Microstructure of MgAl5Ca3Sr alloy after creep deformation and high‐temperature heat treatment

The paper presents results of microstructural investigations of MgAl5Ca3Sr magnesium alloys in the as‐cast condition, after creep tests at 180 °C, and after heat treatment at 450 °C for 4.5 hours. The microstructure of MgAl5Ca3Sr alloy is composed of α‐Mg solid solution, irregular shaped (Mg,Al)₂Ca phase with C36 crystal structure, bulky (Mg,Al)₁₇(Sr,Ca)₂ phase, fine lamellar Mg₂Ca phase with C14 structure, needle‐shaped Al₂Ca precipitates with the C15 crystal structure. The precipitation of the needle‐shaped Al₂Ca phase in the α‐Mg grains and spheroidization of the C14 phase were found after heat treatment at 450 °C in argon atmosphere. The (Mg,Al)₂Ca (C36) and (Mg,Al)₁₇(Sr,Ca)₂ phases seems to be stable at 450 °C, however, the increasing of aluminum content in C36 compound was observed suggesting the initial stage of C36 → C15 transformation. After creep deformation at 180 °C precipitates of the Al₂Ca phase were found in α‐Mg phase. The intermetallic compounds are stable at 180 °C. The MgAl5Ca3Sr alloy exhibits good creep resistance up to 75 MPa. Tensile properties are comparable to those of Mg‐RE‐Zn–Zr alloys.     

Microstructural stability and creep properties of die casting Mg–4Al–4RE magnesium alloy

The AE44 (Mg–4Al–4RE) alloy was prepared by a hot-chamber die casting method. The microstructure, microstructural stability and creep properties at 175 °C were investigated. The microstructure was analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and the Rietveld method. The results show that die cast AE44 magnesium alloy consists of α-Mg, Al₁₁RE₃, Al₂RE and Al_2.12RE_0.88 phases. The Al₁₁RE₃ phase is thermally stable at 175 °C whereas the metastable Al_2.12RE_0.88 phase undergoes a transition into the equilibrium Al₂RE phase. The alloy investigated is characterized by good creep properties at temperatures of 175 °C and 200 °C.     

Mikrostrukturelle Veränderungen von Magnesium‐Druckgusslegierungen nach langzeitiger thermischer Beanspruchung

Microstructural Changes of Pressure Die Cast Magnesium Alloys after Long‐Term Thermic Loading The expansion of the application of pressure die cast magnesium alloys for automobiles requires the development of new alloys and the comprehensive assessment of available alloys on aggravated conditions, too. Such conditions are also given at higher temperatures, which can cause the creep of the material and lead to the component failure. Because the microstructural stability decisively depends on the thermic loading, this paper deals with the change of the microstructure and the hardness of the alloys AZ91, AM50 and AE42 after a long‐term annealing at 150 °C and 200 °C in comparison to the pressure die as‐cast condition. The results reveal clear differences of the microstructural stability of the alloys AZ91 and AM50 on the one hand and the alloy AE42 on the other hand. Due to the long‐term annealing at 150 °C the alloys AZ91 and AM50 show chiefly an intense precipitation of Mg₁₇Al₁₂ from the Al‐rich eutectic α‐phase. Furthermore at 200 °C, it is observed the coagulation and coarsening of these precipitates, too. The last appearances are connected with a weakening of the material. Regarding the alloy AE42, the changes of the precipitation state are not so intensely and do yet not lead to a microstructural weakening.     

Precipitates effect on microstructure of as-deformed and as-annealed AZ41 magnesium alloys by adding Mn and Ca

The microstructure and precipitates effect of AZ41 alloys by adding Mn and Ca during hot compression (300 °C/0.1 s⁻¹) followed by annealing (300 °C/1000 s) have been studied. A kind of bimodal structure consist of a large fraction of the original and deformed grains as well as a small fraction of newly formed grains is found in as-deformed alloys. Equiaxed and fine-grained microstructures occur in as-annealed samples. For Mn-free and Ca-free alloys, Zener pinning effect is minor. Particle stimulated nucleation promotes recrystallization. For Mn- and Ca-containing alloys, fine particles retard dislocation glide forming high density dislocation to inhibit recrystallization.     

Ca-modified Al–Mg–Sc alloy with high strength at elevated temperatures due to a hierarchical microstructure

Al-Mg alloys are normally prone to lose part of their yield and tensile strength at high temperatures due to insufficient thermal stability of the microstructure. Here, we present a Ca-modified Al–Mg–Sc alloy demonstrating high strength at elevated temperatures. The microstructure contains Al₄Ca phases distributed as a network along the grain boundary and Al₃(Sc,Zr) nano-particles dispersed within the grains. The microstructure evolution and age-hardening analysis indicate that the combination of an Al₄Ca network and Sc-rich nano-particles leads to excellent thermal stability even upon aging at 300 °C. The tensile strength of the alloy for temperatures up to 250 °C is significantly improved by an aging treatment and is comparable with the commercial heat-resistant aluminum alloys, i.e., A356 and A319. At a high temperature of 300 °C, the tensile strength is superior to the above-mentioned commercial alloys, even more so when expressed as the specific strength due to the low density of Ca-modified Al–Mg–Sc alloy. The excellent high-temperature strength results from a synergistic effect of solid solution strengthening, grain boundary strengthening and nanoparticle order strengthening.

     

Production of TiC reinforced-aluminum composites with the addition of elemental carbon

TiC reinforced Al matrix composites were produced by the additions of elemental carbon to both Al + 4%Ti and Al + 5%Ti alloys. It is shown that the microstructure, phase composition as well as fracture behavior of the composites produced are controlled by the processing parameters, such as temperature, amount of excess carbon and duration. Composite microstructure subjected to 1300 °C for 15 min includes only TiC particles where the fracture occurs in a ductile manner whilst composites subjected to 1200 °C for 30 min contain Al₃Ti and TiC particles which show mixed mode of fracture behavior where Al₃Ti particles resulted in brittle fracture due to their coarser size.     

Design and microstructural analysis of magnesium alloys for dynamical applications

A microanalytical characterization of cast magnesium alloys of eutectic origin based on the Mg–Al–Ca ternary matrix system has been carried out in order to investigate the influence of alloying elements on their microstructure as well as microchemistry-processing-microstructural relations using structure-sensitive techniques of electron microscopy, mechanical spectroscopy (internal friction), X-ray diffractometry, and advanced microanalytical methods including electron probe compositional analysis. Following the data obtained here there is direct correlation of microstructure with creep properties of the new experimental magnesium alloys. The creep and heat-induced properties of the multicomponent magnesium alloys containing low range of inexpensive additions of titanium (0.07–0.2%) or strontium (of about 1.8%) are defined by resulting structure dynamically formed during creep strain (up to 200 h). It is noteworthy that Ti as novel alloying element competes for creep resistance and cost with Sr and attracts as-cast desirable properties minimizing solute effects at ambient temperatures because of the pinning of slowly moving dislocations with the binding energy no more then 0.3 eV as well as because of stress-induced strengthening. The Ti and Sr solute atmosphere dragging is believed to be the rate-controlling mechanism responsible for radical improvement of creep resistance and long-term strength in the newly developed magnesium alloys at elevated temperatures. The new experimental alloys are superior to commercial alloys AZ91D, AE42, and AS21 following their creep resistance, long-term strength, heat resistance, and castability because of their novel microstructure having desirable engineering properties for structural applications (creep strain ε_c less than 0.3–0.4% at 423 K and 70 MPa for 200 h; έ_c ~ 10⁻⁹ s⁻¹). The newly developed magnesium alloys with improved castability could be used in die-casting technology and automobile (powertrain) industry for manufacturing of components and parts which are difficult to cast with more desirable microstructure.     

Effects of Steam Environment on Creep Behavior of Nextel™610/Monazite/Alumina Composite at 1,100°C

The tensile creep behavior of a N610™/LaPO₄/Al₂O₃ composite was investigated at 1,100°C in laboratory air and in steam. The composite consists of a porous alumina matrix reinforced with Nextel 610 fibers woven in an eight-harness satin weave fabric and coated with monazite. The tensile stress-strain behavior was investigated and the tensile properties measured at 1,100°C. The addition of monazite coating resulted in ~33% improvement in ultimate tensile strength (UTS) at 1,100°C. Tensile creep behavior was examined for creep stresses in the 32–72 MPa range. Primary and secondary creep regimes were observed in all tests. Minimum creep rate was reached in all tests. In air, creep strains remained below 0.8% and creep strain rates approached 2 × 10⁻⁸ s⁻¹. Creep run-out defined as 100 h at creep stress was achieved in all tests conducted in air. The presence of steam accelerated creep rates and significantly reduced creep lifetimes. In steam, creep strain reached 2.25%, and creep strain rate approached 2.6 × 10⁻⁶ s⁻¹. In steam, creep run-out was not achieved. The retained strength and modulus of all specimens that achieved run-out were characterized. Comparison with results obtained for N610™/Al₂O₃ (control) specimens revealed that the use of the monazite coating resulted in considerable improvement in creep resistance at 1,100°C both in air and in steam. Composite microstructure, as well as damage and failure mechanisms were investigated.     

Creep deformation mechanism of magnesium-based alloys

Two heat-resistant magnesium alloys AJC421 and Mg-2Nd were prepared. Both as-cast Mg-2Nd and AJC421 alloys exhibited good creep resistance in comparison with commonly used magnesium alloys. The improvement in creep properties through Nd addition to pure magnesium is attributed to both solid solution and precipitation hardening. The stress exponents of 4.5–5.5 and activation energies of 70.0–96.0 kJ/mol obtained from the as-cast Mg-2Nd alloy at low temperatures and low stresses indicate the five power law can be used for predicting the creep mechanism. The additions of alkaline earth elements Sr and Ca into Mg–Al alloys suppress the discontinuous precipitation of Mg₁₇Al₁₂ and form thermal-stable intermediate phases at grain boundaries, leading to effective restriction to grain boundary sliding and migration. However, the mechanism responsible for creep deformation of Mg–Al based alloys with Ca and Sr additions is not consistent with the results of microstructure observations performed on the alloys before and after creep tests.     

Effect of Ca additions on microstructure and microhardness of an as-cast Mg-5.0 wt.% Al alloy

The effects of Ca additions (0.5-2.0 wt.%) on the microstructure and the microhardness of an as-cast Mg-5.0 wt.% Al alloy have been investigated. The coarse microstructure of the base alloy can be refined through adding Ca. DSC and TEM results showed that, as Ca additions increased up to 1.5 wt.% Ca, the β-Mg₁₇Al₁₂ phase was completely replaced by a (Al, Mg)₂Ca phase. The Vickers microhardness of the as-cast Mg-Al-Ca alloys increased with increasing Ca content. Tests on the Mg-5.0Al-2.0Ca (wt.%) alloy showed an indentation size effect, which was well described by Meyer's Law.     

Effect of bonding temperature on microstructure and intermetallic compound formation of diffusion bonded magnesium/aluminum joints

The diffusion bonding of two dissimilar alloys Aluminum 5083 and Magnesium AZ31 was carried out at 420 °C, 430 °C,440 °C and 450 °C for bonding time of 60 min. In order to characterize the microstructure evolution in the joint zone, scanning electron microscopy, energy dispersive spectroscopy and x-ray diffraction were applied. The results show that joint formation is attributed to the solid-state diffusion of magnesium and aluminum into Aluminum 5083 and Magnesium AZ31 alloys followed by eutectic formation and constitutional liquation along the interface. At bonding temperature of 430 °C diffusion induced grain coarsening was observed at the interface. With increase in bonding temperature, the atomic diffusivity increases, results in easier and speeder chemical bonding. In bonding temperature of 440 °C the weld had an irregular shaped region in the weld center, having a different microstructure from the two base materials. The irregular shaped region contained a large volume of intermetallic compound Al₁₂Mg₁₇ and showed significantly higher hardness in the weld center. The present study suggests that constitutional liquation resulted in the intermetallic compound Al₁₂Mg₁₇ in the weld center.     

Precipitation in cold-rolled Al–Sc–Zr and Al–Mn–Sc–Zr alloys prepared by powder metallurgy

The effects of cold-rolling on thermal, mechanical and electrical properties, microstructure and recrystallization behaviour of the AlScZr and AlMnScZr alloys prepared by powder metallurgy were studied. The powder was produced by atomising in argon with 1% oxygen and then consolidated by hot extrusion at 350 °C. The electrical resistometry and microhardness together with differential scanning calorimetry measurements were compared with microstructure development observed by transmission and scanning electron microscopy, X-ray diffraction and electron backscatter diffraction. Fine (sub)grain structure developed and fine coherent Al₃Sc and/or Al₃(Sc,Zr) particles precipitated during extrusion at 350 °C in the alloys studied. Additional precipitation of the Al₃Sc and/or Al₃(Sc,Zr) particles and/or their coarsening was slightly facilitated by the previous cold rolling. The presence of Sc,Zr-containing particles has a significant antirecrystallization effect that prevents recrystallization at temperatures minimally up to 420 °C. The precipitation of the Al₆Mn- and/or Al₆(Mn,Fe) particles of a size ~ 1.0 μm at subgrain boundaries has also an essential antirecrystallization effect and totally suppresses recrystallization during 32 h long annealing at 550 °C. The texture development of the alloys seems to be affected by high solid solution strengthening by Mn. The precipitation of the Mn-containing alloy is highly enhanced by a cold rolling. The apparent activation energy of the Al₃Sc particles formation and/or coarsening and that of the Al₆Mn and/or Al₆(Mn,Fe) particle precipitation in the powder and in the compacted alloys were determined. The cold deformation has no effect on the apparent activation energy values of the Al₃Sc-phase and the Al₆Mn-phase precipitation.     

Creep behaviour and microstructural evolution of AlxCrMnFeCoNi high-entropy alloys

The creep deformation behaviour of thermo-mechanically treated Al_xCrMnFeCoNi high-entropy alloys was investigated in the high-temperature range of 600–700°C. Compared with the Al-free CrMnFeCoNi alloy, the Al_xCrMnFeCoNi alloys showed higher creep resistance under the same creep conditions due to strengthening contribution from elemental Al. The Al_0.4CrMnFeCoNi alloy exhibited a stress-dependent transition from the low-stress region to high-stress region. The Al_0.6CrMnFeCoNi alloy showed decreased creep resistance compared to the Al_0.4CrMnFeCoNi alloy, which was ascribed to the higher activation volume and stacking-fault energy in the former alloy. Specifically, the intragranular formation of Cr-rich σ precipitation was widely found in the matrix grain among testing conditions, indicating a striking atomic diffusion process due to the limited entropic stabilisation in Al-containing CrMnFeCoNi alloys.     

The tensile and creep behavior of Mg–Zn Alloys with and without Y and Zr as ternary elements

Tensile–creep experiments were conducted in the temperature range 100–200 °C and stress range 20–83 MPa for a series of magnesium–zinc–yttrium (Mg-Zn-Y) and mangnesium-zinc–zirconium (Mg-Zn-Zr) alloys ranging from 0 to 5.4 wt% Zn, 0 to 3 wt% Y, and 0 to 0.6 wt.% Zr. The greatest tensile–creep resistance was exhibited by an Mg–4.1Zn–0.2Y alloy. The room-temperature yield strength increased with increasing Y content for Mg–1.6–2.0Zn alloys. The greatest tensile strength and elongation was exhibited by Mg–5.4Zn–0.6Zr. This alloy also exhibited the finest grain size and the poorest creep resistance. The measured creep exponents and activation energies suggested that the creep mechanisms were dependent on stress. For applied stresses greater than 40 MPa, the creep exponents were between 4 and 8. For applied stresses less than 40 MPa, the creep exponent was 2.2. The calculated activation energies (Qapp) were dependent on temperature where the Q _app values between 100 and 150 °C (65 kJ/mol) were half those between 150 and 200 °C for the same applied stress value (30 MPa). Deformation observations indicated that the grain boundaries were susceptible to cracking in both tension and tension-creep, where at low applied stresses grain boundary sliding was suggested where strain accommodation occurred through grain boundary cracking. Thus grain size and grain boundaries appeared to be important microstructural parameters affecting the mechanical behavior. Microstructural effects on the tensile properties and creep behavior are discussed in comparison to other Mg-based alloy systems.

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Microstructural stability and mechanisms of fatigue and creep crack growth in Ti–24Al–11Nb

Titanium intermetallics are being developed for long term applications at elevated temperatures, particularly those alloys based on the alloys Ti₃Al and TiAl. Typical approaches include the design of appropriate microstructures for room and elevated temperature fatigue and creep resistance. However, a little explored area is the stability of these microstructures at elevated temperature and its effect on fatigue crack growth. The present investigation documented the microstructural stability, fatigue crack behaviour, and stress rupture of Ti-24Al-11Nb, a Nb modified Ti₃Al alloy. A coarse two phase α₂+β Widmanstatten microstructure was found to exhibit the best resistance to fatigue crack growth. Microstructural stability and elemental segregation were studied as a function of exposure time for up to 500 h at 800°C using transmission electron microscopy (TEM). Results indicate that the Widmanstatten microstructure is metastable and the β phase breaks up into particles. The absence of a continuous β phase surrounding the α₂ phase reduces the resistance of the microstructure to fatigue crack growth at room temperature. At elevated temperature the microstructure stability does not play a role in determining the fatigue resistance. A fine Widmanstatten microstructure has the best resistance to creep deformation. Stress rupture tests were conducted in vacuum and air at 649°C and 760°C. Two types of failure mechanisms were seen in stress rupture; these include transgranular and intergranular failure within prior β grains. When tested in air at 760°C a combination of transgranular and intergranular failure occurred. Specimens that exhibited a higher proportion of transgranular failure had longer lives. When tested in vacuum at 760°C the predominant failure mode was intergranular. At 760°C extensive microstructural changes like breakup and spherodization of the β phase occurred under stress while the rate of coarsening without any stress was much slower. At 649°C the specimens tested in vacuum consistently exhibited longer lives. The creep crack growth when tested in air at 649°C was always a brittle transgranular mode while the specimens tested in vacuum always failed by an intergranular mode. This revised version was published online in November 2006 with corrections to the Cover Date.     

Wetting of grain boundaries in Al by the solid Al<Subscript>3</Subscript>Mg<Subscript>2</Subscript> phase

The microstructure of binary Al_100−x –Mg _x (x = 10, 15, 18 and 25 wt%) alloys after long anneals (600–4000 h) was studied between 210 and 440 °C. The transition from incomplete to complete wetting of Al/Al grain boundaries (GBs) by the second solid phase Al₃Mg₂ has been observed. The portion of completely wetted GBs increases with increasing temperature beginning from T _wsmin = 220 °C. Above T _wsmax = 410 °C all Al/Al GBs are completely wetted by the Al₃Mg₂ phase.     

Constructing two-scale network microstructure with nano-Ti5Si3 for superhigh creep resistance

The improvement of mechanical properties must be achieved by designing and constructing more suitable microstructure, such as hierarchical microstructure. In order to significantly enhance the creep resistance of titanium matrix composites (TMCs), two-scale network microstructure was constructed including the first-scale network (<150 μm) with micro-TiB whisker (TiBw) reinforcement and the second-scale network (<30 μm) with nano-Ti₅Si₃ reinforcement by powder metallurgy and in-situ synthesis. The results showed that the creep rate of the composite was remarkably reduced by an order of magnitude compared with the Ti6Al4V alloy at 550 °C, 600 °C, 650 °C under the stresses between 100 MPa and 350 MPa. Moreover, the rupture time of the composite was increased by 20 times, compared with that of the Ti6Al4V alloy at 550 °C/300 MPa. The superior creep resistance could be attributed to the hierarchical microstructure. The micro-TiBw reinforcement in the first-scale network boundary contributed to creep resistance primarily by blocking grain boundary sliding, while the nano-Ti₅Si₃ particle in the second-scale network boundary mainly by hindering phase boundary sliding. In addition, the nano-Ti₅Si₃ particle was dissolved, and precipitated with smaller size than the primary Ti₅Si₃. This phenomenon was attributed to Si element diffusion under high temperature and external stress, which could further continuously enhance the creep resistance. Finally, the creep rate during steady-state stage was significantly decreased, which manifested superior creep resistance of the composite.     

Microstructural change and precipitation hardening in melt-spun Mg–X–Ca alloys

Mg–Al–Si–Ca and Mg–Zn–Ca base alloys were rapidly solidified by melt spinning at the cooling rate of about a million K/s. The melt-spun ribbons were aged in the range 100–400°C for 1 h. The effect of additional elements on microstructural change and precipitation hardening after heat treatment was investigated using TEM, XRD and a Vickers microhardness tester. Age hardening occurred after aging at 200°C in the Mg–Al–Si–Ca alloys mainly due to the formation of Al₂Ca and Mg₂Ca phases, whereas in the Mg–Zn–Ca alloys mostly due to the distribution of Mg₂Ca. TEM results revealed that spherical Al₂Ca precipitate has the coherent interface with the matrix. Considering the total amount of additional elements, Mg–Zn–Ca alloys showed higher hardness and smaller size of precipitates than Mg–Al–Si–Ca alloys. With the increase of Ca content, the hardness values of the aged ribbons were increased. Among the alloys, Mg–6Zn–5Ca alloy showed the maximum value of age hardening peak(H_v:180) after aging at 200°C for 1 h.     

Creep resistance of Fe–Ni–Cr heat resistant alloys for reformer tube applications

Relations between microstructure, phase transformations and creep resistance of austenitic Fe–Ni–Cr alloys are investigated. As-cast alloys with different silicon contents and an ex-service tube are submitted to laboratory agings to trigger specific phase transformations, and subsequently creep-tested at 950°C under stresses of 24–48?MPa. As-cast microstructures contain interdendritic chromium-rich M₇C₃ carbides with niobium-rich MC carbides. After aging at 950°C, primary M₇C₃ carbides transform into chromium-rich M₂₃C₆ carbides, associated to a loss in creep strength. The G phase present in the ex-service alloy is reversed into MC carbides by a heat treatment at 1100°C, associated to a slight decrease in creep resistance. Besides, the addition of silicon is highly detrimental to creep strength. Results can be used for alloy design.     

Microstructure and oxidation behaviors of near-α Ti-6.5Al-4Sn-4Zr-0.5Mo-based alloys with Ir addition

The microstructure and oxidation behaviors of near α-Ti-based alloys with small amount of iridium (Ir) additions were investigated. The microstructure of both Ir-free and Ir-containing alloys was observed to consist of α + β Widmanstätten colonies. The β lamellae gradually became continuous with increasing Ir additions since Ir acted as a β-stabilizer in the alloys. Isothermal oxidation test indicated that Ir addition reduced the oxidation resistance at 650 °C; while at 750 °C, the adherence of thermally grown oxides was enhanced, and a thin Al₂O₃-enriched layer on the oxide scale was promoted in the Ir-containing alloy, which suggests that Ir addition was effective in improving oxidation resistance of near-α-based alloys at 750 °C.