Creep Behavior Of Mg Research Articles

Insights into creep behavior of Mg–14Gd–1Zn–0.4Zr (wt.%) alloy containing β- and γ-type precipitates

Compressive creep tests were performed on sand-cast and peak-aged Mg–14Gd–1Zn–0.4Zr (wt.%) alloys at 250 °C in this study. The microstructures before creep and at the secondary creep stage were analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that plenty of fine precipitates, especially β′-series precipitates or a combination of β-type and γ′ precipitates, could effectively enhance the creep resistance of Mg alloys. Large amounts of β'+β′F precipitate chains in the regions near grain boundaries of the sand-cast alloy blocked the motion of <c + a>-type dislocations, while the interaction of basal and prismatic <a> dislocations could be inhibited by synergy of γ′ and β-type precipitates. In contrast, transformation of β′-series precipitates to β1 or β precipitates in the peak-aged alloy reduced their capacity to impede the <c + a> dislocation movement, seemingly presenting worse microstructures for creep resistance. However, the peak-aged alloy exhibited a uniform distribution of numerous semi-coherent β1 precipitates and dense rectangular networks composed of γ′ and β-type precipitates within the whole grain, which was superior to the uneven distribution of β-type and γ′ precipitates as well as a lack of ample precipitates at the center of grain in the sand-cast alloy. Thus, the peak-aged Mg–14Gd–1Zn–0.4Zr alloy obtained better creep resistance than the sand-cast alloy to some extent.

Effects of Gd/Nd ratio on the microstructures and tensile creep behavior of Mg–8Al–Gd–Nd alloys crept at 423K

The specific Nd/Gd mass ratios were designed for optimizing the microstructure of the Mg–8Al–Nd–Gd alloys, and the strengthening mechanism of Al-Gd-Nd phase in the creep process was revealed. The Mg–8Al-1.0Gd-1.0Nd (T6) at Gd/Nd ratio of 1:1 exhibited excellent tensile creep resistance at 423 K/60 MPa, owing to the diffusely distributed Al 2 RE and the fine Al 11 Nd 3 particles, which hindered the motion of dislocations and grain boundary sliding. The stead-state creep rate and creep strain of the alloy were 1.03 × 10 −8 s −1 and 0.312%, respectively, which were superior to that of AZ91 alloy. It could be verified that Gd showed little aging precipitation strengthening effect, while it made the typical rod-like Al 11 Nd 3 evolved into dot-like phase. A TEM thin foil was extracted by focused ion beam (FIB), and the result showed that Nd occupied the Gd sites to form Al 2 (Nd + Gd) solid solution. The TEM bright-field image was taken along the B = [ 1 ¯ 1 ¯ 01 ] Mg zone axis with diffraction condition g = 10 1 ¯ 1 ¯ and Two-beam dark field image with diffraction condition g = 11 2 ¯ 0 , which demonstrated the activation of <a> and <a+c> dislocations during creep deformation.

Abnormal creep behavior of Mg–12Gd–1MM–0.6Zr (wt.%) alloy at 300°C

The creep behavior of extrusion-solution treated (T4) Mg–12Gd–1MM–0.6Zr (wt.%) alloys with different grain sizes at 300 °C was evaluated. Results showed that the creep rate curves presented an abnormal decelerated–accelerated–decelerated evolution in the initial stage, which was caused by creep aging. The accelerated creep rate is caused by the transformation from thin β' phase into coarse β1 and β phases; and the transformation of the 3D distribution β phase from the initial single variant to network containing three variants is a key reason for the secondary deceleration of creep rate. On the other hand, as the average grain size increased from 44 μm to 90 μm, the steady-state creep rate gradually decreased, but fracture time continuously increased, so coarse grain is beneficial to creep resistance of the present alloy.

Investigation of tensile creep behavior of Mg–Gd–Y–Zr alloy based on creep constitutive model

The tensile creep behavior of as-extruded and T6 treated Mg–8Gd–3Y–0.3Zr (GW83) alloy was estimated under temperature at 170–200 °C and stress at 40–70 MPa. The results showed that the primary creep strain and steady state creep rate increased, while the duration of steady state creep decreased with increasing temperature and stress. The primary creep constitutive model of GW83 alloy kept an Andrade power law ε = βt k with k value of 0.24–0.6 for as-extruded alloy and 0.13–0.49 for T6 alloy, respectively. The steady state creep plastic deformation of as-extruded and T6 state alloys can be expressed by Arrhenius constitutive equations with stress exponents 5.6–5.9 and 8.9–9.8, creep activity energy 124.9–135.6 kJ/mol and 152.2–195.8 kJ/mol, respectively. The tensile creep resistance of GW83 alloy improved significantly by T6 heat treatment, the effect of temperature on steady state creep rate was relatively moderate, and the tensile creep plastic deformation of GW83 alloy mainly was controlled by applied stress. The steady state creep deformation mechanism of as-extruded GW83 alloy was regulated by Mg self-diffusion conducted by dislocation climb before heat treatment, and then transformed to dislocation cross-slip after T6 treated. The recrystallization and texture evolution of (10) , (10) played a non-ignorable effect on the tensile creep deformation. The creep rupture life can be predicted by steady state creep rate that will be controlled by temperature under low and intermediate stress and by stress under larger applied stress, respectively. The creep fracture surfaces with coarse dimples and tear ridges presented as ductile transgranular fracture features with the secondary cracks occurred.

The creep behavior of Mg–9Al–1Si–1SiC composite at elevated temperature

Abstract The creep properties of as-cast Mg–9Al–1Si alloy and Mg–9Al–1Si–1SiC composite were compared. The results show that Mg–9Al–1Si–1SiC composite performs a better creep resistance than that of Mg–9Al–1Si alloy at constant temperature and stress (473 K,70 MPa). Besides, the creep behavior of Mg–9Al–1Si–1SiC composite at various temperature from 448 K to 498 K and under stresses of 70–90 MPa were systematically investigated. The Mg–9Al–1Si–1SiC composite exhibited a stress exponent from 5.5 to 6.9 and the creep activation energy fell within the range of 86–111 kJ/mol. The results showed that the creep mechanism of Mg–9Al–1Si–1SiC composite was mainly attributed to the effects of secondary phase strengthening mechanism and dislocation climb mechanism.

Creep Behavior and Microstructure Evolution of Sand–Cast Mg–4Y–2.3Nd–1Gd–0.6Zr Alloy Crept at 523–573 K

High temperature tensile–creep behavior of Mg–4Y–2.3Nd–1Gd–0.6Zr (wt%, WE43(T6)) alloy at 523–573 K was investigated. The creep stress exponent is equal to 4.6, suggesting the underlying dislocation creep mechanism. The activation energy is (199 ± 23) kJ/mol, which is higher than that for self-diffusion in Mg and is believed to be associated with precipitates coarsening or cross slip. The creep mechanism is further suggested to be dislocation climb at 523 K, while a cross slip at 573 K is possible. The metastable β′ and β1 phases in the WE43(T6) alloy were relatively thermal stable at 523 K and could be effective to hinder the dislocation climb, which contributed to its excellent creep resistance. However, at 573 K it readily transforms into equilibrium βe phase and coarsens within two hours, thereby causing a decrease of creep resistance. In addition, precipitate free zones approximately normal to applied stress direction (directional PFZs) developed during the creep deformation, especially at 573 K. Those zones became preferential sites to nucleate, extend and connect microcracks and cavities, which lead to the intergranular creep fracture. Improving the thermal stability of precipitates or introducing thermally stable fine plate-shaped precipitates on the basal planes of Mg matrix could enhance the high temperature creep resistance.

Creep behavior of Mg–10Gd–xZn (x=2 and 6 wt%) alloys

Creep behavior of Mg–10Gd–xZn alloys was investigated with two Zn contents, 2 and 6wt%. The as cast microstructure of alloys contained (Mg, Zn)3Gd phase at the interdendritic regions and long period stacking ordered (LPSO) phase in the matrix. Increase in the Zn content from 2% to 6%, increased the volume fraction and morphology of second phase [(Mg,Zn)3Gd]. Creep testing at 250 and 300°C with stress levels between 50 and 120MPa indicated that increase in Zn content or temperature reduced the creep performance of the Mg–10Gd–xZn alloys. The minimum creep rate of Mg–10Gd–2Zn was one order less that of Mg–10Gd–6Zn. Dynamic precipitation was found in both the alloys during creep deformation at 250°C and strengthened the alloys: prismatic platelets (Mg5Gd) were observed perpendicular to the LPSO phase in Mg–10Gd–2Zn alloy whereas basal oval precipitates (ternary composition) parallel to the LPSO occurred in Mg–10Gd–6Zn alloy. The dynamic precipitates were not observed in alloys during creep at 300°C. Ternary bulk precipitates at the LPSO phase was observed in both alloys and at all testing conditions which facilitated the cracking during deformation. More continuous second phase at the interdendritic boundaries facilitated easy cracking in Mg–10Gd–6Zn alloy and hence reduced the creep performance.

The impression creep behavior and microstructure evolution of cast and cast-then-extruded Mg–10Gd–3Y–0.5Zr (wt%)

The impression creep behavior of Mg–10Gd–3Y–0.5Zr (wt%, GW103) was investigated by flat cylindrical indenter experiments at temperatures ranging from 200°C to 325°C. The punching stresses, which were calculated based on the indenter dimensions, varied from 50MPa to 505MPa. Samples were examined in the peak-aged cast condition (cast-T6) and in the cast-then-extruded plus peak-aged condition (ex-T5). Using a power–law relationship, the creep stress exponent (n) varied from 1.4 to 5.1 for the cast-T6 alloy, where lower values were obtained at lower temperatures and stresses. The creep activation energy (Q) increased from 87 to 191kJ/mol with increasing stress. For the ex-T5 alloy, the n values were 2.2, 3.4 and 3.4 at 200°C, 250 and 300°C, respectively. The Q value increased from 121 to 165kJ/mol with increasing stress. However, by using a hyperbolic sine relationship, single activation energies of 93 and 134kJ/mol were obtained for the cast-T6 and ex-T5 alloy, respectively. The zone at the edges of the indenter underwent the largest stress and strain, resulting in broken grain boundaries. Extension twinning occurred both in the zone at the edges of the indenter and underneath the indenter face for the cast alloy, while no twinning was observed for the extruded alloy. Intergranular cracking was observed in the zone at the edges of the indenter for the tests under high temperature and high stress.

Effect of Ca and Sr on the compressive creep behavior of Mg–4Al–RE based magnesium alloys

The compressive creep behavior of Mg–4Al–RE–1.2Ca (AEC4112) alloy and Mg–4Al–RE–1.2Ca–0.2Sr (AECJ411202) alloy is investigated on a special apparatus. The stress exponent n under different stresses and the creep activation energy Qc under different temperatures of AEC4112 alloy and AECJ411202 alloy are estimated by an empirical equation. The results reveal that the compressive creep resistance of Mg–4Al–RE (AE41) alloy is improved significantly by a small amount of Ca and Sr. The steady state compressive creep rates of AEC4112 alloy and AECJ411202 alloy are increased with increasing temperature and applied stress. The steady state compressive creep rates of AEC4112 alloy and AECJ411202 alloy are 2.2817×10−7mm/s and 1.3041×10−7mm/s under 150°C and 100MPa, respectively. The compressive creep resistance is improved by intermetallic compounds Al2Ca, Al2RE and Al4Sr inhibiting the climbing of the dislocation, the sliding and diffusion of the grain boundary. The compressive creep constitutive equations of AEC4112 alloy and AECJ411202 alloy are written as ε̇s1=2.0276×10−10×σ5.662exp(−47447/RT) and ε̇s2=5.8208×10−11×σ6.123exp(−68252/RT) respectively. The compressive creep mechanism of AEC4112 alloy and AECJ411202 alloy is the combined effects of grain boundary diffusion and grain boundary sliding determined by dislocation climbing.

Compressive and impression creep behavior of a cast Mg–Al–Zn–Si alloy

Creep behavior of an Mg–6Al–1Zn–0.7Si cast alloy was investigated by compression and impression creep test methods in order to evaluate the correspondence of impression creep results and creep mechanisms with conventional compression test. All creep tests were carried out in the temperature range 423–523K and under normal stresses in the range 50–300MPa for the compression creep and 150–650MPa for impression creep tests. The microstructure of the AZ61–0.7Si alloy consists of β-Mg17Al12 and Mg2Si intermetallic phases in the α-Mg matrix. The softening of the former at high temperatures is compensated by the strengthening effect of the latter, which acts as a barrier opposing recovery processes. The impression results were in good agreement with those of the conventional compressive creep tests. The creep behavior can be divided into two stress regimes, with a change from the low-stress regime to the high-stress regime occurring, depending on the test temperature, around 0.009<(σ/G)<0.015 and 0.021<(σimp/G)<0.033 for the compressive and impression creep tests, respectively. Based on the steady-state power-law creep relationship, the stress exponents of about 4–5 and 10–12 were obtained at low and high stresses, respectively. The low-stress regime activation energies of about 90kJmol−1, which are close to that for dislocation pipe diffusion in the Mg, and stress exponents in the range of 4–5 suggest that the operative creep mechanism is pipe-diffusion-controlled dislocation viscous glide. This behavior is in contrast to the high-stress regime, in which the stress exponents of 10–12 and activation energies of about 141kJmol−1 are indicative of a dislocation climb mechanism similar to those noted in dispersion strengthening mechanisms.

Creep behavior of Mg–9Gd–1Y–0.5Zr (wt.%) alloy piston by squeeze casting

Abstract The Mg–9Gd–1Y–0.5Zr (wt.%) alloy piston was produced by squeeze casting and further heat treated to peak-aged state. The tensile-creep behavior of the specimens taken from the piston top was investigated at temperatures of 200–300 °C (0.54–0.66Tm) and applied stresses of 50–120 MPa. The creep resistance decreases with increasing temperature and applied stress. When tested at 200 °C and 50 MPa, the specimen has a steady-state creep rate of 0.4 × 10− 9 s− 1 and creep strain of 0.014% after 100 h. The creep stress exponent and the activation energy are 1.66–4.47 and 115.9–174.2 kJ/mol, respectively, implying that dislocation creep is the rate-controlling mechanism. At 50–80 MPa the activation energy for creep indicates that dislocation climb is the creep mechanism. At 120 MPa the relatively high activation energy may be associated with the operation of cross slip. After test, the thermal stable β phase appears and its volume fraction increases with testing temperature. The steady-state creep rate and rupture time are modeled by the original and modified Monkman–Grant relationships. The microcracks and cavities preferentially nucleate at the grain boundaries, especially at triple points. Brittle rupture with few plastic tearing ridges is the dominant character of the creep fracture.

Microstructural investigation and the creep behavior of Mg–Sr–Mn alloys

Abstract The Mg–Sr–Mn ternary alloy system exhibits potential for good creep performance in powertrain applications. Consistent with thermodynamic calculations, the microstructure of as-cast (AC) alloys exhibit Mg 17 Sr 2 and α-Mn phases, and, when heat treated (HT), the precipitation of rod-like α-Mn phases from the supersaturated Mg matrix and polygonal α-Mn phases from the supersaturated interdendritic Mg 17 Sr 2 phase is observed. No coarsening of the interdendritic Mg 17 Sr 2 occurs, while some coarsening is seen in the primary α-Mn phases present in the as-cast alloys when the Mn level is ∼2 wt.%. An orientation relationship has been determined between α-Mn precipitated in the interdendritic regions during ageing and the interdendritic Mg 17 Sr 2 . The α-Mn that precipitates from the Mg 17 Sr 2 intermetallic has a morphology which is different from the intradendritic α-Mn precipitating from the α-Mg matrix during ageing. Creep tests were conducted at temperatures between 150 and 200 °C and stresses of 40–50 MPa on Mg–(3–5) wt.%Sr–(0–2) wt.%Mn. The final creep strain ( ɛ final ) values decreased with increasing Sr, and mostly with increasing Mn in the composition range studied.

Creep behavior of Mg–11Y–5Gd–2Zn–0.5Zr (wt.%) at 573K

Abstract The effect of microstructure on the tensile–creep behavior of Mg–11Y–5Gd–2Zn–0.5Zr (wt.%) (WGZ1152) at 573 K (0.64 T m ) and stresses between 30 MPa and 140 MPa was investigated. The minimum creep rate of the peak-aged (T6) alloy was almost two orders of magnitude lower than that for a WE54-T6 (Mg–5.2Y–3.6RE–0.5Zr (wt.%)) alloy. The peak-aged condition (T6) exhibited slightly greater creep resistance than the as-cast condition. The solution treated (T4) material exhibited the lowest creep resistance. The creep stress exponent (∼5) suggested that dislocation creep was the dominant secondary creep mechanism. The minimum creep rate and time-to-fracture could be described by the Monkman–Grant equation. An in-situ creep experiment indicated that intergranular cracking was prevalent in the tertiary creep regime and the crack propagation path tended to follow the grain boundaries.

Creep behaviour of Mg–8% Sn and Mg–8% Sn–3% Al–1% Si alloys

The work highlights the trends recently observed while evaluating the conventional creep behaviour of Mg–8% Sn and Mg–8% Sn–3% Al–1% Si alloys at 150 °C and at four stress levels. The improvement in creep properties in Mg–Sn based alloys is attributed to the presence of thermally stable intermetallics as compared to the AZ-series Mg-alloys. Results revealed that between the two alloys, the creep resistance of Mg–8% Sn–3% Al–1% Si alloy is superior to Mg–8%Sn alloy owing to the presence of Mg2Si intermetallic which retards the grain boundary movement during creep. However, the ductility of the former alloy is lower than that of the later. It is also evident that creep life and creep resistance of all the alloys significantly reduce with increase in applied stress.

Microstructure, tensile properties and creep behavior of Mg–4Al–2Sn containing Ca alloy produced by different casting technologies

Abstract The microstructure, tensile properties and creep behavior of Mg–4Al–2Sn (AT42) and Mg–4Al–2Sn–1Ca (ATX421) alloy produced by ingot and die-cast were investigated. The die-cast alloy exhibits superior yield and tensile properties at room temperature and 200 °C. The strengthening mechanism of die-cast alloys was mainly attributed to the grain refinement and fine morphology of intermetallic phases. The die-cast ATX421 alloy exhibited the best creep resistance, i.e. the lowest minimum creep rate and the longest creep rupture time, while the poorest creep resistance was observed in the ingot cast AT42 alloy.

Creep behavior of Mg–2Nd–0.5Zn–0.4Zr at ambient temperature

The creep behavior of Mg–2Nd–0.5Zn–0.4Zr alloy was investigated at ambient temperature in the present work. It was observed that creep strain is significant dependence of the thermal fluctuation at low temperature and the minimum creep rate was extrapolated. The stress exponent was determined to be 2.8 and the apparent activation energy for creep deformation was determined to be 24 kJ/mol. However, the activation energy controlled by the threshold stress was determined to be 10.8 kJ/mol, which indicates creep deformation can occur at ambient temperature in magnesium alloys. TEM results show that dislocation glide on basal plane is a major deformation mechanism and mechanical twinning was also observed during creep deformation process.

The microstructure, mechanical properties and creep behavior of Mg–3Sm–0.5Zn–0.4Zr (wt.%) alloy produced by different casting technologies

The microstructure, mechanical properties and creep behavior of Mg–3Sm–0.5Zn–0.4Zr (wt.%) alloy produced by ingot-casting and die-casting were investigated. The die-cast alloy exhibits superior tensile properties at room temperature, while the higher tensile strength at 200 °C is obtained in the ingot-cast alloy. The results indicate that the creep behavior in the two alloys is discussed in terms of the microstructural features. The creep data and microstructural analysis reveal that dislocation creep is the dominant creep rate-controlling mechanism at 200 °C/60 MPa for the two alloys. The excellent creep resistance of the die-cast alloy is mainly attributed to the dynamic precipitation in the α-Mg matrix of a high density of fine platelets, which served as obstacles to dislocation motion.

The microstructure, tensile properties, and creep behavior of AZ91, AS52 and TAS652 alloy

The microstructure, mechanical properties and creep behavior of Mg–6Sn–5Al–2Si (TAS652) alloy were compared with those of AZ91 and AS52 alloys. The as-cast microstructure of the TAS652 alloy revealed the presence of α-Mg, Mg 17Al 12 and Mg 2Sn and Mg 2Si phases. The Mg 2Sn phases precipitated near the Mg 17Al 12 phases, and the polygonal Mg 2Si phases formed within the matrix. The tensile tests showed that the addition of Sn contributed to the formation of dispersed short rod-like Mg 2Sn phases and improved the room and elevated temperature strength. The creep properties of the TAS652 alloy were also superior to those of the AZ91 and AS52 alloys. An analysis of the creep behavior at elevated temperatures suggested that the presence of thermally stable dispersoids in the TAS652 alloy increases the resistance to dislocation movement, which improves the creep properties.

Effect of Sm on the microstructure, mechanical properties and creep behavior of Mg–0.5Zn–0.4Zr based alloys

The present study aims at investigating the effect of additions (0.4, 0.5, 1.5, 3, and 5 wt.%) of samarium on the microstructure, mechanical properties of Mg–0.5Zn–0.4Zr alloy. Experimental results show that a small amount of samarium addition can bring about precipitation of eutectic Mg 41Sm 5 phases at the grain boundaries and refine the as-cast grains. When samarium content increasing to 3%, a great number of fine plate-shaped precipitates form in the regions close to grain boundaries, and could effectively hinder dislocation slip and play important roles in improving tensile and creep properties. Furthermore, the transition of fracture mode as a function of samarium content can be generally described as follows: trans-granular fracture in Mg–(0.4–0.7)Sm–0.5Zn–0.4Zr alloys, both trans-granular and inter-granular fracture in Mg–1.5Sm–0.5Zn–0.4Zr alloys, and inter-granular fracture in Mg–(3–5)Sm–0.5Zn–0.4Zr alloys. Stress and temperature dependence of the steady-state creep rate were studied over a temperature range of and stress ranges of 200–225 °C and stress range of 60–100 MPa, and creep mechanisms corresponding to different conditions were proposed for the Mg–3Sm–0.5Zn–0.4Zr alloy.

The effect of cooling rates on the refinement of microstructure and the nanoscale indentation creep behavior of Mg–Al–Ca alloy

A series of casting experiments were carried out with Mg–5.0 wt.% Al–2.0 wt.% Ca (AC52) alloy with different cooling rates from 0.5 to 65.0 °C/s. A significant refinement of the grain size of the alloy had been achieved at higher cooling rates of 20.0–65.0 °C/s. The decrease in cooling rates resulted in continuous coarsening of the eutectic phases from nanoscale particle to microscale needle within the primary grain of the alloys. EDS results showed that there were the higher solute content of alloying elements (Al and Ca) within the primary grain of the alloys resulting from the quenching effect of fast cooling. The nanoscale creep resistance of the as-cast AC52 alloys increased with an increase in cooling rate except that the alloy cast at 20 °C/s showed the highest creep resistance. The creep curves indicated that the interaction between the dramatically increased dislocations and the surrounding solute and dispersed fine eutectics caused a high plastic penetration depth in the primary stage after loading. Then creep deformation changed from the high stress exponent regime ( n ≈ 7) to the low stress exponent regime ( n ≈ 3) at a threshold stress (∼140 MPa) controlled by the dislocation and twining controlled mechanisms. The creep behavior was correlated with the microstructure including solute content of alloying elements and dispersed eutectic phases within the primary grain to explain the underlying creep mechanisms.