等轴细晶Ti-45Al-7Nb合金的高温机械性能及变形机制

采用粉末冶金法制备了双相等轴细晶Ti-45Al-7Nb（原子分数）合金,研究了该合金在温度为900、950和1000 ℃以及应变速率为1×10^-3、1×10^-4和5×10^-5 s^-1条件下的高温力学性能,并讨论了相应的变形机理。结果表明,在高温或低应变率下,Ti-45Al-7Nb合金的极限拉伸强度逐渐降低,但伸长率显著增加。由于细小晶粒容易实现变形和协调,其伸长率明显高于粗晶粒合金。高温拉伸后,合金在裂缝处形成大量的空洞,并在裂缝前部形成大量垂直于拉伸方向的长裂纹。此外,晶界的滑动、晶粒的孪生和动态再结晶也导致了合金变形,从而提高了微观组织的延展性。     

Environmental embrittlement of two-phase Fe30Ni20Mn35Al15

It is shown that the ductility of lamellae-structured Fe₃₀Ni₂₀Mn₃₅Al₁₅ (in at. %), which consists of B2 and f.c.c. phases, is influenced by testing environment. Tensile tests performed in air at strain rates ranging from 3 × 10^?6 to 3 × 10^?1 s^?1 showed that the elongation to fracture and ultimate tensile strength (UTS) increased with increasing strain rates below 3 × 10^?3 s^?1, and were independent of strain rate at ～10.5% and 840 MPa for strain rates ≥ 3 × 10^?3 s^?1. In order to understand this strain-rate sensitive behavior, tensile tests were also performed in either dry oxygen or 4% hydrogen + nitrogen at different strain rates. The elongation and UTS in oxygen were insensitive to strain rate and close to those tested at 3 × 10^?3 s^?1 in air, whereas the elongation in hydrogen was 4% for strain rates ≤3 × 10^?3 s^?1 and increased to ～10.8% at 3 × 10^?1 s^?1. The reduction of ductility in air and hydrogen-charged environment at low strain rate is attributed to hydrogen embrittlement.     

Temperature-Dependent Flow Behavior and Microstructural Evolution During Compression of As-Cast Mg-7.7Al-0.4Zn

The microstructure and mechanical properties improve substantially by hot working. This aspect in as-cast Mg-7.7Al-0.4Zn (AZ80) alloy is investigated by compression tests over temperature range of 30-439°C and at strain rates of 5 × 10^?2, 10^?2, 5 × 10^?4 and 10^?4 s^?1. The stress exponent (n) and activation energy (Q) were evaluated and analyzed for high-temperature deformation along with the microstructures. Upon deformation to a true strain of 0.80, which corresponds to the pseudo-steady-state condition, n and Q were found to be 5 and 151 kJ/mol, respectively. This suggests the dislocation climb-controlled mechanism for deformation. Prior to attaining the pseudo-steady-state condition, the stress-strain curves of AZ80 Mg alloy exhibit flow hardening followed by flow softening depending on the test temperature and strain rate. The microstructures obtained upon deformation revealed dissolution of Mg₁₇Al₁₂ particles with concurrent grain growth of α-matrix. The parameters like strain rate sensitivity and activation energy were analyzed for describing the microstructure evolution also as a function of strain rate and temperature. This exhibited similar trend as seen for deformation per se. Thus, the mechanisms for deformation and microstructure evolution are suggested to be interdependent.     

A Preliminary Study of Deformation Behavior of Friction Stir Welded Ti-6Al-4V

A preliminary study of deformation behavior of friction stir welded (FSW) Ti-6Al-4V was performed using two different tools with cylindrical and stepped spiral pin design for the welding process. The nugget regions experienced temperature above β transus and the matrix transformed to fine acicular α during cooling of the nugget. By using stepped spiral pin design, a local region with much refined grain structure and significant tool debris particles were observed. Room temperature tensile test showed increased strength and decreased ductility in the material from this region. Fractographic analysis revealed that tool debris particles served as void nucleation sites. Tensile tests of FSW material were carried out at 625 °C in the strain rates of 3 × 10^?4 and 1 × 10^?3 s^?1. The strength was higher as compared to the as-received material. Microstructural evolution during tensile test was also investigated. Results showed that dynamic globularization occurred during the high temperature tensile test.     

Research on the Hot Deformation Behavior of Al-Zn-Mg-Sc-Zr Alloy During Compression at Elevated Temperature

The flow behavior of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation was studied by isothermal compression test using Gleeble-1500 thermo-mechanical equipment. Compression tests were performed in the temperature range of 340-500 °C and in the strain rate range of 0.001-10 s^?1.The results indicate that the flow stress of the alloy increases with increasing strain rate at a given temperature, and decreases with increasing temperature at a given imposed strain rate. The relationship between flow stress and strain rate and temperature was derived by analyzing the experimental data. The constitutive equation of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation can be described by the Arrhenius relationship of the hyperbolic sine form. The values of A, n, and α in the analytical expression of strain rate are fitted to be 1.49 × 10¹⁰ s^?1, 7.504, and 0.0114 MPa^?1, respectively. The hot deformation activation energy of the alloy during compression is 150.25 kJ/mol. The temperature and strain rate have great influences on microstructure evolution of Al-Zn-Mg-Sc-Zr alloy during hot compression deformation. According to microstructure evolution, the dynamic flow softening is mainly caused by dynamic recovery and dynamic recrystallization in this present experiment.     

The hot formability of an Al-Cu-Mg-Fe-Ni forging disk

The high temperature formability of AA2618-T61 forged disk was studied by means of tensile test over temperatures and strain rates ranging from 100 to 400°C and 3 × 10⁻⁵ −3 × 10⁻³ s⁻¹, respectively. The constitutive equations of the material were calculated based on an Arrhenius-type equation and the ductility of the material was evaluated considering elongation and percent reduction of area. The results showed that both kinds of softening mechanisms, dynamic recovery and dynamic recrystallization, occurred during high temperature deformation of the alloy. Strain rate sensitivity of the material was evaluated in all the deformation conditions and the obtained values were used to calculate the apparent activation energy.     

Tensile ductility of an Fe–40Al–0.6C alloy

Binary Fe–40Al and ternary Fe–40Al–0.6C alloys were cast, hot-extruded into rods, annealed at low temperatures to reduce the non-equilibrium vacancy concentration and tested in uniaxial tension at room temperature in air, over a range of strain rates from 4.2×10⁻¹ s⁻¹ to 4.2×10⁻⁸ s⁻¹. Yield strength, fracture strength, tensile ductility and the work-hardening behavior in the 0.2–1.0% plastic deformation range were monitored. Resulting fracture surfaces were examined at low and high magnifications, and the change in the fraction transgranular cleavage as a function of test strain rate was correlated with the observed mechanical properties. Prior to testing, both alloys exhibited fairly coarse grain size (∼80–100 μm); whereas the binary alloy was single phase, the ternary alloy contained a dispersion of lath-shaped perovskite carbides (Fe₃AlC_0.5) in the grain interior and at grain boundaries. In the binary alloy, ductility decreases continuously with decreasing strain rate and this behavior has been previously attributed to an environmental effect. For a given strain rate, over the range of strain rates examined, the ternary alloy demonstrates improved ductility over the binary alloy; furthermore, at the extremely slow strain rates (<4×10⁻⁷ s⁻¹), the ductility of the ternary alloy increases with decreasing strain rate after reaching a minimum. Whereas in the binary alloy, fracture mode remains completely intergranular over the entire strain rate regime, in the ternary alloy, fracture mode is completely intergranular at the fastest strain rate but gradually transitions to a predominantly transgranular cleavage mode with decreasing strain rate. A maximum in the fraction transgranular cleavage is reached coincident with the ductility minimum, beyond which (i.e. lower strain rates) the fraction transgranular cleavage decreases sharply. These observations are discussed in terms of the possible role of these carbides as hydrogen traps and their consequential effects on mechanical properties.     

A Modified Constitutive Equation for Aluminum Alloy Reinforced by Silicon Carbide Particles at Elevated Temperature

In this paper, the constitutive relationship of an aluminum alloy reinforced by silicon carbide particles is investigated using a new method of double multivariate nonlinear regression (DMNR) in which the strain, strain rate, deformation temperature, and the interaction effect among the strain, strain rate, and deformation temperature are considered. The experimental true stress-strain data were obtained by isothermal hot compression tests on a Gleeble-3500 thermo-mechanical simulator in the temperature range of 623-773 K and the strain rate range of 0.001-10 s^?1. The experiments showed that the material-softening behavior changed with the strain rate, and it changed from dynamic recovery to dynamic recrystallization with an increase in the strain rate. A new constitutive equation has been established by the DMNR; the correlation coefficient (R) and average absolute relative error (AARE) of this model are 0.98 and 7.8%, respectively. To improve the accuracy of the model, separate constitutive relationships were obtained according to the softening behavior. At strain rates of 0.001, 0.01, 0.1, and 1 s^?1, the R and AARE are 0.9865 and 6.0%, respectively; at strain rates of 5 and 10 s^?1, the R and AARE are 0.9860 and 3.0%, respectively. The DMNR gives an accurate and precise evaluation of the flow stress for the aluminum alloy reinforced by silicon carbide particles.     

A Study on the Hot Deformation Behavior of Cast Mg-4Sn-2Ca (TX42) Alloy

The hot deformation behavior of as-cast Mg-4Sn-2Ca (TX42) alloy has been studied using compression tests in the temperature range of 300°C to 500°C, and strain rate range of 0.0003 s^?1 to 10 s^?1. Based on the flow stress data, a processing map has been developed, which exhibited two domains of dynamic recrystallization in the temperature and strain rate ranges: (I) 300°C to 380°C and 0.0003 s^?1 to 0.001 s^?1, and (II) 400°C to 500°C and 0.004 s^?1 to 6 s^?1. While hot working may be conducted in either of these domains, the resulting grain sizes are finer in the first domain than in the second. The apparent activation energy values estimated by kinetic analysis of the temperature and strain rate dependence of flow stress in the domains 1 and 2 are 182 kJ/mol and 179 kJ/mol, respectively. Both the values are much higher than that for self-diffusion in pure magnesium, indicating that the thermally stable CaMgSn particles in the matrix cause significant back stress during the hot deformation of this alloy. The alloy exhibits a regime of flow instability at lower temperatures and higher strain rates, which manifested as flow localization.     

Study on the Hot Workability of SiCp/Al Composites Based on a Critical Strain Map

We used the isothermal compression test (conducted in a Gleeble-3500 system) to study the hot deformation behaviors of SiCp/Al composites over a wide range of temperatures (623-773 K) and strain rates (0.001-10 s^?1). A 3D hot-processing map was constructed based on the Malas stability criteria and experimental data. An artificial neural network model of four hot work quality characteristic parameters (strain rate sensitivity m, its derivative m′, temperature sensitivity s, and its derivative s′) were established. A new hot-processing map, known as a hot-processing critical strain map, has been proposed based on the smallest strain prior to instability. Two optimized processing regions at 623-660 K, 0.05-0.075 s^?1 and 720-773 K, 0.04-0.18 s^?1 were determined based on this map.     

Superplastic behavior of a TiAl-based alloy

Superplastic behavior under the conditions of a temperature range from 850 to 1075°C and strain rates varying from 8×10⁻⁵ to 1×10⁻³ s⁻¹ was investigated for Ti–33Al–3Cr–0.5Mo (wt%) alloy with a very fine grain size obtained by the multi-step thermal mechanical treatment. The results show that the TiAl-based alloy with a hot-deformed fine grain size possesses good superplasticity. It exhibits a strain rate sensitivity coefficient of 0.9 at a strain rate of 3×10⁻⁵ s⁻¹ and temperature from 1000 to 1075°C. Moreover, the strain rate sensitivity coefficient is stable during the hot deformation, and a tensile elongation of 517% was obtained at 1075°C and a strain rate of 8×10⁻⁵ s⁻¹. The superplastic behavior of the present fine-grained TiAl-based alloy can be explained by the local strain hardening and high m value during the tensile deformation. Microstructure evolution in the superplastic deformation was also discussed.     

Effect of Strain Rate on Tensile Properties of Carbon Fiber Epoxy-Impregnated Bundle Composite

The tensile tests for high tensile strength polyacrylonitrile (PAN)-based (T1000GB) carbon fiber epoxy-impregnated bundle composite at various strain rates ranging from 3.33 × 10^?5 to 6.0 × 10² s^?1 (various crosshead speeds ranging from 8.33 × 10^?7 to 1.5 × 10¹ m/s) were investigated. The statistical distributions of the tensile strength were also evaluated. The results clearly demonstrated that the tensile strength of bundle composite slightly increased with an increase in the strain rate (crosshead speed) and the Weibull modulus of tensile strength for the bundle composite decreased with an increase in the strain rate (crosshead speed), there is a linear relation between the Weibull modulus and the average tensile strength on log-log scale.     

Hydrogen embrittlement of Ti–49Al at various strain rates

The mechanical properties of a Ti–49Al alloy were investigated by tensile tests at different strain rates in vacuum, air, or a flowing hydrogen gas, before or after the specimens were exposed to a high temperature hydrogen gas. The results indicate that the strain rate hardly affect the tensile properties in a strain rate range of 5.6×10⁻⁶ to 2×10⁻⁴ s⁻¹. A large amount of the internal hydrogen decreases the ductility more than the external hydrogen. The internal hydrogen induces embrittlement by decreasing the cohesive strength of the lattice, whereas the external hydrogen embrittlement may be attributed to hydrogen-enhanced localized plastic deformation. The presence of a large amount of internal hydrogen increases the width of the stacking fault; that is, it decreases the stacking fault energy.     

Mg-6Zn-1Al-0.3Mn 镁合金的热压缩变形行为及变形组织

采用Gleeble热模拟方法研究Mg?6Zn?1Al?0.3Mn 变形镁合金在温度为200~400°C,应变速率为0.01~7 s?1条件下的热压缩变形行为。结果表明,变形温度和应变速率显著影响其热变形行为。通过计算获得了热变形激活能及应力指数分别为Q=166 kJ/mol,n=5.99,且其本构方程为ε&=3.16×1013[sinh(0.010σ)]5.99exp [?1.66×105/(RT)]。热压缩显微组织观察表明：在应变速率为0.01~1 s?1的条件下,在250°C热压缩变形时初始晶粒晶界及孪晶处发生了部分动态再结晶,而在高温(350~400°C)条件下,发生了完全动态再结晶且再结晶晶粒尺寸随着应变速率的增加而减小。获得的较优的变形条件为温度330~400°C、应变速率为0.01~0.03 s?1以及350°C、应变速率为1 s?1。     

Physical Simulation of Hot Deformation and Microstructural Evolution of Fe-0.05C-0.13P Steel

High-phosphorus steels are important for structural applications where corrosion resistance is required and are subjected to hot deformation processing. Therefore, hot deformation behavior of Fe-0.05C-0.13P steel is studied by conducting hot compression tests in the temperature range 750-1050 °C after austenitization at 1050 °C for 10 s. The strain rates ranged from 0.001 to 10 s^?1. Optical and scanning electron microscopy was performed to determine the microstructural evolution. EBSD measurement on selected samples was used to determine the microstructural changes in the ferrite phase. Processing windows were determined using modified dynamic material model in order to determine the safe hot working domains and these are correlated with the microstructural developments.     

Plastically deforming amorphous ZrO2-Al2O3

We report for the first time non-viscous, plastic deformation in an amorphous oxide: ZrO₂-Al₂O₃. Dense samples of amorphous ZrO₂-Al₂O₃ made by hot pressing spray-pyrolysed powder were deformed in uniaxial compression at 600–700°C at strain rates from 6×10⁻⁵ s⁻¹ to 10⁻³ s⁻¹. A transition from elastic to plastic deformation occurred at a critical stress ~360 MPa. The onset of plastic deformation was associated with a drop in the stress by 20–25%. Little influence of strain and strain rate on the flow stress was observed. The non-viscous, plastic deformation is related to the open structure of the amorphous phase as indicated by its low true density.     

Effect of hot deformation on α→β phase transformation in 47Zr-45Ti-5Al-3V alloy

The effect of strain rate and deformation temperature on the α→β phase transformation in 47Zr-45Ti-5Al-3V alloy with an initial widmanstatten α structure was investigated. At the deformation temperature of 550 °C, the volume fraction of α phase decreased with increasing strain rate. At 600 and 650 °C, the volume fraction of α phase firstly increased to a maximum value with increasing strain rate from 1×10^?3 to 1×10^?2 s^?1, and then decreased. At 700 °C, the microstructure consisted of single β phase. At a given strain rate, the volume fraction of α phase decreased with increasing deformation temperature. With decreasing strain rate and increasing deformation temperature, the volume fraction and size of globular α phase increased. At 650 °C and 1×10^?3 s^?1, the lamellar α phase was fully globularized. The variation in the volume fraction and morphology of α phase with strain rate and deformation temperature significantly affected the hardness of 47Zr-45Ti-5Al-3V alloy.     

High temperature deformation and processing map of a NiTi intermetallic alloy

The deformation behavior of a 49.8 Ni-50.2 Ti (at pct) alloy was investigated using the hot compression test in the temperature range of 700 °C–1100 °C, and strain rate of 0.001 s^?1 to 1 s^?1. The hot tensile test of the alloy was also considered to assist explaining the related deformation mechanism within the same temperature range and the strain rate of 0.1 s^?1. The processing map of the alloy was developed to evaluate the efficiency of hot deformation and to identify the instability regions of the flow. The peak efficiency of 24–28% was achieved at temperature range of 900 °C–1000 °C, and strain rates higher than 0.01 s^?1 in the processing map. The hot ductility and the deformation efficiency of the alloy exhibit almost similar variation with temperature, showing maximum at temperature range of 900 °C–1000 °C and minimum at 700 °C and 1100 °C. Besides, the minimum hot ductility lies in the instability regions of the processing map. The peak efficiency of 28% and microstructural analysis suggests that dynamic recovery (DRV) can occur during hot working of the alloy. At strain rates higher than 0.1 s^?1, the peak efficiency domain shifts from the temperature range of 850 °C–1000 °C to lower temperature range of 800 °C–950 °C which is desirable for hot working of the NiTi alloy. The regions of flow instability have been observed at high Z values and at low temperature of 700 °C and low strain rate of 0.001 s^?1. Further instability region has been found at temperature of 1000 °C and strain rates higher than 1 s^?1 and at temperature of 1100 °C and all range of strain rates.     

Hot Workability and Flow Characteristics of Aluminum-5 wt.% B4C Composite

Flow behavior of aluminum-5 wt.% boron carbide (Al-B₄C) composite was investigated by carrying out compression tests over a range of strain rates (10^?4-10⁰ s^?1) and temperatures (200-500 °C). The flow stress data obtained from these tests at true strain 0.5 were used to develop processing map. The stable and instable flow regimes in the map were characterized by the microstructural examination using Scanning Electron Microscopy and Electron Backscattered Diffraction. The optimum condition for processing of Al-5%B₄C composite was found to lie between 425 and 475 °C at the strain rate of around 10^?4 s^?1. A strain-compensated Sellars-McG Tegart constitutive equation was established to model high-temperature deformation behavior of the material.     

Effect of Various SPD Techniques on Structure and Superplastic Deformation of Two Phase MgLiAl Alloy

MgLiAl alloy containing 9 wt% Li and 1.5% Al composed of hexagonal α and bcc β phases was cast under protecting atmosphere and hot extruded. Various methods of severe plastic deformation were applied to study their effect on structure and grain refinement. Rods were subjected to 1–3 passes of Twist Channel Angular Pressing TCAP (with helical component), cyclic compression to total strain ε?=?5 using MAXStrain Gleeble equipment, both performed at temperature interval 160–200 °C and, as third SPD method, KOBO type extrusion at RT. The TCAP pass resulted in grain refinement of α phase from 30 μm down to about 2 μm and that of β phase from 12 to 5 μm. Maxstrain cycling 10?× up to ε?=?5 led to much finer grain size of 300 nm. KOBO method performed at RT caused average grain size refinement of α and β phases down to about 1 μm. Hardness of alloy decreased slightly with increasing number of TCAP passes due to increase of small void density. It was higher after MAXStrain cycling and after KOBO extrusion. TEM studies after TCAP passes showed higher dislocation density in the β region than in the α phase. Crystallographic relationship (001) α|| (110) β indicated parallel positioning of slip planes of both phases. Electron diffraction technique confirmed increase of grain misorientation with number of TCAP passes. Stress/strain curves recorded at temperature 200 °C showed superplastic forming after 1st and 3rd TCAP passes with better superplastic properties due to higher elongation with increasing number of passes. Values of strain rate sensitivity coefficient m were calculated at 0.29 after 3rd TCAP pass for strain rate range 10^?5 to 5?×?10^?3 s^?1. Deformation by MAXStrain cycling caused much more effective grain refinement with fine microtwins in α phase. Superplastic deformation was also observed in alloy deformed by KOBO method, however the value of m?=?0.21 was obtained at lower temperature of deformation equal to 160 °C and deformation rate in the range 10^?5 to 5?×?10^?3. Tensile samples deformed superplastically showed grain growth and void formation caused by grain boundary slip. Summarizing, all methods applied resulted in sufficient grain refinement to obtain the effect of superplastic deformation for alloys of two phase α?+?β structure.