Mg98Ni1Y1, Mg96Ni2Y2, Mg94Ni3Y3, Mg97Ni1Y2 and Mg94Ni2Y4 rolled sheets exhibited 0.2% proof stress (σ0.2) of 232MPa, 255MPa, 358MPa, 337MPa and 393MPa, and elongation (δ) of 6%, 5%, 7%, 15% and 7% at R.T, respectively. The σ0.2 of the Mg-Ni-Y rolled sheet tend to increase with increasing of area fraction of the LPSO phase. After annealing at 773K for 0.6ks, the δ of Mg-Ni-Y rolled sheet tend to increase, while the σ0.2 decreased due to randomization of the Mg phase by re-crystallization. The Mg-Ni-Y rolled sheets exhibited high σ0.2 above 200MPa at 473K. Additionally, it was noted that σ0.2 of the Mg94Ni2Y4 rolled sheet exhibited 329MPa at 473K and 211MPa at 573K. Thus, the LPSO phase have high thermal stability and is attribute to strengthening of the Mg-Ni-Y alloy sheet at high temperature.

Influence of Grain Size on Work-Hardening Behavior in 12Cr Stainless Steel

Grain refinement is attracting attention as a strengthening method which does not depend on the alloying elements added to steels. Many reports have described the manufacturing methods and mechanical properties of ultra-fine grained steels. In ultra-fine grained steels, it is well known that yielding stress drastically increases in accordance with the Hall-Petch relationship, while uniform elongation significantly decreases. These tendencies imply that grain size affects not only yielding but also work-hardening behavior. However, the influence of grain size on work-hardening behavior has not been clearly understood. Therefore, in this study, we investigated the work-hardening behavior during tensile deformation of 12Cr stainless steel with various grain sizes. Grain refining was conducted by cold-rolling of annealed and quenched steel specimens, followed by recrystallization annealing. The grain size of the specimens decreased as the cold-rolling reduction rate increased. The minimum grain size obtained by this method was approximately 5 μm. With decreasing grain size, 0.2% proof stress increased and the strain which reached the plastic instability condition decreased. In the session, we report the dislocation accumulation behavior estimated by grain hardness and XRD and the dynamic recovery behavior assessed by the Kocks-Mecking model.

Nonlinear analysis of circular high strength concrete-filled stainless steel tubular slender beam-columns

Concrete-filled stainless steel tubular (CFSST) slender columns are increasingly used in composite structures owing to their distinguished features, such as aesthetic appearance, high corrosion resistance, high durability and ease of maintenance. Currently, however, there is a lack of an accurate and efficient numerical model that can be utilized to determine the performance of circular CFSST slender columns. This paper describes a nonlinear fiber-based model proposed for computing the deflection and axial load-moment strength interaction responses of eccentrically loaded circular high-strength CFSST slender columns. The fiber-based model incorporates the accurate three-stage stress-strain relations of stainless steels, accounting for different strain hardening characteristics in tension and compression. The material and geometric nonlinearities as well as concrete confinement are included in the computational procedures. Existing experimental results on axially loaded CFSST slender columns are utilized to verify the proposed fiber-based model. A parametric study is conducted to examine the performance of high-strength slender CFSST beam-columns with various geometric and material parameters. It is shown that the fiber-based analysis technique developed can accurately capture the experimentally observed performance of circular high-strength CFSST slender columns. The results obtained indicate that increasing the eccentricity ratio, column slenderness ratio and diameter-to-thickness ratio remarkably decreases the initial flexural stiffness and ultimate axial strength of CFSST columns, but considerably increases their displacement ductility. Moreover, an increase in concrete compressive strength increases the flexural stiffness and ultimate axial strength of CFSST columns; however, it decreases their ductility. Furthermore, the ultimate axial strength of CFST slender columns is found to increase by using stainless steel tubes with higher proof stresses.

Stress-strain hysteresis and strain hardening during cyclic tensile test of Mg-0.6at%Y alloy

The Mg-0.6at%Y alloy having an average grain size of 180µm and random texture was subjected to cyclic tensile tests at room temperature. Tensile tests were performed at a strain rate of 1×10−3s−1 for 10 cycles with the peak stress of 40, 70, and 100MPa with reference to the 0.2% proof stress of 68MPa. Hysteresis loops and cyclic strain hardening were observed in all peak stress cases. The observation of slip traces and twin types suggested that the hysteresis behavior was due to synchronous movement of basal slip and anomalous {101̅2} twinning, and that the cyclic strain hardening was due to non-basal slip. Prismatic slip led to cyclic strain hardening when peak stress was below the proof stress, and both prismatic and pyramidal slip promoted larger cyclic strain hardening when peak stress was increased over the proof stress.

Influence of particulate reinforcement on microstructure evolution and tensile properties of in-situ polymer derived MMC by friction stir processing

The authors recently reported a novel in-situ method of fabricating nano-polymer derived metal matrix composite (PD-MMC) by friction stir processing (FSP) and addressed the issues of tool wear and particle agglomeration. In the present work, the microstructural evolution and tensile properties of the processed composite are reported. The microstructure during FSP evolved by discontinuous dynamic recrystallization. In the composite, fine ceramic particles pin the grain boundaries, preventing grain growth resulting in a fine grain (2μm) structure being retained. FSPed Cu (processed with the same process parameters as that of the composite) exhibited a grain size of 100μm compared to 400μm in the base Cu. The composite microstructure was characterized by equiaxed grains with narrow grain size distribution and a high fraction (>80%) of high angle grain boundaries. The combined effect of grain refinement and ceramic particle incorporation lead to a twofold improvement in the proof stress of the composite (201MPa compared to 98MPa of base copper). The ultimate tensile strength improved by 33% and there was small drop in the ductility of the composite when compared to base Cu. Kocks-Mecking plot of the composite showed stage III of work hardening.

Proof stress measurement of die-cast magnesium alloys

Magnesium and its alloys exhibit not only elastic and plastic behaviour but also anelastic behaviour upon loading. The presence of anelastic strain poses a challenge to the measurement of proof stress using conventional methods. As such, the proof stress measurement methods specified by American Society for Testing and Materials (ASTM) and International Standards Organization (ISO) are reviewed and applied to three common die-cast magnesium alloys including AE44, AM60 and AZ91. The methods in the standards give inconsistent results due largely to the inherent anelastic behaviour of these alloys. The widely used 0.2% offset strain method tends to underestimate proof stress while the 0.2% permanent plastic strain method requires repeated loading and unloading. In view of the fact that the non-equivalence between the offset strain and the residual plastic strain for magnesium alloys is a key obstacle to the accurate proof stress measurement, a conversion chart is constructed to enable the determination of the appropriate offset strain for a desired residual plastic strain for a range of magnesium alloys. It is also shown that employing a higher offset strain than 0.2% has an advantage in reproducibility of proof stress measurement.

The Welded Joint Strength Reduction Factors of Modified 9Cr–1Mo Steel for the Advanced Loop-Type Sodium Cooled Fast Reactor

Creep strength enhanced ferritic (CSEF) steels including ASME Gr.91 are widely used in fossil power plants. In the advanced loop-type sodium-cooled fast reactor (SFR), modified 9Cr–1Mo steel (ASME Gr.91) is going to be adopted as a structural material. Modified 9Cr–1Mo steel was registered in the Japan Society of Mechanical Engineers (JSME) code as a new structural material for SFRs in the year 2012. The creep-rupture curve of the base metal of this steel was standardized using region splitting analysis method. According to this method, creep-rupture data were divided into two regions, high-stress and low-stress regimes, and those regions were individually evaluated by regression analyses with the Larson–Miller parameter (LMP). The difference in the creep failure mechanisms between the high-stress and low-stress regions was considered in this method. The boundary between these regions was half of the 0.2% proof stress of the base metal at the corresponding temperature. In the modified 9Cr–1Mo steel welded joint, creep strength may markedly degrade, especially in the long-term region. This phenomenon is known as “type-IV” damage due to creep voids and cracks in the fine-grained heat-affected zone (HAZ). There is no precedent for indicating the obvious creep strength degradation of welded joints under SFR temperatures (550 °C or less). Although obvious strength degradation of the welded joints has not yet been observed at 550 °C, it is fair to assume that the strength degradation will occur due to very long-term creep. Therefore, considering strength degradation due to “type-IV” damage is necessary. This paper proposes the creep-rupture curve and the welded joint strength-reduction factor (WJSRF). The creep-rupture curve of the welded joint was proposed by employing a second-order polynomial equation with LMP using region splitting analysis method, which is used for the base metal as well. The WJSRFs were proposed on the basis of design creep-rupture stress strength. The resulting allowable stress was conservative compared with that prescribed in ASME code and the Japan domestic regulation for thermal plants. In addition, the design of the hot-leg pipe in SFR was reviewed considering the WJSRFs.

Studying recovery processes in a strain-hardened Al–Mg–Mn–Fe–Si alloy

The material of a shell structure subjected to 20-year use under ambient conditions has been studied. The structure and mechanical characteristics of a strain-hardened AMg6 alloy, as well as the effect of subsequent holdings of this alloy for 10–3000 h at temperatures of 50, 70, 80, 100, 130, 150, 180, and 220°C, on changes in its dislocation structure and mechanical characteristics have been investigated. It has been shown that, in the structures under study, the AMg6 alloy has a cellular structure with a high density of dislocations and the ultimate strength σu = 445.5 ± 2.5 MPa, the proof stress σ0.2 = 326.5 ± 3.5 MPa, and the relative elongation δ = 11.7 ± 0.5%. Polygonization in the alloy occurs at a temperature of 220°C and the initial stage of the recovery process corresponds to a temperature range of 50–100°С in which the softening process can be divided into two stages, i.e., stage (1) of active softening due to the interaction of point defects with each other and stage (2) of the stabilization of the characteristics of the alloy.

Experimental investigation of cold-formed high strength steel tubular beams

This paper presents a test program on cold-formed high strength steel tubular beams. The nominal 0.2% proof stresses of the high strength steel were 700, 900 and 1100MPa in this study. Twenty-five four-point bending tests on circular, rectangular and square hollow structural sections were conducted. Load-deformation histories and failure modes of the beams were reported. Experimental results were compared against design values calculated from European code, Australian standard and North American specification for hot-rolled and cold-formed steel structures. In addition, the test strengths were also compared with the Direct Strength Method predictions. The compactness criteria were assessed by comparing the section slenderness to the slenderness limits in codes.