Room Temperature Elongation Research Articles

Effect of thermal exposure on microstructure and mechanical properties of Ti65 high-temperature titanium alloy deposited by laser direct energy deposition

Industrial design and new product development benefit from the adaptable processing approach offered by laser direct energy deposition (LDED). In this paper, a novel Ti65 alloy was successfully prepared by LDED and subjected to thermal exposure at 650 °C and 750 °C, respectively. The results showed that the as-deposited microstructure varies in different regions due to the cooling rate. Silicide precipitation was observed at the α/β interface, and a coherent α2 phase was generated in the matrix after thermal exposure at 650 °C. Two-scale silicides were observed at the α/β interface and inside the α laths after thermal exposure at 750 °C. The as-deposited ultimate tensile strength (UTS) was 1058 MPa and 690 MPa at room temperature and 650 °C, respectively. After thermal exposure at 650 °C, the room temperature UTS was 1135 MPa, and the elongation (EL) at 650 °C was 27.5 %. After thermal exposure at 750 °C, the room temperature EL was 11.05 %, and both UTS and EL at 650 °C showed a decrease. The quantitative analysis showed that silicide precipitation was primarily responsible for the strengthening mechanism of the specimens thermally exposed at 650 °C at room temperature. The results provide scientific data for the manufacture of Ti65 alloy with excellent mechanical properties, laying the foundation for its application in aerospace.

Effect of La2O3 on the properties of molybdenum rhenium alloy

Adding rhenium to molybdenum can improve room temperature processing performance and reduce the ductile brittle transition temperature. Adding lanthanum oxide to molybdenum can refine grain size and improve strength. This work analyzes the effect of adding different amounts of lanthanum oxide to molybdenum rhenium alloys on their properties. The results indicated that with the increase of lanthanum oxide, the tensile strength increased, but the room temperature elongation only increased at 0.6%; as the temperature increases, the trend of tensile strength and elongation at 900 °C and 1100 °C is similar; at 1300 °C, the strength decreases significantly but the elongation increases sharply; the corresponding changes can be explained by the fracture morphology.

The Effect of Replacing Ni with Mn on the Microstructure and Properties of Al2O3-Forming Austenitic Stainless Steels: A Review.

Al2O3-forming austenitic steel (AFA steel) is an important candidate material for advanced reactor core components due to its excellent corrosion resistance and high temperature strength. Al is a strong ferrite-forming element. Therefore, it is necessary to increase the Ni content to stabilize austenite. Ni is expensive and highly active, and so increasing the Ni content not only increases the costs but also damages the radiation resistance. Mn is a low-cost austenitic stable element. Its substitution for Ni will not only help to improve the irradiation resistance of austenitic steel, but also reduce the cost. In order to explore the feasibility of Mn-substituted Ni-stabilized austenite in AFA steel, this paper summarized the research progress of Mn-added AFA steels, whilst the research status of traditional Mn-added austenitic steels are also referred to and compared herein. The effect of the addition of Mn on the microstructure and properties of AFA steel was analyzed. The results show that Mn can promote the precipitation of the M23C6 phase and inhibit the precipitation of the B2-NiAl phase and secondary NbC phase. With the increase in Mn content, the strength of AFA steel at room temperature and high temperature decreased slightly, the room temperature elongation increased slightly, while the high temperature elongation and creep resistance decreased obviously. In addition, for austenitic steel free of Al, the addition of Mn will destroy the oxide layer of Cr2O3, which will decrease the oxidation resistance of the steel. But the preliminary study shows that Mn has little effect on the Al2O3 oxide layer. It is worth studying the effect of Mn-substituted Ni on the oxidation resistance of AFA steel. In summary, more efforts are necessary to investigate the optimal Mn content to balance the advantages and disadvantages of introducing Mn instead of Ni.

Microstructure evolution and properties of powder metallurgy Ti43Al9V0.3Y alloy sheets at different rolling temperatures

TiAl alloy is a new type of lightweight high temperature structural material with excellent properties. Its room temperature brittleness can be improved by refining the microstructure and adjusting the composition. In this study, the pre-alloyed powder was sintered and densified by hot isostatic pressing, and TiAl alloy sheets were prepared using hot pack rolling. The present work establishes a fundamental understanding of the microstructure and properties during hot rolling. An ultrahigh ductility TiAl alloy was prepared by a combination of introduction of twins, purifying matrix and high sphericity grains. The room temperature elongation of the TiAl sheet is 3.0% and the elongation reaches 65% at 700 °C. The high uniform malleability might be due to several reasons: (i) Some twins are formed during the rolling deformation, in the process of dislocation slip, twins can adjust the crystal orientation to facilitate the continued dislocation slip. In this way, twinning slightly improves the room temperature plasticity of TiAl alloys; (ii) Y purifies matrix of the TiAl alloy; (iii) the grain has the high sphericity, and the spherical grain could minimize the stress concentration.

Surface-Roughness-Induced Plasticity in a Biodegradable Zn Alloy.

Improving plasticity has been an eternal theme of developing metallic materials. It is difficult to increase room-temperature elongation of metallic materials over 100% without sacrificing strength using existing methods. Herein, surface-roughness-induced plasticity (SRIP) is discovered in biodegradable Zn-0.4Mn alloy. Surprisingly, in the good surface range that meets the international standard ISO 6892, reducing surface roughness results in significant increase in plasticity without loss of strength. From unground to 5000# sandpaper ground states, the surface roughness Ra of the alloy decreases from 0.63 to 0.05µm, while its room temperature elongation increases from 74% to 143%. SRIP is the synergistic result of increased microstructure damage tolerance and decreased surface roughness. It provides a new method for improving plasticity.

Effect of B2 ordering on the tensile mechanical properties of refractory AlxNb40Ti40V20−x medium-entropy alloys

Most of refractory high/medium-entropy alloys show promising compressive mechanical properties at elevated temperatures. However, scarce tensile testing data hinder the estimation of the applicability of these alloys for potential high-temperature service. It is also unclear how an ordered structure, which improves high-temperature compressive strength, will affect the tensile performance. This work demonstrated that the effect of B2 ordering on the tensile mechanical properties of new refractory AlxNb40Ti40V20−x (x = 0; 15; 20 at%) medium-entropy alloys was manifold, and it depended on the chemical composition. In an Al15Nb40Ti40V5 alloy having a weak degree of B2 ordering, large uniform room-temperature elongation and moderate high-temperature strength, comparable with a bcc Nb40Ti40V20 alloy, were found. An Al20Nb40Ti40 alloy with the high degree of B2 ordering was brittle at room-temperature and sensitive to testing environment at elevated temperatures. Meantime, it softened notably slower up to 0.4 Tm, albeit losing strength at> 0.4 Tm faster than the bcc Nb40Ti40V20 or weakly B2-ordered Al15Nb40Ti40V5 alloys. Factors responsible for the resulted mechanical properties were thoroughly discussed.

Thermal stability and mechanical properties of Ti–22Al–25Nb alloy with different initial microstructures

Abstract In order to better understand the thermal stability and mechanical properties of the nominal Ti–22Al–25Nb alloy with different initial microstructures, microstructural evolution after long-term aging, and mechanical properties at room temperature and 650 °C of four differently heat-treated Ti–22Al–25Nb alloy were investigated. The investigation results indicated that α2 platelets formed by aging at (α2+B2) region were not as stable as previous research claimed and spontaneously decomposed into fine (O+β/B2) mixture during the annealing at 650 °C. As a result, the alloy's steady-state creep strain rate dropped dramatically at the secondary creep stage and tensile strength grew. On the contrary, long time aging exposure resulted in the connection of original dispersed lenticular O precipitation and thus degraded the ductility of all specimens. Besides, the discontinuous precipitation B2→β+O caused by supersaturation of solutes in the matrix also harmed the mechanical properties. Under the condition of 650 °C/250 MPa, the steady-state creep strain rates of the Ti–22Al–25Nb specimens show a significant dependence on their grain size. Crept samples showed that most dislocation motion was restrained in the β channels under a small strain and then extended into O phase when creep strain grew over 10%. Finally, by comparing the room temperature tensile property and creep resistance of four samples, supertransus solution and furnace cooling ensured the alloy most balanced mechanical properties among lamellar samples. The supertransus then furnace-cooled sample did not only have an excellent creep resistance but also kept a moderate room temperature elongation to failure of nearly 6%.

Influences of thermal exposure on the microstructural evolution and subsequent mechanical properties of a near-α high temperature titanium alloy

The microstructural evolution with respect to the precipitation of silicides and ordered α2 phase in near-α TA29 Ti alloy during thermal exposure at 650 °C for different time has been investigated in this work, and the subsequent mechanical properties have been evaluated by tensile and fracture toughness tests. The results indicated that the (Ti, Zr)6Si3 silicides firstly precipitated in residual β films, and then began to precipitate within α plates. A large number of fine α2 precipitates showing sphercial shape concurrently formed in the α plates at a short time. The ordered α2 precipitates asymmetrically grew up along different crystallographic directions with increasing exposure time, leading to their spherical shape change to ellipsoidal shape and finally olive shape, in which the long axis was perpendicular to [0001]α as viewed along [2‾110]α direction. The number density of ordered α2 was obviously decreased after 1000 h due to Ostwald ripening. The precipitation of silicides and ordered α2 phase improved the tensile strength but deteriorated the ductility and fracture toughness, whereas the strength was decreased and the room temperature elongation was recovered slightly after long-term exposure because of α2 ripening.

Mechanical properties and microstructure of 9Cr-ODS-CLF-1 steel

A kind of oxide dispersion strengthened (ODS) steel with addition of 0.3 % Y2O3 and 0.3 % Ti was manufactured via mechanical alloying (MA) process from CLF-1 steel raw powder that was prepared by Ar gas atomization. This 10 kg ingot of 9Cr-ODS-CLF-1 steel was obtained by hot isostatic pressing (HIP) and subsequent forging. The as-HIPed and forged 9Cr-ODS-CLF-1 steel exhibits high Vicker hardness to 384 Hv which is much higher than that of CLF-1 steel (224 Hv). The ultimate tensile strength (UTS) is about 1100 MPa with elongation of 7 % at room temperature (RT). After 90 min tempering at 1013 K, the RT elongation increased to 11 % and the ultimate tensile strength decreased to 670 MPa. The creep results showed that the minimum strain rate for 9Cr-ODS-CLF-1 steel as received and 1013 K tempered state tested at 873 K under 160 MPa were low as 1.6 × 10-8 /s and 8.42 × 10-9 /s, respectively. Typical small dispersion particles were observed in the matrix. The grains showed inhomogeneous size distribution for both the as received and the 1013 K tempering state. The method of mechanical alloying combined with HIP proved to be an effective way to strengthen CLF-1 steel in large scale.

Effect of Heat Treatment on Microstructures and Mechanical Properties of a Novel β-Solidifying TiAl Alloy.

The effect of heat treatment on the microstructures and mechanical properties of a novel β-solidifying Ti–43Al–2Cr–2Mn–0.2Y alloy was investigated. A fully lamellar (FL) microstructure with a colony size of about 100 μm was obtained by heat treatment at 1320 °C/10 min/furnace cooling (FC). A duplex (DP) microstructure with globular γ grains and γ/α2 lamellae was obtained by heat treatment at 1250 °C/4 h/FC. The residual hard–brittle β0 phase was also eliminated after heat treatment. The mechanical properties of the β-solidifying TiAl alloy depended closely on the heat treatment. The FL alloy had better fracture toughness, and the fracture toughness (KIC) value was 24.15 MPa·m1/2. The DP alloy exhibited better ductility, and the room temperature (RT) elongation of the alloy could reach 1%. The elongation of the alloy with different microstructures sharply increased when the temperature increased from 700 to 750 °C, indicating that the microstructure had no effect on the ductile–brittle transition temperature of the β-solidifying TiAl alloy. The fracture morphologies of different tensile specimens were observed. Interlamellar and translamellar fractures were the main fracture features of the FL alloy. Intergranular, translamellar, and interlamellar fractures were the main fracture features of the DP alloy.

The effects of carbon on the phase stability and mechanical properties of heat-treated FeNiMnCrAl high entropy alloys

This work systematically investigates the effect of carbon on the phase stability and room- temperature tensile performance of an annealed Fe40.4Ni11.3Mn34.8Al7.5Cr6 (at%) high entropy alloy without (HEA) and with 1.1% carbon (CHEA). Four annealing conditions were investigated: 773 K for 13 d and 42 d, 973 K for 20 d, and 1073 K and 1423 K for 24 h. The resulting microstructures were analyzed using a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive spectroscopy (EDS), and atom probe tomography (APT). Vickers hardness testing and room- temperature tensile testing were used to determine the mechanical properties of the annealed HEA and the CHEA. APT composition profiles revealed fine Ni,Mn,Al-enriched matrix precipitates in the CHEA annealed at 773 K for 13 d. After ageing at 773 K for 42 d, colonies of (Ni,Fe)2MnAl- enriched Heusler phase lamellae were observed at the grain boundaries (GBs) and in the matrix for the HEA, while GB lamellar colonies of Mn,Cr-enriched M23C6 carbides were observed for the CHEA. Due to the presence of the GB carbides, the resulting room-temperature elongation to fracture for the CHEA annealed at 773 K for 42 d was ~1% compared to ~11% for the HEA given the same anneal. At higher annealing temperatures, the microstructure contains Ni,Al-rich and Mn,Cr,C-rich precipitates that alternate along the GBs and appear to be associated with each other in the matrix. Electron diffraction analysis indicates the aforementioned Ni,Al-precipitates and Mn,Cr-carbides have b.c.c. and f.c.c. crystal structures, respectively. Once again, a low elongation to fracture (2%) was seen for the carbon-containing material compared to its un-doped counterpart (23%) which solely contained Ni,Al-rich b.c.c. precipitates.

Effects of Heat Treatment on Unique Layered Microstructure and Tensile Properties of TiAl Fabricated by Electron Beam Melting

In the present study, effects of heat treatment on microstructures and tensile properties of the cylindrical bars of Ti-48Al-2Cr-2Nb (at.%) alloy with unique layered microstructure consisting of equiaxed γ grains region (γ band) and duplex-like region fabricated by electron beam melting (EBM) were investigated. We found that it is possible to control width of the γ bands (Wγ) by heat treatments at 1100°C and 1190°C. The Wγ increases with decreasing heat treatment temperature. The bars heat-treated at 1190°C exhibit high elongation of 2.9% at room temperature (RT) with maintaining high strength. The RT elongation increases with increasing the Wγ because of increasing deformable regions. In contrast, the RT elongation of the bars decreases with increasing the Wγ when Wγ is very large. This is because the large γ band leads intergranular fracture. These results indicate that there is appropriate width for the γ band to obtain excellent tensile properties at RT.

Optimization of RT superplasticity of UFG Zn-22Al alloy by applying ECAP at different temperatures and phase regions

Zn–22Al alloy was subjected to either one-step or two-step equal channel pressing (ECAP) to investigate the effect of processing temperature on its microstructure and room temperature (RT) superplasticity. In one-step ECAP processes, 4 passes ECAP were applied to the alloy at different temperatures: RT, 100°C and 250°C in two-phase region below eutectoid temperature and 350°C in single-phase region above eutectoid temperature. In two-step ECAP processes, one-step ECAP-processed samples were subjected to four more passes ECAP at RT. Considering the one-step ECAP processing, RT superplasticity increased with decreasing ECAP temperature as expected, and the highest RT superplasticity was achieved as 350% after 4 passes ECAP at RT. On the other hand, application of 4 more passes ECAP at RT to the sample showing the lowest superplastic elongation after one-step ECAP (the sample processed at 350°C) resulted in the maximum RT elongation of 400% at a high strain rate of 5×10−2 s−1. These results suggest that first step temperature of two-step ECAP process is needed to increase above the eutectoid point of Zn-22Al alloy to achieve high RT superplasticity. These results were attributed to the changes in microstructure inside the single-phase and two-phase regions during the processes.

Tensile behavior of Ti-22Al-24Nb-0.5Mo in the range 25–650 °C

The tensile and fracture behavior of an orthorhombic alloy Ti-22Al-24Nb-0.5Mo (at%) with three types of microstructure were investigated in the temperature range between 25°C and 650°C. The O+bcc microstructure (basketweave lath type) possesses an elongation maximum at 500°C and its drop at 650°C has been attributed to dynamic strain aging as suggested by previous investigators. The α2+bcc microstructure has the highest room temperature elongation (εf >18%) and the cause of the elongation drop at 650°C is suggested to be the fine O phase precipitates from the bcc phase. An optimal balance between strength and plasticity was achieved in the α2+O+bcc microstructure.

Fabrication of in situ Ti5Si3/TiAl composites with controlled quasi-network architecture using reactive infiltration

Fully dense Ti5Si3/TiAl composites were successfully in situ synthesized by reactive infiltration using Ti powders and Al–Si alloys as raw materials. Fine Ti5Si3 particles exhibit a unique quasi-network distribution, remarkably hindering the coarsening of α2/γ lamellar colonies. Compared to the as-cast samples, the mean size of lamellar colonies is much finer and thus the resulting Ti5Si3/TiAl composites show the enhanced room temperature elongation, up to 2.5%, while retaining the ultimate tensile strength of 414MPa. Moreover, Ti5Si3/TiAl composites display superior high temperature tensile properties. Consequently, the novel Ti5Si3/TiAl composites have potential for high temperature applications in aeronautics and aerospace.

Microstructural characterization and mechanical properties of TiC/near-α Ti composite obtained at slow cooling rate

In this paper, microstructural characterization and mechanical properties of 10vol% TiC/Ti-6Al-3Sn-3.5Zr-0.4Mo-0.75Nb-0.35Si composite obtained at slow cooling rate has been discussed. Equiaxed or near-equiaxed α phase was formed in the composite when cooled from above the β-transus temperature at a cooling rate of 10°C/min. This is due to the fact that α precipitates can heterogeneously nucleate on TiC particles during the slow-cooling process. It was obtained that the lattice disregistry between TiC and α precipitates was only 3.8%, which corresponds to the crystallographic orientation relationship of [2-1-10]α//[011]TiC; (0001)α//(1-11)TiC. The tensile results showed that heat treatment improved UTS and elongation, simultaneously, below 700°C. The enhancement of UTS is mainly attributed to solid solution strengthening of C in matrix since about 5.5vol% TiC dissolved into matrix after heat treatment. The obvious increase in room-temperature elongation is because of the formation of equiaxed microstructure and the reduction in the sizes of spheroidised TiC particles. With the increase of temperature, the discrepancy in UTSs of the as-cast and heat-treated composites became small.

Superior ductility in as-cast TiC/near-α Ti composite obtained by three-step heat treatment

In this paper, three-step heat treatment was developed and performed on 8vol.%TiC/Ti–5.8Al–2Zr–1.3Mo–1V composite prepared by in situ casting route. This heat treatment leads to significant changes in microstructure and tensile properties. Matrix microstructure was refined obviously and fine TiC particles with average diameter of 3.6 μm were obtained. The tensile strength and elongation are all improved at room temperature and 600 °C and room-temperature elongation increases to 5.86%.

Fabrication of a powder metallurgy Ti2AlNb-based alloy by spark plasma sintering and associated microstructure optimization

This study deals with preparing powder metallurgy Ti2AlNb-based alloy via spark plasma sintering (SPS) technique. A pre-alloyed Ti2AlNb-based alloy powder was produced by a plasma rotating electrode process (PREP), and then consolidated using SPS at temperatures between 900°C and 1050°C with a pressure of 50MPa. Most of the pre-alloyed powder particles are characterized by single B2-phase dendrites. When the SPS temperature was 1000°C or higher, the dendritic features and original boundaries of powder particles were removed, resulting in fully dense compacts. The as-sintered microstructure was modified by heat treatment to form fine acicular O-precipitates, leading to a superior yield strength of up to 1220MPa at room temperature (RT). On the other hand, the heat treated microstructure with coarsened O-precipitates, produced via controlled slow cooling within the B2+O phase field, exhibits a good RT elongation to fracture of 9.5%, but a relatively low yield strength of 840MPa.

Effect of intermediate annealing on the microstructure and mechanical property of ZK60 magnesium alloy produced by twin roll casting and hot rolling

Twin roll cast (designated as TRC in short) ZK60 magnesium alloy strip with 3.5mm thickness was used in this paper. The TRC ZK60 strip was multi-pass rolled at different temperatures, intermediate annealing heat treatment was performed when the thickness of the strip changed from 3.5mm to 1mm, and then continued to be rolled until the thickness reached to 0.5mm. The effect of intermediate annealing during rolling process on microstructure, texture and room temperature mechanical properties of TRC ZK60 strip was studied by using OM, TEM, XRD and electronic universal testing machine. The introduction of intermediate annealing can contribute to recrystallization in the ZK60 sheet which was greatly deformed, and help to reduce the stress concentration generated in the rolling process. Microstructure uniformity and mechanical properties of the ZK60 alloy sheet were also improved; in particular, the room temperature elongation was greatly improved. When the TRC ZK60 strip was rolled at 300°C and 350°C, the room temperature elongation of the rolled sheet with 0.5mm thickness which was intermediate annealed during the rolling process was increased by 95% and 72% than that of no intermediate annealing, respectively.

The effect of aging on the microstructure and mechanical behavior of the alumina-forming austenitic stainless steel Fe–20Cr–30Ni–2Nb–5Al

The effect of aging on the microstructure and mechanical behavior of an alumina-forming austenitic stainless steel, Fe–20Cr–30Ni–2Nb–5Al (at%) has been investigated. The alloy was fully solutionized after a 1250°C, 24h heat treatment, and the precipitation of B2 and Laves phases was studied after aging at 800°C for up to 1325h. While after 24h the Laves phase precipitates in the matrix were 205nm in diameter, they showed a further increase in diameter of only 50nm even after aging for 1325h. In contrast, the B2 precipitates in the matrix grew at a faster rate: after first being observed after aging for 24h at an average diameter of 194nm, they more than doubled in size from 330 to 734nm as the aging time increased from 240h to 1325h. Both the Laves and B2 precipitates in the grain boundaries grew at a faster rate and were larger than matrix precipitates. The grain boundary coverage at 2.4h (Laves 192nm, NiAl 126nm) was 56% with Laves phase initially making up the bulk of the precipitates, but after 2.4h Laves phase and B2 precipitates alternated on the grain boundaries and total coverage reached 93% after 1325h. An increase in the volume fraction of precipitates in the alloy was accompanied by an increase in the yield strength from 205MPa after the solutionizing treatment up to 383MPa after aging at 800°C for 1325h. After aging for 1325h, even with extensive intermetallic grain boundary coverage, the alloy showed a room temperature elongation of 19%.