Softening Of Alloy Research Articles

The relationship between flow behavior and initial microstructure of Ti-10V-2Fe-3Al alloy during subtransus deformation

The morphology of alpha phase has a significant effect on the deformation and flow softening of near-β titanium alloys. This study investigated the deformation behavior of Ti-10V-2Fe-3Al alloy with equiaxed and lamellar microstructures by isothermal compression test in α + β region. The results show that lamellar microstructure presented higher peak stresses as well as intenser flow softening than equiaxed microstructure. Constitutive analysis including the flow softening index is consistent with the observed phenomenon. Microstructure characterization was employed to investigate the effect of deformation parameters on microstructural evolution. Then softening mechanisms for equiaxed and lamellar morphologies were summarized comprehensively. The softening mode for beta matrix is dominated by continuous dynamic recrystallization (CDRX) and it can be enhanced by alpha phase particles. With the increase of strain rate and decrease of temperature, the subgrain network in beta phase became denser due to dispersed alpha particles. Alpha lamellar evolutions, including globularization, kinking and rotation, were very sensitive to plastic strain and they all contributed to the flow softening of lamellar microstructure. Since the globularization fraction was generally very limited, the loss the Hall-Petch strengthening resulted from lamellae rotation and kinking was inferred as the main softening mechanism for Ti-10V-2Fe-3Al with colony lamellar alpha phase.

Softening and Jump-Like Deformation of Aluminum Alloys Under Stress Concentration Conditions at a Temperature of 4 K with Decreasing the Stiffness of the Loading System

Softening and Jump-Like Deformation of Aluminum Alloys Under Stress Concentration Conditions at a Temperature of 4 K with Decreasing the Stiffness of the Loading System

Flow stress stabilization of Zn-Cu-Mn-Mg alloys using thermomechanical processing

Zinc-based alloys are potential candidates for bioabsorbable metallic devices due to their application-appropriate corrosion rates and biocompatibility. However, strain softening and rate sensitivity in tensile testing remain as challenges for their use in load bearing applications. In this study, three different Zn-xCu-yMn-0.05Mg (x = 0.5, 1.0 wt%, y = 0.4, 0.6 wt%) alloys were formulated and their microstructure and tensile properties in the room-temperature rolled condition were characterized. Additionally, the effect of short-time annealing at 320 °C on the strain softening and strain rate sensitivity of alloys was studied. The results indicate that dissolution of secondary phases and grain coarsening lead to the suppression of strain softening and strain rate sensitivity. The evolution of microstructure during the room-temperature tensile testing indicates that dynamic recrystallization is responsible for strain softening and can be eliminated by tuning the fraction of secondary phases and underlying grain size. The formulated alloys are not susceptible to natural aging and show good thermal stability during aging up to 200 °C for 60 h due to the pinning effect of MnZn13 precipitates on the grain boundaries.

Effect of high temperature deformation on the deformation behavior and thermodynamic properties of a Zr-based bulk amorphous alloy

Zr 55.7 Cu 22.4 Ni 7.2 Al 14.7 alloy rods with a diameter of 3 mm were prepared by copper mold suction casting. The structure, thermodynamic properties and plastic deformation behavior of the specimens before and after compression in supercooled liquid region were studied by X-ray diffractometer (XRD), differential scanning calorimeter (DSC) and uniaxial compression machine (equipped with a high-temperature oven). The range of thermal forming process parameters was also discussed. The results show that the alloys have obvious glass transition and crystallization kinetics, and the crystallization activation energies obtained by the Kissinger and Ozawa equations are 297.1 kJ/mol and 294.9 kJ/mol, respectively. The amorphous alloys exhibit stress overshoot in low temperature region at high strain rates and deformation behavior of alloy softening in high temperature region at medium to high strain rates. Meanwhile, the viscosity of the alloy remains approximately constant at 740 K and 750 K, showing Newtonian fluid characteristic. It can be concluded that the optimal thermoplastic forming process ranges from 740 K to 750 K, 5 × 10 −4 s −1 to 5 × 10 −3 s −1 . The crystallization activation energies of the specimens after deformation are slightly higher than before, and they increase with the increase of strain rate. • Zr-based bulk amorphous alloy has a good thermodynamic stability, which facilitates thermoplastic forming. • The amorphous alloy shows superplastic deformability at high temperatures and strain rates. • The crystallization activation energies of the deformed specimens are higher than that of the undeformed. • The crystallization activation energies of the alloys after deformation increase with increasing strain rate.

Anisotropic damage constitutive model of AA5754 aluminum alloy considering warm forming history

A multipath two-stage loading test for AA5754 aluminum alloy is carried out. In the first loading stage, the tensile tests of big samples were conducted with different preforming conditions. Then small samples were cut off from the pre-strained big samples. In the second loading stage, the mechanical properties of small samples were measured, and the influence of preforming factors such as prestrain value, temperature, strain rate and stress state on the mechanical properties of the sheets has been studied. With increasing prestrain, the yield stress of the sheet increases and the failure strain decreases linearly. The higher the preforming temperature is, the lower the yield strength of the sheet is under the same prestrain, and the greater the hardening index and failure strain. Moreover, the mechanical properties of the sheet are not sensitive to the forming strain rate and stress state. In addition, a transient softening effect appeared during the two-stage loading test. A damage constitutive model considering the history of warm forming is established by VUMAT subroutine of ABAQUS commercial software. The parameters of the constitutive model are determined by the two-stage loading experimental data and a genetic algorithm. This model can describe the transient softening and Bauschinger effect of AA5754 aluminum alloy after preforming at different temperatures. Good agreement is also observed between the stress-strain responses of AA5754 aluminum alloy after warm preforming and the predictions of the model.

Mechanical Properties of Titanium Alloys at Operating and Pressure Shaping Temperatures Depending on the Aluminum and Molybdenum Equivalents

The mechanical properties of different classes of titanium alloys have been compared during tensile tests and upset tests in the temperature range of 20-1200 °C. It has been established that at temperatures of 20–500 °C, the maximum strength is observed for the α+β-transition alloys, and at 600–800 °C – for heat-resistant near α-and α+β-alloys. At temperatures exceeding 900 °C and up to the polymorphic transformation point, the softening of titanium alloys is mainly defined by the content of α-stabilizers and neutral hardeners. At temperatures above the polymorphic transformation point, the tensile strength does not depend on the aluminum and molybdenum equivalents.

Cold spray deposition characteristic and bonding of CrMnCoFeNi high entropy alloy

Particle bonding physics remains an important area of study in cold spray (CS) deposition. It has been argued that for pure metals, large interfacial strains in particles triggered either by adiabatic shear instability (ASI) or hydrostatic plasticity promote bonding while the bonding mechanisms of high entropy alloys (HEA) have not been explored. HEAs are an emerging class of materials that have superior work-hardening ability and are usually resistant to softening, a behavior that is different from pure elements and conventional alloys. In this study, equiatomic CrMnCoFeNi HEA feedstock powder is produced and CS experiments are conducted to explore the deformation evolution of these HEA particles and their bonding characteristics. The shear localization strain of the HEA is theoretically estimated and compared to conventional alloys and elements to explain the deposition behavior of the HEA. Results show that the particle impact morphology is strain rate dependent in addition to being material and microstructure dependent. Electron channelling contrast imaging reveals severe plastic deformation at the lower half of the particles due to the dynamic inertia effect. Deformation nano-twins with a thickness of about 200 nm were observed. HEA/HEA pair has a higher critical velocity compared to HEA/Nickel, HEA/Inconel625 and HEA/Stainless steel 304 pairs. This is attributed to the excellent strain hardening and intermediate softening of the HEA alloy used, which potentially postpones shear localization and impedes bonding, requiring a higher deposition velocity. Finally, it is shown that the ASI based criterion for bonding can predict the deposition mechanism of HEAs more inclusively.

THERMOPHYSICAL PROPERTIES OF ALLOY COMPOSITIONS (2Bi2O3∙B2O3)100-x(2Bi2O3∙3GeO2)x (x=0, 10, 50)

The termophisical properties, namely, the temperature dependence of thermal conductivity, thermal resistance and heat capacity of the allays compositions (2Bi2O3∙B2O3)100-x (2Bi2O3∙3GeO2)x in the (300–600) K temperature range have ligated been invest. An increase in thermal conductivity χ(T) above 500 K is probably associated with the softening of alloys and the presence of blurred phase transitions, which are accompanied by partial breaking of chemical bonds. It was revealed that the heat capacity in alloys of the compositions (2Bi2O3∙B2O3)100-x (2Bi2O3∙3GeO2)x increases with an increase in the GeO2 concentration. In the studied samples, that showed their own disorder during solidification, the thermal conductivity is strongly reduced due to the enhancement of the anharmonicity of phonon – phonon interactions. İn turn a small "disorder" introduced by defects due to the difference in masses is not noticeable against the background of the huge "disorder" inherent in oxide substances

Influence of Cold Rolling Process and Chemical Composition on the Mechanical Properties and Corrosion Behavior of Zr-Based Metallic Glasses

Cold rolling (CR) with thickness reduction of 10%, 30%, and 50% was applied to Zr50Cu44Al6 and Zr49.5Cu44Al6Nb0.5 metallic glassy ribbon samples. The XRD patterns showed the amorphousness of all samples after casting and CR processes. The SEM images indicated the formation of multiple shear bands (SBs) owing to plastic deformation during CR. However, the addition of 0.5 at% Nb to the alloy changed the SBs’ density and spacing characteristics. The characterization of free volume changes caused by CR was carried out by measuring the density of Archimedes. The micro-hardness of samples was studied by investigating SBs and free volume during plastic deformation. Alloy softening occurred due to the formation of free volume during CR. X-ray photoelectron spectroscopy (XPS) confirmed the presence of oxides ZrO2, CuOx, and AlOx on the surface of the ribbons. The anodic polarization curves of the cast and R50 samples in solutions of NaCl and Na2SO4 (0.5 M) were obtained using potentiodynamic polarization measurements. Compared to CR ribbons, melt-spun ribbons after casting showed better corrosion resistance with lower anodic current densities in Na2SO4 solution.

The evolution of dynamic recrystallization and recrystallization texture during isothermal compression of NiTi shape memory alloy

Isothermal compression tests, electron backscatter diffraction (EBSD) observations and quantitative analysis are performed to investigate the effect of process parameters on the microstructure variables (i.e. recrystallized fraction, size of austenite grain and grain boundary misorientation) during isothermal compression of NiTi shape memory alloy. And the evolution of dynamic recrystallization and recrystallization texture for this alloy is thoroughly discussed in the present study. The results show that the dynamic recovery is a dominated softening mechanism at less than 0.5 strain. As the strain increases, the discontinuous dynamic recrystallization (original grain boundary bulging and strain-induced grain boundary migration) and the subsequent continuous dynamic recrystallization (subgrain rotation) also participate in the softening of NiTi shape memory alloy, leading to a steady state of flow behavior. In the deformation end stage, the grain refinement of the austenite phase due to the effect of dynamic recrystallization improves the flow stress of NiTi shape memory alloy. The texture intensity increases significantly with increasing strain during isothermal compression of NiTi shape memory alloy, and the occurrence of dynamic recrystallization during deformation promotes the formation of a strong γ-fiber texture {111}<uvw>//CA.

Microstructure and properties of an equiatomic TaTiZr alloy

Abstract Microstructure and compression mechanical properties of a ternary TaTiZr alloy, after hot isostatic pressing (HIP) at 1400 °C, are reported. In the slow cooled condition, the alloy had a coarse-grained structure consisting of Zr-rich BCC matrix and Ta-rich submicron-sized BCC precipitates, which were coherent to the matrix phase. Ti was almost equally distributed between these two phases. omega-phase nano-precipitates were also detected inside the Zr-rich matrix. According to CALPHAD analysis, the observed microstructure is a metastable one, which was formed in the temperature range of 800–1130 °C during cooling after HIP. The HIP'd alloy had high yield stress of 1671 MPa at 25 °C and 1440 MPa at 400 °C, but low ductility, likely due to the presence of the omega phase. Considerable alloy softening, accommodated by a noticeable increase in ductility, occurred between 400 °C and 600 °C, with the yield stress decreasing to 346 MPa at 600 °C, 157 MPa at 800 °C and 103 MPa at 1000 °C. Such dramatic change in the mechanical behavior was correlated with dissolution of the omega phase and approaching phase equilibrium at T ≥ 600 °C.

Effects of electropulsing on the microstructure and microhardness of a selective laser melted Ti6Al4V alloy

Optimizing the microstructure and mechanical properties of the selective laser melted (SLM) Ti6Al4V alloy is an important issue both in the scientific and engineering interests. In this study, the rapid hardening and softening of the SLM-Ti6Al4V alloy is realized due to the obvious evolution of microstructures during the electropulsing treatment (EPT) with different discharge voltages. The microhardness of the EPT-7.5 kV sample decreased about 7% while the EPT-8 kV sample increased about 10% compared with the as-received sample. Scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis were used to characterize the evolutions of α and β grains and dislocation structures which are corresponding with the microhardness changes induced by EPT. Analysis shows that EPT could reduce the thermodynamic barrier for α → β solid-state phase transformation, further arise microstructure refinement in the SLM-Ti6Al4V alloy. This research shed some new light on rapidly and efficiently improving the microstructure and mechanical properties of the additive manufactured metal components.

Avoiding abnormal grain growth in thick 7XXX aluminium alloy friction stir welds during T6 post heat treatments

Abstract Friction stir welding (FSW) is becoming an essential welding technique in the aerospace industry. However, friction stir welds are usually weaker than the parent material. The dissipated heat causes a softening of the 7XXX series aluminium alloys. Post-welding heat treatments are used to restore the highest strength after welding. The reference treatment is composed of two-steps heating: a solution heat treatment typically at 470 °C for 30 min, quench in water followed by artificial aging 120 °C for 24 h. These treatments are responsible for very significant grain coarsening in the weld nugget zone known as secondary recrystallization, sometimes leading to abnormal grain growth (AGG). This is particularly true for thick plates. This secondary recrystallization phenomenon is detrimental for tensile performances. A detailed analysis of the conditions leading to secondary recrystallization during post-welding heat treatments is provided. A procedure to avoid secondary recrystallization in FSWed joints is then highlighted. Thermal cycles during welding are controlled by welding on a low thermal conductivity backing plate (i.e. stainless steel) which avoids very fine grains in the bottom of the weld nugget. This new procedure opens the path to excellent welds presenting at least the strength and ductility of the T6 base material.

Diffusion bonding of copper alloy to nickel-based superalloy: effect of heat treatment on the microstructure and mechanical properties of the joints

Successful joining of heat conducting materials, to high-temperature components is of significant importance for heat management in nuclear power plants and liquid propellant launch vehicles. Solid-state processes are considered for joining such dissimilar materials. Diffusion bonding of a copper alloy (C18150) to a Ni-based superalloy (GH4169) was investigated in this work. The bonding trials at 900°C for 60 min under a constant 10 MPa pressure led to formation of sound joints free from intermetallic, pores, voids and discontinuities. Conventional tensile testing led to the failure within the copper alloy and away from the joint. Due to the softening of the copper alloy during the bonding process, the maximum tensile strength of the as-bonded copper alloy was only 48% of its as-received strength. Post-bonding solution treatment at 960°C for 60 min followed by ageing at 450°C for 3.5 h, restored the tensile strength of the copper alloy up to 77%.

In-situ reactions and mechanical properties of 6061 aluminum alloy weld joint with SiCp by laser melting injection

In order to overcome weld joint softening of aluminum alloy, laser melting injection (LMI) methodology with 6H-SiC particles (SiCp) and 6061 aluminum alloy was introduced. In this manufacturing, the molten pool morphology and temperature were gained by high speed camera and thermal imager. After LMI, a graded distribution of SiCp with 4.6%–4.8% volume fraction was obtained along the depth direction of weld. The aluminum grain under LMI (12.80 μm) was much finer than that under single laser welding (SLW) (40.94 μm). It was novel that the in-situ reactant Al4C3 platelets would mainly locate discontinuously on the SiC/Al interface whereas acicular Al4SiC4 was found primarily in the aluminum matrix. In addition, the nano-spherical simple substance carbon was precipitated around the surface of SiCp. Eventually, due to these particular in-situ reactions during laser welding with SiCp injection, the Vickers hardness of weld with SiCp was larger than that under SLW; a gradual hardness distribution in the depth direction of PRW was also observed; the tensile strength of weld with SiCp (192 MPa ~ 243 MPa) became much higher without sacrificing ductility than SLW (165 MPa); the failure mechanism of weld with SiCp would change from SiCp cracking to the SiCp decohesion with the increasing welding speed.

Influence of aluminium alloys relative positioning on dissimilar friction stir lap welds properties

Very thin sheets of heat treatable (AA6082-T6) and non-heat treatable (AA5754-H22) aluminium alloys were used to produce dissimilar joints by friction stir lap welding. Joint strength and mechanical heterogeneity were assessed by performing lap tensile-shear and microhardness testing, respectively. Optical microscopy was used to analyse the weld morphology. Welding machine outputs, such as torque and axial force, were analysed in order to capture differences in the alloys strength during dissimilar welding. From the process outputs analysis, it was concluded that higher Z-Forces were necessary while positioning the AA5754 alloy at the top of lap configuration, indicating that this alloy offered higher resistance to the tool stirring than the AA6082 alloy. These results were associated with the flow softening of the AA6082 alloy during welding, which was explained using finite element analysis to illustrate the welding thermal cycles and the precipitation behaviour of this alloy. The lap welds defect formation was also related to the base materials plastic properties at high temperatures, which was inferred from process outputs data analysis. Based on the results it was shown that the lap welds defects might be supressed by a proper positioning the dissimilar base materials in the lap joint.

Laser-induced softening analysis of a hardened aluminum alloy by physical simulation

The study focuses on the analysis of the softening effects of the work-hardened aluminum alloy sheets EN AW 5754 H32 1.5 mm thick, through the physical simulation of thermal cycles induced in the material by laser heat treatments (LHTs). A numerical-experimental approach was implemented to define the laser thermal cycles and to subsequently reproduce them on the GleebleTM 3180 physical simulator. The obtained softening was measured by microhardness and metallographic analysis tests. For the definition of laser thermal cycles, preliminary tests with a 2.5 kW CO2 laser source have been realized, and a three-dimensional transient finite element thermal models were developed and calibrated with the experimental results. The investigated laser heat treatment parameters explored thermal cycles with different shape, interaction time, and peak temperature. Physical simulation tests were performed using laser thermal cycles that showed the maximum softening of the aluminum alloy. A three-dimensional transient finite element thermoelectric model was developed to design the shape of the Gleeble specimens, which satisfy the heating and cooling rate required by laser thermal cycles. Results obtained show that it is possible to physically simulate the investigated laser thermal cycles, reducing the cross section of the shaped part of the specimen. Softening effects depend on the thermal cycle shape. Greater softening is observed by increasing the interaction time and the peak temperature, but beyond a peak temperature threshold value, negligible effects are detected.

Prototypic Lightweight Alloy Design for Stellar-Radiation Environments.

The existing literature data shows that conventional aluminium alloys may not be suitable for use in stellar‐radiation environments as their hardening phases are prone to dissolve upon exposure to energetic irradiation, resulting in alloy softening which may reduce the lifetime of such materials impairing future human‐based space missions. The innovative methodology of crossover alloying is herein used to synthesize an aluminium alloy with a radiation resistant hardening phase. This alloy—a crossover of 5xxx and 7xxx series Al‐alloys—is subjected to extreme heavy ion irradiations in situ within a TEM up to a dose of 1 dpa and major experimental observations are made: the Mg32(Zn,Al)49 hardening precipitates (denoted as T‐phase) for this alloy system surprisingly survive the extreme irradiation conditions, no cavities are found to nucleate and displacement damage is observed to develop in the form of black‐spots. This discovery indicates that a high phase fraction of hardening precipitates is a crucial parameter for achieving superior radiation tolerance. Based on such observations, this current work sets new guidelines for the design of metallic alloys for space exploration.

Nickel-Based Alloy Dry Milling Process Induced Material Softening Effect.

Improving the cutting efficiency is the major factor for improving the processing of nickel-based alloys. The novelty of this research is the calibrated SiAlON ceramic tool dry milling nickel-based alloy process. Firstly, the nickel-based alloy dry milling process was analyzed through the finite element method, and the required milling force and temperature were deduced. Then, several dry milling experiments were conducted with the milling temperature, and the milling force was monitored. The change in cutting speeds was from 400 m/min to 700 m/min. Experimental results verified the reduction of the dry milling force hypothesized by the simulation. The experiment also indicated that with a cut depth of 0.3 mm, cut width of 6 mm, and feed per tooth of 0.03 mm/z, when milling speed exceeded 527.52 m/min, the milling force began to decrease, and the milling temperature exceeded the nickel-based alloy softening temperature. This indicated that easy cutting could be realized under high-speed dry milling conditions. The interpolation curve about average temperature and average milling forces showed similarity to the tensile strength reduction with the rise of temperature.

A modified Johnson-Cook model considering strain softening of A356 alloy

The compression experiments of as-received A356 alloy were conducted at temperatures ranges from 300 to 500 °C with varying strain rates. The influence of deformation parameters on the strain softening of flow stress were researched. The observation of microstructure indicated that high temperature and large strain were favorable for dynamic recrystallization behavior. In order to improve the predictability, a modified Johnson-Cook (JC) model considering strain softening mechanism was proposed. The JC model and modified JC model were calculated according to experimental data. In contrast with the JC model, the new proposed model had the better correlation and the smaller average absolute error (1.46%). Furthermore, thermal compression experiments at constant reduction speed and finite element simulations based on the new model were performed. High consistency of load-displacement curve indicated that new model had good prediction accuracy for the A356 aluminum alloy.