Effect of rolling conditions on the structure and shape memory properties of Fe–Mn–Si alloys

Lattice defects play an important role in controlling the γ → ε martensitic transformation in shape memory ferrous alloys. This work focuses on the relation between various rolling and annealing processes, the microstructure resulting from the processes, and strain recovery of two Fe–Mn–Si alloys with different stacking fault energies (SFEs). Rolling experiments, conducted over a temperature range from 20 °C to 1000 °C, produce quite different microstructures, which vary from a high dislocation density to a structure containing only few isolated dislocations. In addition, annealing temperature has a very important influence not only on the dislocation arrays but also on the stacking faults remaining in the austenite, whose density depends on the SFE value for the alloy. Within the framework of the processing parameters selected for this work, i.e. roll speed, rolling reductions, processing temperatures and schedules, rolling at intermediate temperatures and annealing at a temperature of 650 °C seem to be the most appropriate methods to obtain a microstructure favorable for a nearly full degree of shape recovery.     

Microstructure evolution and its influence on deformation mechanisms during high temperature creep of a nickel base superalloy

The interaction of dislocation with strengthening particles, including primary and secondary γ′, during different stages of creep of Rene-80 was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). During creep of the alloy at 871 °C under stress of 290 MPa, the dislocation network was formed during the early stages of creep, and the dislocation glide and climb process were the predominant mechanism of deformation. The density of dislocation network became more populated during the later stages of the creep, and at the latest stage of the creep, primary particles shearing were observed alongside with the dislocation glide and climb. Shearing of γ′ particles in creep at 871 °C under stress of 475 MPa was commenced at the earlier creep times and governed the creep deformation mechanism. In two levels of examined stresses, as far as the creep deformation was controlled by glide and climb, creep curves were found to be at the second stage of creep and commence of the tertiary creep, with increasing creep rate, were found to be in coincidence with the particles shearing. Microstructure evolution, with regard to γ′ strengthening particles, led to particles growth and promoted activation of other deformation mechanisms such as dislocation bypassing by orowan loop formation. Dislocation-secondary γ′ particles interaction was detected to be the glide and climb at the early stages of creep, while at the later stages, the dislocation bypassed the secondary precipitation by means of orowan loops formation, as the secondary particle were grown and the mean inter-particle distance increased.     

The effects of ruthenium additions on tensile deformation mechanisms of single crystal superalloys at different temperatures

The tensile behavior of two experimental nickel-base single crystal superalloys has been studied from room temperature to 1100 °C. Emphasis is placed on elucidating the effects of ruthenium (Ru) additions on the deformation mechanisms using transmission electron microscopy (TEM). Furthermore, the partitioning behavior of the alloy elements between the γ and γ′ phases for both experimental alloys has been studied using three-dimensional atom probe (3DAP). Detailed analysis demonstrates that at low and medium temperature ranges, the stacking faults present in the γ matrix of the 3Ru alloy but no trace of stacking fault in the γ matrix of the 0Ru alloy have been observed; during high temperature range, as a result of Ru additions, the γ/γ′ interfacial dislocation space of the 3Ru alloy is smaller than that of the 0Ru alloy due to further decreasing the lattice misfit. Apart from that, Ru additions result in more Re partitioning to the γ′ phase, and thus the solution strengthening for the γ phase is decreasing. Thus, during tests below and at the temperature corresponding to the peak strength, the yield strength of the 3Ru alloy is lower than that of the 0Ru alloy. At last, in the light of the TEM observations, the changing trends of the stacking fault energy in the γ matrix and the transformation points (the temperature related to the stacking faults formation) for the two experimental alloys have been drawn. The temperature range of the stacking faults formation in the γ matrix is expanded after Ru additions. The energy conditions of the stacking faults formation of the 0Ru and 3Ru alloys have been analyzed in detail. The changing of lattice misfit with temperature can be considered as one of the principal reasons for the stacking faults formation.     

Effect of stacking fault energy on mechanical behavior of cold-forging Cu and Cu alloys

Cu, Cu–2.87 wt% Mn, Cu–4.40 wt% Mn and Cu-10.19 wt% Mn were prepared by cold-forging. The deformation behavior of Cu–Mn alloys is consistent with the Cu-Al alloys and Cu–Zn alloys but without lowering the stacking fault energy to simultaneously increase the strength and ductility. A series of analysis demonstrate that Cu–Mn alloys have a much smaller twin density than low stacking fault energy (SFE) metals, and dislocation strengthening is the major reason for the higher strength. The role of short range order (SRO) in promoting the mechanical properties has also been briefly discussed.     

Effects of Co content on tensile properties and deformation behaviors of Ni-based disk superalloys at different temperatures

Tensile properties of three Ni-based disk superalloys with 5 wt.%, 15 wt.% and 23 wt.% Co contents were investigated from room temperature (RT) to 725 °C with a constant strain rate of 3 × 10^− 4 s^− 1. It is found that addition of Co enhances the yield strength and the strain hardening capacity of the alloy in the studied temperature regime. It is due to the following two reasons: i.e. the interactions between a large volume fraction of fine secondary and tertiary γ′ precipitates with the dislocations in slip bands at lower temperatures and the formation of deformation microtwins at higher temperatures. However, the formation of deformation microtwins in the high Co-containing content alloy sharply decreases the ductility at higher temperatures.     

Stacking fault energy and tensile deformation behavior of high-carbon twinning-induced plasticity steels: Effect of Cu addition

Three experimental fully austenitic high-carbon twinning-induced plasticity (TWIP) steel grades were produced and the stacking fault energy (SFE) was investigated based on the thermodynamic modeling approach. The SFE of Fe–20Mn–xCu–1.3C (x = 0, 1.5 and 3.0) steels varied from 24.36 to 28.74 mJ m⁻² at room temperature. In order to study the correlation between the SFE and the mechanical behavior of TWIP steels, tensile tests were performed at room temperature and the deformed microstructures were examined at different strain levels by transmission electron microscopy. The Cu additions resulted in a remarkable increase in total elongation without a slight loss of tensile strength. In addition, the critical strain for serration start on the tensile stress–strain curves (i.e. required strain to generate mechanical twinning) was found to increase with increasing Cu content. Transmission electron microscope (TEM) observations also indicated that the occurrence of mechanical twinning was suppressed by increasing the Cu addition. The strain hardening mechanism and the superior ductility in deformation are dominated by the interaction of twins and dislocations. The mechanical behavior of TWIP steels is related to the Cu addition, the SFE, the interaction of twins and dislocations.     

A research on the creep properties of titanium matrix composites rolled with different deformation degrees

(TiB + La₂O₃)/Ti composites were in situ synthesized and deformed with different deformation degrees. The influence of TiB whisker orientation and grain refinement on the creep properties of titanium matrix composites (TMCs) are discussed. The creep test reveals that the steady state creep rate of TMCs first decreases and then increases with the increase of deformation degree, which can be attributed to competing effects: TiB whisker rotating to the rolling direction, α plate grain boundary hindering and pinning dislocations can all decrease the creep rate, however, dislocation movement on the α plate grain boundary and dislocation emitting from the α plate grain boundary can both increase the creep rate.     

The effects of Co and Ti additions on microstructures and compressive strength of Udimet710

The effects of Co and Ti additions on the microstructures and compressive strength of the Ni-based superalloy Udiemt710 (U710) were investigated. The preliminary results showed that the Ni₃Ti-type (η) phase was observed in the alloys with low Co and Ti content; while a (Ni_xCo_1?x)₃Ti phase with a hexagonal structure was detected in the alloys with high Co and Ti content. The γ′ → η → (Ni_xCo_1?x)₃Ti phase transformation was discussed in terms of Co and Ti contents. The γ′ morphology changed from spherical to cuboidal with increasing Co and Ti content. Compressive tests showed that the alloys with Co and Ti addition possessed higher yield strength than the base alloy, U710. The strength increase could be ascribed to solid–solution strengthening of the γ and γ′ phases as a result of the Co and Ti additions, and the higher volume fraction of γ′ in the new alloys.     

Stacking faults in B2-structured magnesium alloys from first principles calculations

The stacking faults of 17 B2-structured magnesium alloys have been studied systematically by means of first-principles calculations. After structure optimization and stability analysis, the generalized stacking fault energy surfaces for two possible slip planes {0 0 1} and {1 1 0} have been calculated using a super-cell tilling technique, and the main feature of generalized stacking fault energy surfaces was analyzed. Then the most likely slip directions were determined, and the stable and unstable stacking energies were obtained. The dissociation of perfect dislocation was further discussed. The electronic structures were also investigated to reveal the underlying mechanism for stability and stacking faults.     

Effect of Mo on microstructural characteristics and coarsening kinetics of γ' precipitates in Co–Al–W–Ta–Ti alloys

The solvus temperature,volume fraction,coarsening behavior of Y' precipitates and the partitioning behavior of alloying elements as well as lattice misfit of Y/Y' phases influence the creep behavior of Ni-and Co-base superalloys.However,few investigations about the microstructural characteristics and the coarsening behavior of Y' precipitates were reported in multicomponent novel Co-base superalloys during thermal exposure.Two alloys containing different contents of molybdenum and tungsten have been investigated to explore the effect of molybdenum on Y' solvus temperature,Y + Y' microstructure and Y' coarsening in Co–Al–W–Ta–Ti-base alloys.The results showed that the Y' solvus temperature decreases with the addition of Mo.Mo addition reduces the Y' volume fractions after aging above 1000?C,but results in negligible influence on the Y' volume fractions aging at 900?C.Meanwhile,Y' coarsening is controlled by diffusion in experimental alloys after aging at 900?C and 1000?C,and the kinetics of Y'growth in experimental alloys are consistent with the predictions of LSW theory.     

Age hardening behaviors,mechanical and corrosion properties of deformed Mg–Mn–Sn sheets by pre-rolled treatment

The age behaviors, mechanical and corrosion properties of Mg–1.5Mn–xSn (x = 1 and 5 wt.%) alloys under three aging conditions have been investigated. The results reveal that both age behaviors and mechanical properties are improved with the increment of Sn. Meanwhile, the mechanical properties of EA (extrusion + artificial aging) state Mg–1.5Mn–5Sn alloy are higher than those of SA (solid solution + artificial aging) state sample, which are mostly attributed to fine grain and high density secondary precipitate. In addition, an accompanying improvement in age hardening response and strength is achieved in ERA (extruded + rolled + aging) state alloy compared with EA state one. The main reasons are related to the formations of a large number of dislocations and deformation twins, which provide effective nucleation sites to form fine β-Mg₂Sn strengthening precipitates during the following aging process. In addition, compared with EA state sample, a lower corrosion rate of ERA alloy is confirmed by Tafel curves and electrochemical impedance spectroscopy results, which is mainly related to the formation of a thick anodic passivation film on the surface.     

Mechanical properties and phase composition of potential biodegradable Mg–Zn–Mn–base alloys with addition of rare earth elements

Mechanical properties and creep resistance of the MgY4Zn1Mn1 alloy in the as cast as well as in the T5 condition were compared to those of the MgCe4Zn1Mn1 alloy in the same conditions. Yield tensile stress and ultimate tensile strength of the MgY4Zn1Mn1 alloy are slightly better in the temperature range 20 °C–400 °C than these of the MgCe4Zn1Mn1 alloy. Better thermal stability of ultimate tensile strength was observed in the T5 treated MgCe4Zn1Mn1 alloy than in this material in the as cast condition. An outstanding creep resistance at 225 °C–350 °C found in the MgY4Zn1Mn1 alloy is due to the existence of the 18R long period stacking structure persisting in this alloy even a long heat treatment of 500 °C/32 h. No similar stacking effects happen when Ce substitutes Y in approximately the same concentration. The creep resistance deteriorates considerably in the MgCe4Zn1Mn1 alloy. Rectangular particles of the equilibrium Mg₁₂Ce phase dominate in the microstructure of as cast as well as of high temperature heat-treated MgCe4Zn1Mn1 alloy. A population of small oval particles containing Mg and Zn develops additionally during annealing of this alloy. These particles pin effectively dislocations and can be responsible for the better thermal stability of the T5 treated material.     

The microstructure and impression creep behavior of cast AZ80 magnesium alloy with yttrium additions

The effects of 0.5, 1.0 and 2.0 wt.% Y additions on the microstructure and creep behavior of the as-cast AZ80 alloy were investigated by impression tests. The tests were performed at temperatures in the range 423–523 K, under punching stress in the range 150–650 MPa. At low temperatures up to 473 K, the AZ80 + 0.5Y alloy had the highest creep resistance among all materials tested, whereas with increasing temperature from 473 K to 523 K, the AZ80 + 1.0Y alloy had a better performance. This can be attributed to the fact that at low temperatures the presences of β-Mg₁₇Al₁₂ and Al₂Y phases together with solid solution hardening effects of Al in the Mg matrix strengthen the AZ80 + 0.5Y alloy. At higher temperatures, AZ80 + 1.0Y with a higher volume fraction of the more thermally stable Al₂Y and lower amounts of the less stable β-Mg₁₇Al₁₂ exhibits better creep behavior. The stress exponents and activation energies were almost the same for all alloy systems studied, 6.0–8.8 and 90–119 kJ/mol, respectively. The observed decreasing trend of creep-activation energy with stress suggests that two parallel mechanisms of lattice and pipe-diffusion-controlled dislocation climb are competing. Climb of dislocations with an additional particle strengthening effect controlled by dislocation pipe diffusion is dominant at high stresses, whereas climb of dislocations is the controlling mechanism at low stresses.     

Microstructure and mechanical properties of Al-7075 alloy processed by equal channel angular pressing combined with aging treatment

In order to examine the combined effect of plastic deformation and aging process, the Al 7075 alloy was subjected to equal channel angular pressing (ECAP) deformation by route B_C in various ECAP and aging conditions: pre-ECAP aging, post-ECAP aging and dynamic aging during ECAP at 393 K and 423 K. Followed by ECAP and aging treatment, Vickers microhardness and tensile test were performed and microstructural observations were undertaken using transmission electron microscopy (TEM) and X-ray diffractometer (XRD). TEM investigation showed that ultrafine-grained (UFG) materials with grain size less than 500 nm could be obtained after three or four passes of ECAP. Precipitates characterization revealed that maximum mechanical properties are achieved when the microstructure mainly consists of fine dispersion of small η^′ precipitates and minor quantities of GP zones. Dynamic aged specimens at 393 K and 423 K represented maximum and minimum mechanical properties, respectively, due to formation of fine η^′ precipitates plus GP zones and η^′ plus η precipitates, respectively. Dynamic aging during ECAP at 393 K appeared preferable to other procedures for attaining maximum mechanical properties as well as saving time and energy.     

Effects of nitrogen on microstructural evolution of biomedical Co–Cr–W alloys during hot deformation and subsequent cooling

The present study investigated how nitrogen affected the high-temperature deformation and microstructural evolution of biomedical Ni-free Co–Cr–W alloys during hot deformation. Hot compression tests of undoped and N-doped Co–28Cr–9W–1Si–0.05C (mass%) alloys were performed at deformation temperatures ranging from 1323 to 1473 K at strain rates of 10⁻³ to 10 s⁻¹. The microstructures, which were subjected to a true strain of 0.92 (60% in compression), were characterized using electron backscatter diffraction (EBSD) analysis and transmission electron microscopy (TEM). Dynamic recrystallization (DRX) was found to occur in both alloys during hot deformation. The grain size (d) decreased considerably with an increase in the Zener–Hollomon (Z) parameter. Although adding nitrogen to the alloys barely affected dynamic-recrystallization-induced grain refinement, it increased the magnitude of the flow stress and delayed static recrystallization during post-deformation cooling. Consequently, the N-doped alloy contained bulk nanostructures whose average grain size was 0.9 μm.     

Mechanical and microstructural characterizations of ultrafine grained Zircaloy-2 produced by room temperature rolling

The effect of deformation strain at room temperature on the microstructural and mechanical properties of Zircaloy-2 was investigated in the present work. The sample was initially heat treated at 800 °C in argon environment and quenched in mercury prior to rolling. The deformed alloys were characterized by using EBSD and TEM. It reveals the misorientation of incidental grain boundaries (IDBs) due to large plastic strain induced in the sample. The recovery of deformed alloy upon annealing leads to the formation of ultrafine and nanostructured grains in the alloy. The hardness achieved after 85% room temperature rolling (RTR) is found to be 269 HV, while the tensile strength is 679 MPa and 697 MPa in the rolling and transverse direction, respectively. The improvement in strength is due to generation of high dislocation density and ultrafine grains in the deformed alloy with 85% thickness reduction, during rolling. The deformed alloy subjected to annealing at 400 °C for 30 min sample shows increase in ductility (6% and 7.2%) in rolling and transverse direction, respectively, due to the annihilation of dislocations as evident from the TEM study.     

Enhancement of creep resistance and thermal behavior of eutectic Sn–Cu lead-free solder alloy by Ag and In-additions

The eutectic Sn–0.7Cu solder alloy is widely used in electronic packaging in which the creep property of the solder joint is essential to meet the global demand for longer operating lifetime in their applications. In this study, the influence of Ag and In additions on tensile creep behavior and thermal properties of bulk eutectic Sn–Cu solder alloy is reported. Results show that addition of Ag and In resulted not only in the formation of new Ag₃Sn and γ-SnIn₄ intermetallic compounds (IMCs), but also in the refinement of grain size of Sn–0.7Cu solder from ∼0.50 to ∼0.15 μm. Accordingly, the creep properties of the Ag or In-containing solder alloys are notably improved. The creep strain rate increases and creep lifetime decreases as the applied stress level and temperature increase. Room and elevated-temperature creep rate of bulk Sn–Cu solder was reduced by 521.0% after Ag addition, but for In addition the reduction was about 200.7%. These differences are attributed to the presence of new Ag₃Sn and γ-SnIn₄ precipitates and their rules in classical dispersion strengthening as a separate phases. Moreover, the eutectic temperature of Sn–0.7Cu is decreased from 227.4 to 217.8 and 224.0 °C with the addition of Ag and In, respectively.     

Investigation of the compressive creep behavior of AZ91D magnesium alloy

The creep resistance of AZ91D alloy has been studied in uniaxial compression tests at temperature ranges from 275 °C to 325 °C. The initial microstructure of the alloy consists of α phase and β phase precipitated in the grain boundary. The minimum creep rate dependence on applied stress and the temperature is also analyzed in detail. We find that the stress exponent n is close to the theoretical values (3 or 5) and the activation energy Q for creep varies from 121 kJ/mol to 171 kJ/mol. Creep could be controlled by high-temperature climb and cross-slip of dislocation at different temperatures.     

Impression creep behavior of a cast MRI153 magnesium alloy

Creep behavior of a cast MRI153 magnesium alloy was investigated using impression creep technique. The tests were carried out under constant punching stress in the range of 360–600 MPa at temperatures between 425 and 490 K. Microstructure of the alloy was composed of α(Mg) matrix phase besides Mg₁₇Al₁₂ and Al₂Ca intermetallic compounds. Stress exponent of minimum creep rate, n, was found to vary between 6.45 and 7. Calculation of the activation energy showed a slight decrease with increasing stress such that the creep activation energy of 115.2 kJ/mol under σ_imp/G = 0.030 decreased to 99.5 kJ/mol under σ_imp/G = 0.040. The obtained stress exponent and activation energy data suggested that the pipe diffusion dislocation climb controlled creep as the dominant mechanism during the creep test.     

Design and mechanical characterization of a Zr–Nb–O–P alloy

In this study, Sn-free Zr–1.5Nb–O–P alloys were manufactured and their mechanical properties were characterized. The ultimate tensile strength (UTS) of cold rolled Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (680 MPa) were close to that of a commercially available Zr–1Nb–1Sn–0.1Fe alloy (720 MPa), achieving a good mechanical strength without the addition of Sn, an effective solution strengthening element. The UTS of recrystallized Zr–1.5Nb–O–P alloy with 160 ppm phosphorous (533 MPa) was far greater than that of a commercially available Zr–1Nb–O (323 MPa) because of the strengthening due to higher Nb and oxygen content combined with phosphorous strengthening. The activation volumes for the cold rolled Zr–1.5Nb–P alloys were not much different from those of annealed Zr–1.5Nb–P alloys despite the higher dislocation density in the cold rolled alloys. Insensitivity of the activation volume to the dislocation density and the decrease of the activation volume with the addition of phosphorous support the suggestion linking the activation volume with the activated bulge of dislocations limited by segregation of oxygen and phosphorous atoms.