The effect of Ca and RE elements on the precipitation kinetics of Mg17Al12 phase during artificial aging of magnesium alloy AZ91

The effect of simultaneous alloying with Ca and rare earth (RE) elements on the age hardening kinetics of AZ91 was studied through the fitting of the Johnson-Mehl-Avrami (JMA) equation. The results showed that the addition of both Ca and RE elements not only suppress discontinuous precipitation of the Mg₁₇Al₁₂ phase during the age hardening process, but also decrease the alloy hardness. Fitting the JMA equation to the experimental data indicated that the phase transformation during age hardening of an alloy variant containing both Ca and RE (at 170 °C and 190 °C) and standard AZ91 (at 170 °C) takes place by the nucleation of precipitates on dislocations. In contrast, the precipitation during age hardening of AZ91 at 190 °C occurs via nucleation at grain boundaries. Although it was observed that the creep strength of age hardened specimens are lower than that of the as cast specimens, but age hardening treatment has lower deleterious influence on the creep resistance of the alloy containing Ca and RE in comparison with conventional AZ91. This may be ascribed to the decreased precipitation rate resulting from the addition of both Ca and RE elements.     

Effects of high-temperature ageing on the creep-rupture properties of high-tungsten cobalt-base superalloys

The effects of high-temperature ageing on creep-rupture properties were studied using cobalt-base superalloys containing about 14–20 wt% tungsten (W) at 1089 K (816 °C) and 1 311 K (1038 °C) in air. A high-temperature ageing for 1080 ks at 1273 K after solution treatment caused grain-boundary and matrix precipitates of W solid solution and carbide phases in these alloys, and grain boundaries were serrated especially in the alloys with higher W content. The high-temperature ageing largely improved the rupture life in the alloys with higher W content, particularly under lower stresses at 1089 K, whereas it caused the creep ductility to decrease a little in the alloy containing 20% W. The high-temperature ageing also improved the rupture life without decreasing creep ductility in these alloys under higher stresses at 1311 K. Under the same ageing conditions of 1080 ks at 1273 K, the initiation of grain-boundary cracks was retarded in the solution-treated and aged specimens, as well as in the aged specimens with serrated grain boundaries, for the alloys with higher W content at both 1089 and 1311 K. A large amount of grain-boundary serration also occurred in the non-aged specimens of the alloys with higher W content during creep at 1311 K, and contributed to the strengthening of the alloys. The solution-treated and aged specimen had almost the same rupture strength as the aged specimens with serrated grain boundaries in these cobalt-base alloys. The rupture strength of the solution-treated and aged specimens largely increased with increasing W content under the lower stresses at 1089 K and under the higher stresses at 1311 K. A ductile grain-boundary fracture surface, which was composed of dimples and grain-boundary ledges associated with grain-boundary precipitates, was observed in the solution-treated and aged specimens, as well as in the aged specimens with serrated grain boundaries at both 1089 and 1311 K. The fracture surface of the non-aged specimens was a brittle grain-boundary facet at 1089 K, but it became a ductile grain-boundary fracture surface, as serrated grain boundaries were formed owing to grain-boundary precipitates occurring during creep at 1311 K.     

Microstructure evolution in a 3%Co modified P911 heat resistant steel under tempering and creep conditions

Microstructure evolution was studied in a 3%Co modified P911 heat resistant steel during creep tests at 873 and 923 K to failure, which occurred in 4103 and 4743 h, respectively. The tempered martensite lath structure consisted of packets, blocks and laths. The average spacing of high-angle boundaries and the mean transverse lath size were about 1.9 μm and 360 nm, respectively. Various second phase particles precipitated upon tempering. Fine M(C, N) carbonitrides with an average size of about 30 nm were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M₂₃C₆ carbide particles (average size 120 nm) were located at internal boundaries. The tempered martensite lath structure was rather stable upon aging for about 4 × 10³ h. The boundary precipitates of M₂₃C₆ and Laves phases, which appeared during creep tests, exerted a high pinning pressure on low-angle lath boundaries and high-angle packet/block boundaries. The growth of martensite structural elements during the tests correlated with the coarsening of second phase particles. Quantitative relations of pinning and driving pressures for low- and high-angle boundaries are discussed.     

Creep behaviour and microstructural evolution in AISI 316LN + Nb steels at 650 °C

The paper deals with the effect of niobium in the wrought AISI 316LN steels on the long-term creep characteristics at 650 °C. Casts B and C contained 0.1 and 0.3 wt.%Nb, respectively. As a reference material the niobium free Cast A was used. Small additions of niobium to the AISI 316LN steel resulted in a significant reduction of the minimum creep rate and shortening of the tertiary creep stage. At time to rupture exceeding 10⁴ h the creep rupture strength of the niobium-bearing Casts B and C was slightly inferior to the Cast A. Two nitrides formed in the Casts B and C: Z-phase and M₆X. The minimum creep rate in niobium-bearing casts was favourably affected by precipitation of the Z-phase. The dimensional stability of Z-phase particles was very high, but niobium additions also accelerated the formation and coarsening of η-Laves and σ-phase. Coarse σ-phase particles at grain boundaries contributed significantly to the shortening of the tertiary creep stage.     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.     

Effect of Laves phase strengthening on the mechanical properties of high Cr ferritic steels for solid oxide fuel cell interconnect application

Two types of high chromium ferritic steels envisaged as construction materials for SOFC interconnects, were investigated in respect to microstructure and creep in the proposed application temperature range from 700 to 800 °C. The steel compositions mainly differed in the amounts of the Laves phase forming elements Nb, W and Si. The steel containing these alloying additions exhibited substantially higher creep resistance in the temperature range 700-800 °C than the high purity steel. The Laves phase formation occurred trans- as well as intragranular whereby the extent and size of grain boundary precipitates increased with increasing exposure time. Especially at 800 °C the precipitates inside the grains virtually completely vanished after longer exposure times and only intergranular precipitates remained. This change in precipitate morphology resulted especially at 800 °C in a decrease of creep resistance with increasing exposure time, although the Laves phase containing steel still exhibited higher creep strength than the high purity steel.     

Formation of Re-containing Carbides in a Second Generation Directionally Solidi¯ed Ni Base Superalloy

The precipitates at grain boundary in a directionally solidified Ni base superalloy after heat treatment, aging at 975°C, and creep rupture test have been characterized. Besides the primary MC carbides and fine particles of μ phase, the Re-containing M₂₃C₆ was observed. The precipitation kinetics revealed that the formation of M₂₃C₆ was associated with the dissolution of μ phase and MC carbides. TEM image shows that the continuous precipitation of M₂₃C₆ particles effectively hinders the dislocation movement and strengthens the grain boundaries. The high strength of the alloy suggests that M₂₃C₆ carbides are beneficial to the properties although Re as an important matrix strengthening element was consumed.     

Comparison of ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel

This paper compares the ferritic and austenitic plasma nitriding and nitrocarburizing behavior of AISI 4140 low alloy steel carried out to improve the surface corrosion resistance. The gas composition for plasma nitriding was 85% N₂–15% H₂ and that for plasma nitrocarburizing was 85% N₂–12% H₂–3% CO₂. Both treatments were performed for 5 h, for different process temperatures of 570 and 620 °C for ferritic and austenitic plasma treatment, respectively. Optical microscopy, X-ray diffraction and potentiodynamic polarization technique in 3.5% NaCl solution, were used to study the treated surfaces. The results of X-ray analysis revealed that with increasing the treatment temperature from 570 to 620 °C for both treatments, the amount of ε phase decreased and γ′ phase increased. Nitrocarburizing treatment resulted in formation of a more amount of ε phase with respect to nitriding treatment. However, the highest amount of ε phase was observed in the ferritic nitrocarburized sample at 570 °C. The sample nitrided at 620 °C exhibited the thickest layer. The potentiodynamic polarization results revealed that after plasma nitriding and nitrocarburizing at 570 °C, corrosion potential increased with respect to the untreated sample due to the noble nitride and carbonitride phases formed on the surface. After increasing the treatment temperature from 570 to 620 °C, corrosion potential decreased due to the less ε phase development in the compound layer and more porous compound layer formed at 620 °C with respect to the treated samples at 570 °C.     

Development of sputtered Shape Memory Alloy (SMA) Ni-Ti films for actuation in ice cooled environments

Due to the high sensitivity of Ni-Ti films to environmental changes, e.g. thermal, and/or to stress, they are ideal materials for applications on micro-sensors.It was aimed to obtain Ni-Ti films exhibiting the beginning of the B2 ⇔ R-phase transformation between room temperature (RT) and 0 °C. Thus, films with a slightly Ni-rich composition were prepared by sputtering, without intentional heating of the substrate. The Ni-Ti films were deposited on an Si₃N₄ intermediate layer previously deposited on naturally oxidized Si(100). The crystallization behaviour of the samples (at a constant temperature of 430 °C) was studied by X-ray diffraction in grazing incidence geometry off-plane (GIXD) at a synchrotron-radiation beamline. The GIXD patterns obtained during the annealing process of the Ni-Ti polycrystalline films revealed mainly an austenitic structure (B2 phase) and the precipitation of Ni₄Ti₃. The results have also shown that the presence of an intermediate layer of Si₃N₄ enhances the crystallization process of the Ni-Ti sputtered films when compared to the films deposited directly on single-crystal Si (with native oxide).The phase transformation behaviour of the Ni-Ti film on Si₃N₄ was evaluated by XRD in off-plane Bragg-Brentano geometry during cooling (RT → −40 °C) and heating (−40 °C → RT). It has been observed that a high fraction of the Ni-Ti film is already transformed to R-phase at 9 °C (austenitic at RT), as well as a very small temperature hysteresis for the B2 ⇔ R-phase transformation.After the characterization described above, the film was removed from the substrate. The free-standing film showed a pronounced “two-way” shape memory effect (SME). In the austenitic state the film presents a flat shape. During cooling, by reducing its distance from ice cubes (i.e., decreasing the surrounding temperature), the film starts bending exhibiting a final curled shape (yet without touching the ice). On heating it recovers its flat shape. The authors attribute the nature of this “two-way” SME to the Ni₄Ti₃ precipitates that formed during the heat treatment.     

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.     

Aging behavior of a 2024 Al alloy-SiCp composite

In the present research work the 2024 aluminum alloy was reinforced with SiC particles via powder metallurgy method. The effect of heat treatment conditions on artificial aging kinetics was investigated. The solution treatment of the composite sample and the unreinforced alloy was carried out at 495 °C for 1, 2 and 3 h followed by aging at 191 °C for various aging times between 1 and 10 h. The existence of SiC particles led to increasing the peak hardness of the alloy. The peak hardness of the composite sample took place at shorter times than that of the unreinforced alloy for the samples solution treated for 2 and 3 h, but took place at longer times for the samples solution treated for 1 h. The suitable solution treating time was about 2 h for both the composite and the unreinforced alloy that led to the fastest aging kinetics and the maximum hardness. At the solution treating time shorter than 2 h due to incomplete dissolution of precipitates, the aging kinetics decelerated and the hardness values decreased. X-ray diffraction studies indicated the presence of precipitation phases such as CuAl₂ and CuMgAl₂ in the composite in both as-extruded and solutionized conditions. For the samples solution treated more than 2 h, hardness values decreased due to the grain growth of matrix but no change occurred in the aging kinetics.     

Formation of nano-grained structure in a 301 stainless steel using a repetitive thermo-mechanical treatment

In this work effects of the thermo-mechanical parameters were investigated in order to achieve nanocrystalline structure in the as-cast AISI 301 stainless steel. In order to get nanocrystalline structure the repetitive cold rolling and subsequent annealing were used. The cold rolling was carried out at temperatures of 0, - 10 and - 196 °C with strain rate of 0.5 s^− 1 and reduction of 95%, while the annealing treatment was conducted at temperature 600 to 850 °C for 0.5 to 50 min. The results showed that the nanocrystalline austenitic structure with grain size of about 30-40 nm was obtained by annealing at 850 °C for 0.5 min after totally 95% cold rolling reduction at - 10 °C.     

Microstructure and properties of austenitic stainless steel reinforced with in situ TiC particulate

Austenitic stainless steel reinforced with 5 vol.% TiC particulate was in situ synthesized by in situ reaction during melting process successfully and its microstructure, mechanical properties as well as oxidation behavior were investigated. Microstructure observations revealed that in situ TiC particulates with an average size of 2–10 μm distributed uniformly in the matrix and the interface boundaries between TiC particulates and austenite matrix were clean without any impurities and contaminations. Addition of TiC particulates refined the grain structure of austenitic matrix, but did not cause formation of any new phases in microstructure. Beneficial effects of TiC addition to austenitic stainless steel on both mechanical properties and oxidation resistance were found. Both at ambient and elevated temperature, tensile strengths of the steel with TiC addition were notably higher than those of its matrix alloy, however, a decrease in ductility also appeared, as exhibited by other particulate reinforced alloys. Besides tensile strengths, creep resistance of austenitic stainless steel was also significantly increased by TiC addition at elevated temperature of 923 K. Oxidation test at 1073 K revealed that TiC addition to austenitic stainless steel raised the oxidation resistance of the steel remarkably.     

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys

Effects of high-temperature ageing on the creep-rupture properties of cobalt-base L-605 alloys were investigated at 1089 and 1311 K in air. The specimens with serrated grain boundaries and those with normal straight grain boundaries were aged for 1080ksec at 1273 or 1323 K to cause the matrix precipitates of tungsten-rich b c c phase and M₆C carbide. The creep-rupture strength of both specimens were improved by the high-temperature ageing. The rupture strength at 1311 K was the highest in the specimens with serrated grain boundaries aged at 1273 K, while the specimens with straight grain boundaries aged at 1273 K of the highest matrix hardness had the highest rupture strength at 1089 K. The high-temperature ageing did not decrease the rupture ductility of specimens. The ruptured specimens with serrated grain boundaries exhibited a ductile grain-boundary fracture surface which consisted of dimple patterns and steps, regardless of whether high-temperature ageing was carried out. The fracture mode of the specimens with straight grain boundaries was changed from the brittle grainboundary fracture to the ductile one similar to that of the specimens with serrated grain boundaries by high-temperature ageing, since large grain-boundary precipitates which gave nucleation sites of dimples were formed during the ageing. The grain-boundary cracks initiated in the early stage of creep (transient creep regime) in both non-aged and aged specimens of L-605 alloys in creep at 1089 and 1311 K, although the time to crack initiation is shorter in the specimens with straight grain boundaries than in those with serrated grain boundaries. Thus, the period of crack growth and linkage occupied most of the rupture life. The strengthening mechanisms of the aged specimens were also discussed.     

Hot tensile properties and microstructural evolution of as cast NiTi and NiTiCu shape memory alloys

Hot tensile properties of as cast NiTi and NiTiCu shape memory alloys were investigated by hot tensile test at temperature range of 700–1100 °C using the strain rate of 0.1 s⁻¹. The NiTi alloy exhibited a maximum hot ductility at temperature range of 750–1000 °C, while the NiTiCu alloy showed it at temperature range of 800–1000 °C. It was found that at temperatures less than 750 °C, diffusion-assisted deformation mechanism was inactive leading to semi-brittle type of failure and limited ductility in both alloys. Also it was found that at temperature range of 800–1000 °C, dynamic recrystallization is dominant leading to high ductility. Likewise, the fracture surface of the specimens presenting the maximum hot ductility showed an ideal type of ductile rupture in which they gradually pulled out to a fine point. On the other hand, the decline in ductility occurred at the temperatures above 1000 °C was attributed to the liquid phase formation leading to interdendritic and intergranular type of fracture.     

Static recovery of tempered lath martensite microstructures during long-term aging in 9-12% Cr heat resistant steels

Static recovery of tempered lath martensite microstructures of strength enhanced ferritic steels has been investigated during very long-term aging up to 2 × 10⁴ h at 650 °C for 3 steels containing 9 to 12% Cr. Static recovery of tempered lath martensite microstructure occurs after an incubation period for 1-2 × 10³ h in the steels. The static recovery is controlled by the loss of strengthening due to M₂₃C₆ precipitates, and disappearance of MX carbonitrides cannot be the main cause of the static recovery.     

Influence of HIP processing on microstructure and mechanical properties of superalloy Udimet 720LI

Abstract

Inert gas (high purity argon) atomised powder of composition conforming to that of the superalloy Udimet 720 of low interstitial grade was hipped at 1200°C/120 MPa/3 h. The hipped alloy has shown near theoretical density and consisted of equiaxed grains with an average diameter of ～45 μm. While primary γ′-Ni₃ (Ti, Al) precipitates with an acicular morphology were found at the grain boundaries, finer secondary γ′ precipitates with near cuboidal morphology were present in the austenite γ matrix. The yield strength (YS) of the as hipped alloy was found to be the same as that of the wrought alloy heat treated for creep applications (termed as creep resistant alloy) at room temperature (RT) as well as at 650°C. However, the ultimate tensile strength (UTS) and ductility were found to be higher than those of the wrought creep resistant alloy. On the other hand, the YS and UTS of the as hipped material were lower than those of the wrought alloy heat treated for high strength applications (termed as high strength alloy), although the ductility of the former was comparable to that of the latter. In order to improve the strength, the hipped alloy was subjected to a heat treatment consisting of solution treatment followed by two-step aging. Extensive precipitation of fine and coarse γ′ precipitates with cuboidal morphology during duplex aging treatment has led to a considerable improvement in YS and UTS of the alloy, although the corresponding ductility dropped moderately at RT and at 650°C. Fractography of the tensile tested specimens has shown ductile transgranular mode of fracture in the as hipped alloy at RT and at 650°C, while the hipped+heat treated alloy exhibited a mixed mode of fracture at those temperature. The stress rupture properties of the as hipped alloy compare well with those of the wrought alloy and have been found to improve significantly after heat treatment. The present investigation reveals that the hipped superalloy Udimet 720LI has substantial potential for use in the development of near net shaped components for aerospace applications.     

Enhanced mechanical properties in an Al-Cu-Mg-Ag alloy by duplex aging

A type of duplex aging heat treatment was developed to improve the mechanical properties at room temperature and elevated temperatures in a pre-strained Al-Cu-Mg-Ag alloy. In contrast to the conventional T8 temper at 165 °C and 200 °C, the hardening response of the alloy to aging was increased by duplex aging treatment, the ultimate tensile strength and yield strength of duplex aging temper were improved by approximately 3-7%, which was attributed to the fact that the recovery of dislocations occurred and the precipitation of θ′ phase was restrained effectively at high aging temperature, and more Ω precipitates were formed during secondary aging.     

Production of nano/ultrafine grained AISI 201L stainless steel through advanced thermo-mechanical treatment

In recent years, great attention has been focused on the development of nanostructured stainless steels to improve their mechanical properties. This work reports the formation of nano/ultrafine grain structure in the AISI 201L stainless steel using advanced thermo-mechanical treatment. The cast specimens were first homogenized and then hot forged to provide a suitable microstructure prior to the treatment. Cold rolling was carried out with the reductions of 10-95% followed by annealing at temperature of 850 °C for 15-1800 s. X-ray diffraction, optical and scanning electron microscopy, and tensile and hardness tests were used to characterize the processed specimens. The results showed that the nanocrystalline austenitic structure with a grain size of about 65 nm was obtained by annealing at 850 °C for 30 s after the cold reduction of 95%. The yield strength, total elongation and hardness of this specimen were measured as 1485 MPa, 33% and 386 Vickers, respectively.     

Characterization of the mechanical properties changes in an Al–Zn–Mg alloy after a two-step ageing treatment at 70° and 135 °C

Fine-scale precipitation of the η′ phase and its precursors are essential for the mechanical properties of Al-4.6 wt%Zn-1.2 wt%Mg alloy. This paper deals with an investigation of precipitation in an industrial Al–Zn–Mg alloy at various stages of a conventional two-step ageing treatment at 70 °C and 135 °C. The effect of microstructure on the mechanical properties was performed using microhardness and tensile tests, together with optical, scanning and transmission electron microscopy. After ageing at 135 °C, corresponding to the maximum value of hardness, small η′ precipitates were observed in the alloy matrix. After two-step ageing at 70 °C plus at 135 °C, the volume fraction of this precipitate becomes higher. Consequently, the yield strength of the material increases and it maintains its ductility. This high precipitate density slows the dislocation movement and thus a higher stress was required for its bowing.