Dominant Creep Mechanism Research Articles

New Ferritic Stainless Steel for Service Temperatures up to 1050 °C Utilizing Intermetallic Phase Transformation

A large number of thermodynamic simulations has been used to design a new Nb-Ti dual stabilized ferritic stainless steel with excellent creep resistance at 1050 °C through an optimal volume fraction of Laves (η) phase stabilized by the alloying elements Nb, Si and Mo. By raising the dissolution temperature of the phase, which also corresponds to the onset of rapid grain growth, the steel will better maintain the mechanical properties at higher service temperature. Laves phase precipitates can also improve creep resistance through precipitation strengthening and grain boundary pinning depending on the dominant creep mechanism. Sag tests at high temperatures for the designed steel showed significantly better results compared to other ferritic stainless steels typically used in high temperature applications at present.

The Role of Second Phases on the Creep Behavior of As-Cast and Hot-Extruded Mg-Ca-Zr Alloys

The effect of second phases on the creep behavior of Mg-Ca-Zr alloys was investigated. Casting and hot-extrusion processes were performed to prepare Mg-xCa-0.5Zr (x = 0 wt.%, 0.3 wt.% and 0.6 wt.%) alloys with different morphologies and distributions of second phases. The as-cast microstructures of Mg-Ca-Zr alloys consisted of an Mg matrix and coarse Mg2Ca intermetallic compounds distributed at the grain boundaries. The hot-extruded microstructures exhibited finer Mg2Ca spherical particles and precipitates distributed homogeneously inside the matrix and along the grain boundaries. The results of creep tests suggested that the viscous glide of dislocation and dislocation climb were the dominant creep mechanisms of Mg-Ca-Zr alloys crept at 200°C. The creep property was strongly related to the morphology and distribution of the Mg2Ca phases. The finer and more homogeneously distributed particles showed a better strengthening effect than the coarser intermetallic compounds distributed along the grain boundaries.

Effect of Cu on the Creep Behavior of Cast Al-15Si-0.5Mg Alloy

In the present article, the effect of Cu on the microstructure and creep properties of Al-15Si-0.5Mg cast alloy was studied by using optical microscopy, scanning electron microscopy, energy dispersion spectrometry and the impression creep test. The microstructure is changed considerably in the presence of copper. The number of the Cu-rich phases increased noticeably with rising Cu content. Formation of a dendritic structure of α(Al) and modification of the eutectic Si were the other effects of increasing the Cu amount. The results showed that creep properties of the alloy increase considerably with the Cu addition. Calculating the stress exponent (n) and creep activation energy (Q) revealed that pipe diffusion climb-controlled creep is the dominant creep mechanism of the main alloy. Although Cu had no effect on the dominant creep mechanism at the lower stress, it was changed to lattice diffusion climb-controlled dislocation creep with increasing stress.

Effects of Zn and Sb additions on micro-structure, creep behavior and thermal properties of binary eutectic Sn-0.7%Cu lead-free solder

The effects of Zn and Sb additions on microstructure, creep behavior and thermal properties of binary eutectic Sn-0.7%Cu lead-free solder were investigated. Results show that Zn-addition reduced the melting temperature of Sn-0.7Cu and enhanced the creep resistance due to the formation of Cu5Zn8 intermetallic compound (IMC) and reduction of Cu6Sn5 phase in β-Sn matrix. Conversely, the Sb-addition showed negative effect on both the creep resistance and melting temperature compared to the plain Sn-0.7Cu and Sn-0.7Cu-2.0Zn alloys due to the formation of SnSb IMC particles and coarse Cu6Sn5 phase through the β-Sn rich phase. The average stress exponents of 4.9 - 6.0 and activation energies of 50-60 KJ/mol were obtained for three solders; suggesting that the dislocation climb controlled by lattice diffusion is the dominant creep mechanism. X-ray diffraction (XRD) analysis revealed that the lattice parameters (a), (c), the axial rate (c/a) and the peak height intensities (hkl) of some crystallographic planes are changed with alloying additions and the crystallite size is decreased.

Superior creep behavior of n-SiCp/Mg–9%Al composites fabricated by ultrasonic-assisted semi-solid hot pressing of powder

The n-SiCp/Mg–9%Al composites reinforced with 7.5 wt% SiC nanoparticles were fabricated by semi-solid powder hot pressing technique-assisted with ultrasonic, which show finer grains and a more uniform microstructure compared to Mg–9%Al alloys. The steady-creep rates of the nanocomposite were approximately 12 times lower than those of the Mg–9%Al matrix alloy at 200 °C under an applied stress of 70 MPa due to the strengthening effect of the nano-sized SiCp. The creep resistance of the nanocomposite was investigated at temperatures between 175 and 225 °C under an applied stress in the range of 70–90 MPa. The creep stress exponents were 5.86–7.95, implying that the dominant creep mechanisms in the nanocomposite are dislocation climb and particle strengthening-controlled creep. The activation energies obtained for creep were in the range of 102–146 kJ/mol, which are higher than those for boundary diffusion and are believed to be associated with self-diffusion and the diffusion of Al atoms in the Mg matrix.

EVALUATION OF CREEP BEHAVIOR OF TI-6AL-4V ALLOY WITH THERMAL BARRIER COATING DEPOSITED BY AIR PLASMA SPRAY

Titanium alloys have attractive properties for application in aeronautical components submitted to high temperature. Yttria-stabilized zirconia (YSZ) of thermal barrier coating systems (TBC) have become an alternative to increase the operational lifetime of turbine blades. The aim of this work was to evaluate the creep behavior of Ti-6Al-4V with TBC deposited by air plasma spray. Constant load creep tests were conducted in air on equiaxed Ti-6Al-4V (uncoated) and Ti-6Al-4V with TBC and the experimental parameters related to secondary creep were determined. All the coated samples showed a reduction of secondary creep rate values and, thereafter, higher creep resistance, which could be associated with oxidation protection and thermal insulation provided by the coating. Creep rate analysis suggests that the dominant creep mechanisms in all cases is the dislocation climb.

Microstructure and impression creep characteristics of squeeze-cast AZ91 magnesium alloy containing Ca and/or Bi

The effects of Ca and/or Bi additions on the microstructural modification and creep characteristics of the squeeze-cast AZ91 alloy have been addressed. Both the single as well as the mixed contents of Ca and Bi in the AZ91 alloy refine the grain size of primary α-Mg and reduces the β-Mg17Al12 phase considerably. The effect is more noticeable in combined additions than in the individual additions. The reticular-shaped Al2Ca and needle-shaped Mg3Bi2 phases additionally form with the α-Mg and β-Mg17Al12 phases because of the sole Ca and Bi addition in the AZ91 alloy. The Al2Ca and Bi3Ca5 phases are formed when Ca and Bi are added together, suppressing the Mg3Bi2 phase formation. The modified AZ91-based alloys containing Ca and/or Bi exhibit improved creep behaviour than the AZ91 alloy at all the stress and temperature levels tested. The individual additions of the elements in the AZ91 alloy shows higher creep rate than the combined additions. The individual Ca addition is better than the Bi addition for resisting the creep deformation in the AZ91 alloy as the Al2Ca phase in the AZX911 alloy has superior thermal stability compared to that of the Mg3Bi2 phase in the AZB910 alloy. The AZXB9120 (AZ91 with 2.0Ca and 0.5Bi (wt%)) exhibits the best creep performance owing to the lower volume fraction of β-Mg17Al12 phase and existence of larger quantity of thermally stable Al2Ca and Bi3Ca5 phases. The values of stress exponents and activation energies concludes that the dominant creep mechanism for all the alloys is dislocation climb aided by pipe diffusion. Microstructural investigation following creep indicates that the β-Mg17Al12 phase was broken into small pieces. In contrast, the thermally stable Al2Ca, Mg3Bi2 and Bi3Ca5 phases preserved their continuity, which resulted in piled-up dislocations and tangling of dislocations in the interior of the α-Mg grains that leads to the improved resistance to creep deformation of the modified AZ91 alloys.

Stress dependence of the creep behaviors and mechanisms of a third-generation Ni-based single crystal superalloy

Elevated temperature creep behaviors at 1100 °C over a wide stress regime of 120–174 MPa of a third-generation Ni-based single crystal superalloy were studied. With a reduced stress from 174 to 120 MPa, the creep life increased by a factor of 10.5, from 87 h to 907 h, presenting a strong stress dependence. A splitting phenomenon of the close- (about 100 nm) and sparse- (above 120 nm) spaced dislocation networks became more obvious with increasing stress. Simultaneously, a0<010> superdislocations with low mobilities were frequently observed under a lower stress to pass through γ′ precipitates by a combined slip and climb of two a0<110> superpartials or pure climb. However, a0<110> superdislocations with higher mobility were widely found under a higher stress, which directly sheared into γ′ precipitates. Based on the calculated critical resolved shear stresses for various creep mechanisms, the favorable creep mechanism was systematically analyzed. Furthermore, combined with the microstructural evolutions during different creep stages, the dominant creep mechanism changed from the dislocation climbing to Orowan looping and precipitates shearing under a stress regime of 137–174 MPa, while the dislocation climbing mechanism was operative throughout the whole creep stage under a stress of 120 MPa, resulting a superior creep performance.

Impression creep behaviour of TiB2 particles reinforced steel matrix composites

ABSTRACTImpression creep behaviour of the powder metallurgy processed steel matrix composites was investigated under constant stress at different temperatures in the range of 873–973 K. By using the power-law relationship, the estimated activation energy for unreinforced steel was found to be 149 kJ mol−1 and steel reinforced with 2 and 4 vol.-% TiB2 was found to be 298 and 338 kJ mol−1, respectively indicating better creep resistance of the reinforced steel matrix composites. Dislocation creep is the dominant creep mechanism based on the calculated values of stress exponent and activation energy. Hence, this method can be used to assess the potential of steel matrix composites for use as structural materials for high-temperature application.

Enhancing creep properties of a hot-rolled Mg-4Y binary alloy via a new thought of inhibiting cross-slip

A new thought of enhancing the creep properties of a hot-rolled Mg-4 wt%Y binary alloy was reported in this work. Using various characterization methods, it was indicated that cross-slip composed of basal ⟨a⟩ and prismatic ⟨a⟩ dislocations was the origin of creep rupture for the homogeneous annealed alloy. But amazingly, cross-slip was successfully inhibited by performing a pre-compression and introducing {10-12} twins into this alloy. As a result, pyramidal ⟨c + a⟩ slip became the dominant creep mechanism and enhanced creep properties were obtained in the pre-compressed alloy. It is hoped that this new thought of inhibiting cross-slip can enrich the basic theories and extend the application fields of Mg alloys.

Creep behaviors of a highly concentrated Mg-18 wt%Gd binary alloy with and without artificial aging

In this work, a highly concentrated Mg-18 wt%Gd binary alloy was prepared and its tensile creep behaviors were studied. Both as-solution (AS) and as-aged (AA) samples were subjected to creep tests at 523 K under various stresses. The results showed that precipitate variants and sizes became similar in AS and AA samples during their secondary creep stages. Strain aging and artificial aging play equivalent roles in the strengthening effects on AS and AA samples. Dislocation climb composed by edge <a> dislocations was the dominant creep mechanism of AS sample, and dislocations bypassing precipitates strengthened AS sample limitedly. By contrast, cross-slip composed by basal <a> and prismatic <a> dislocations was the dominant creep mechanism of AA sample, and dislocations cutting precipitates strengthened AA sample obviously. Furthermore, compared to AS sample, a high density of precipitate-free zones (PFZs) were developed in AA sample during creep testing. These high-density PFZs then relieved local stress concentration and catalyzed a homogeneous stress distribution, which postponed the occurrence of creep rupture rather than accelerated it. Accordingly, AA sample had an increased creep resistance than that of AS sample at 523 K. This unexpected discovery that PFZs could contribute to the increase in creep resistance is believed to extend the application of the Mg-Gd alloys with vast potentials.

Influence of Low Ti-Element Additions on The Microstructure, Melting Properties and Creep Behavior of Sn-Ag-Cu Lead Free Solders

Sn-3.5wt.%Ag-0.5wt.%Cu is one of the alloys considered for replacing SnPb solder alloys in electronic ambiance. In the present study, the effect of minor additions of Ti (0.25, 0.5, 1.0 wt.%) on the microstructure, creep and thermal properties of Sn-3.5wt.%Ag-0.5wt.%Cu lead free solder alloy are investigated by means of scanning electron microscopy (SEM), x-ray diffraction (XRD), creep tests and differential scanning calorimetry (DSC) thermal tests. Results showed that the addition of different wt.% of Ti refined the grain size of the solder alloys. The microstructure parameters obtained from x-ray analysis represented by lattice parameters (a,c), axial ratio (c/a), residual strains (?a/a? &amp; ?c/c?), peak height intensities and crystallite size of some crystallographic planes were found to be sensitive to Ti - additions, applied stresses and working temperatures. The morphological studies using SEM showed refinements of Ag3Sn, Cu6Sn5 and Ti2Sn3 (IMCs) in ?-Sn matrix. Thermal analysis showed that Ti - additions slightly affect melting temperature and pastry range. In terms of creep behavior the addition of 1wt.%Ti gave the highest creep resistance with respect to the others, due to fine dispersion of IMCs. The stress exponents (n) and the activation energies (Q) indicated that the dominant creep mechanism is dislocation climb over all temperature ranges

Effect of Cr and Zr on the Tensile Creep Behavior of Al-5Fe Based Semisolid Alloys

Determining the dominant creep mechanism is the key feature for design and development of Al-Fe alloys. The tensile creep behavior of Al5Fe4Cu2ZnMnMg alloy (AF1) and Al5Fe4Cu2CrZnZrMnMg alloy (AF15) by prepared semisolid forming has been investigated. The results reveal that the steady-state creep rates of the AF1 and AF15 alloys are increased with increasing temperature and applied stress. The creep property of the AF15 alloy is slightly higher than that of the AF1 alloy. There is threshold stress during creep. The creep true stress exponent is close to 5. The creep mechanism of the alloys is controlled by the lattice diffusion of aluminum. The creep resistance of the AF15 alloy is significantly improved by adding a small amount of Cr and Zr elements The reasons for the improved creep life of the AF15 alloy are discussed.

Effects of aging temperature on microstructure, tensile and creep properties of ring rolled AZ80-Ag alloy

In this study, aging behavior, tensile property and creep performance of the ring rolled AZ80-Ag magnesium alloy at different aging temperatures were systematically investigated. Results show that the microstructure is mainly occupied by discontinuous precipitation (DP) and continuous precipitation (CP) of β-Mg17Al12 after T6 treatment. The peak aging at low temperature (175 °C) results in a larger volume fraction of β precipitation and a lower CP/DP ratio than peak aging at high temperature (250 °C). With raising aging temperature, the tensile strength at ambient temperature reveals a descending tendency, whereas the creep resistance at 120–175 °C under 70–90 MPa exhibits an enhancement. Based on the analysis of creep stress exponent and activation energy values, the dominant creep mechanism of both aged specimens is dislocation creep controlled by competing lattice diffusion and pipe diffusion. Basal <a> and non-basal <c + a> dislocations can be activated in the creep deformation.

Investigation of short-term creep deformation mechanisms in MarBN steel at elevated temperatures

This paper reports the short-term creep behaviour at elevated temperatures of a MarBN steel variant. Creep tests were performed at three different temperatures (625°C, 650°C and 675°C) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 h. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ mol−1, suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.), and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures.

Effect of minor Gd addition on the microstructure and creep behavior of a cast Al–15Mg2Si in situ composite

The influence of 0.5 wt% Gd addition on the microstructure and creep behavior of a cast Al–15Mg2Si composite was studied by means of compression tests under different constant stresses in the range of 50–250 MPa, corresponding to the modulus-compensated stress levels of 0.002 ≤ σ/G ≤ 0.011, in the temperature range of 473–563 K. Analysis of the data showed that for all loads and temperatures, the Al–15Mg2Si–0.5Gd composite had lower creep rates, and therefore, higher creep resistances than the base composite. This is ascribed to the morphological modification of the primary Mg2Si particles, refinement of the eutectic structure, significant reduction in the size and volume fraction of the primary Mg2Si particles, and the formation of the fine thermally stable MgGd intermetallic phase at the grain boundary areas. Based on the obtained stress exponents of 4.6–5.9 and stress-dependent activation energies, which are close to that of the lattice self-diffusion in pure aluminum, it can be proposed that the dominant creep mechanism in the present composites is stress-assisted dislocation climb controlled by lattice self-diffusion.

Impression creep behaviour of in-situ Al3Ti reinforced Al alloy composite fabricated by salt-melt reaction technique

In the present study, Al3Ti reinforced Al6061 aluminium alloy matrix composites were fabricated by adding 5 wt.%, 10 wt.% and 15 wt.% K2TiF6 inorganic salt into the melt at 750 °C, respectively. For phase identification, scanning electron microscopy and X-ray diffraction techniques were used. Impression creep tests were performed on the composites at two different temperatures of 543 K and 573 K and three different stresses of 113, 142 and 170 MPa. Creep resistance of the composites increased as the amount of reinforced particles increased. The average value of stress exponent of the composites was in the range from 3 to 6 which suggested that the dislocation creep was the dominant creep mechanism. The creep activation energy of the composites was close to the lattice diffusion (142 kJ/mol.) which suggested that the lattice diffusion controlled dislocation creep is probably the dominant creep mechanism.

Effect of Multi Directional Forging on Impression Creep Behavior of Zn-24Al-2Cu Alloy

Multi directional forging (MDF) of Zn-24Al-2Cu alloy were carried at 100°C and 200°C with total equivalent strain of 0.6 and 1.2 respectively. The average grain size was reduced from 30 to 1 µm after MDF processing. Impression creep behavior of MDF processed samples was studied at 2 kg and 2.5 kg load with temperatures 30, 50, 100 and 150°C. Microstructural characterizations were carried out using optical microscope, transmission electron microscope and X-Ray diffractometer. Steady state creep rate values were recorded in the range of 1.2x10-5 to 5.2x10-7. The four phase transformation occurred in the structure after MDF process and during the impression creep test. Initial stress exponent (n) values are in the range of 2.5 to greater than 5. High values of ‘n’ showed due to the existence of threshold stress. The true activation energy values calculated are in the range of 22-129 kJ/mol. Changes in creep properties of MDF processed samples seem to be more sensitive to change in temperature compared to microstructural differences. Lattice diffusion controlled by dislocation climb is the dominant creep mechanism.

Creep Deformation of Carbon-Based Cathode Materials for Low-Temperature Aluminum Electrolysis

The uniaxial-compression creep behavior of semi-graphitic carbon products was investigated using modified Rapoport equipment in a K3AlF6–Na3AlF6–AlF3 and a Na3AlF6–AlF3 system. The stress exponent is low for the K3AlF6–Na3AlF6–AlF3 system in the steady-state creep stage. With an increase in graphitization degree and grain size, the interlayer space and porosity of the tested samples decrease after aluminum electrolysis. A low temperature can suppress carbon-cathode damage. Based on these stress exponents and a microstructural investigation using transmission electron microscopy, it is proposed that dislocation glide is the dominant creep mechanism for the carbon cathode during aluminum electrolysis in the steady-state creep stage.

Creep resistance of as-cast Mg-5Al-5Ca-2Sn alloy

In the present study, creep properties of as-cast Mg-5Al-5Ca-2Sn (AXT552) alloy were investigated by means of a GWT304 creep testing machine at temperatures of 175 °C and 200 °C in the stress range of 35-90 MPa. Results show that creep rates increase with applied stress at an identical temperature. Creep strain at 100 hours is 0.0518% and 0.083% at creep conditions of 175°C/75MPa and 200°C/60MPa, respectively, which is comparable to MRI230D and much lower than most of AX series alloys. By the observation and analysis for samples before and after creep tests using a Shimadzu XRD-7000 type X-ray diffractometer (XRD) and a Hitachi S-3400N type scanning electron microscope (SEM), it was found that Al2Ca (C15) phase precipitated out of C36 phase or matrix. The cavity formation and connection at the interface of soft matrix and hard intermetallics caused the propagation of cracking along the eutectic phase during creep process and dislocation accommodated grain/ phase boundary sliding is expected to be the dominant creep mechanism.