Low-temperature superplasticity of Al2O3(p)/Ni–Co nanocomposites

Nanocomposites of Al₂O₃/Ni–Co prepared using Al₂O₃ of various particle sizes were fabricated by pulse current electrodeposition. Their superplastic tensile deformation was investigated at strain rates of 8.33 × 10⁻⁴ s⁻¹ and 1.67 × 10⁻³ s⁻¹ and temperatures of 723–823 K. The Al₂O₃ particle sizes and the deformation temperature had significant influence on the elongation of the deposited materials. The optimal superplastic condition and the maximum elongation were determined. A low temperature superplasticity with elongation of 632% was achieved at a strain rate of 1.67 × 10⁻³ s⁻¹ and 823 K. Scanning electron microscopy and transmission electron microscopy were used to examine the microstructures of the deposited and deformed samples. The grains grew to a micrometer dimensions and were elongated along the tensile direction after superplastic deformation. Superplasticity in electrodeposited nanocomposites is related to the presence of S at grain boundaries and to deformation twinning.     

Investigation of high-strength and superplastic Mg–Y–Gd–Zn alloy

The Mg–7Y–4Gd–1Zn (wt.%) alloy was prepared by hot extrusion technology, and the microstructure, tensile properties and superplastic behavior have been investigated. The extruded alloy possesses high tensile strength both at room temperature and 250 °C, and especially the yield strength can remain above 300 MPa at 250 °C. The outstanding microstructure, i.e. bent 18R long period stacking ordered (LPSO) strips and dynamic recrystallization (DRX) Mg grains containing fine lamellae with 14H LPSO or stacking fault structures, is responsible for the excellent mechanical properties, and it is considered that the integrated performance can be further improved by controlling the size of LPSO phase. The alloy shows the maximum elongation of 700% at 470 °C and 1.7 × 10⁻⁴ s⁻¹. The predominant superplastic mechanism is considered to be grain boundary sliding assisted by lattice diffusion. The fracture of superplastic deformation is related to the microstructure evolution, i.e. the disappearance of LPSO phase and the formation of cubic phase. Both high temperature and stress contribute to the phase transformation.     

High strain-rate superplasticity of fine- and ultrafine-grained AA5083 aluminum alloy at intermediate temperatures

Superplastic behavior of fine and ultra fine-grained AA5083 Al alloy was examined using the shear punch test. To achieve fine- and ultra fine-grained microstructures, a relatively new severe plastic deformation (SPD) process, namely Double Equal Channel Lateral Extrusion (DECLE) was employed. The strain rate sensitivity indices (m) of samples were evaluated after 1, 2, 4, and 6 passes for shear strain rates in the range of 3 × 10^− 3 to 3 × 10^− 1 s^− 1 and temperatures in the range of 573 to 673 K. For microstructural observations, TEM images together with the corresponding SAED patterns were prepared and utilized. A considerable increase in the m-value was observed after the first pass of the operation for all testing temperatures. The best condition for achieving a good superplasticity for the alloy was found to be a single pass DECLE at 673 K in the strain rate range of 10^− 2 to 10^− 1 s^− 1. This process condition resulted in an m-value of 0.43, indicative of a high strain rate superplastic deformation behavior. Further passes of the SPD process did not show any sign of superplasticity until the last pass of the operation, during which the m-value slightly increased, compared with the previous pass.     

Superplasticity and microstructure of TC21 titanium alloy during thermomechanical treatment

Abstract

As rolled TC21 titanium alloy was subjected to isothermal constant strain rate tensile tests using an electronic tensile testing machine. After tensile deformation, the alloys were subjected to double annealing. Superplastic behaviour and microstructure evolution were systematically investigated. Experimental results show that as rolled TC21 alloy exhibits good superplasticity at temperatures ranging from 870 to 930°C and strain rates ranging from 3×10^?4 to 3×10^?2 s^?1. A maximum elongation of 373·3% was obtained at 910°C and 3×10^?4 s^?1. In addition, the alloy microstructure comprises α and β phases during plastic deformation. The primary α-grains aggregate and merge to form new crystal grains with irregular grain boundaries because of dynamic recrystallisation. Furthermore, the primary α phase content gradually decreases with increasing temperature. The resulting microstructure after deformation and double annealing is a duplex microstructure comprising a primary equiaxed α phase and a β-transformed lamellar structure. The acicular α phase transformed from the β phase is mutually interlaced as a basketweave structure after deformation at 930°C and double annealing.     

Artificial neural network approach to predict the flow stress in the isothermal compression of as-cast TC21 titanium alloy

Isothermal compression of as-cast TC21 titanium alloy at the deformation temperatures ranging from 1000 to 1150 °C with an interval of 50 °C, the strain rates ranging from 0.01 to 10.0 s^?1 and the height reduction of 60% was conducted on a Gleeble-3500 thermo-mechanical simulator. Based on the experimental results, an artificial neural network (ANN) model with a back-propagation learning algorithm was developed to predict the flow stress in isothermal compression of as-cast TC21 titanium alloy. In the present ANN model, the strain, strain rate and deformation temperature were taken as inputs, and the flow stress as output. According to the predicted and experimental results, the maximum error and average error between the predicted flow stress and the experimental data were 4.60% and 1.58%, respectively. Comparison of the predicted results of flow stress based on the ANN model and those using the regression method, it was found that the relative error based on the ANN model varied from ?1.41% to 4.60% and that was in the range from ?13.38% to 10.33% using the regression method, and the average absolute relative error were 1.58% and 5.14% corresponding to the ANN model and regression method, respectively. These results have sufficiently indicated that the ANN model is more accurate and efficient in terms of predicting the flow stress of as-cast TC21 titanium alloy.     

Optimization of β/near-β forging process parameters of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si by using processing maps

The isothermal and constant strain rate compression tests of titanium alloy Ti-6.5Al-3.5Mo-1.5Zr-0.3Si are conducted by Thermecmaster-Z simulator and the deformation behaviors at the temperature of 990 ～ 1080 °C and strain rate of 0.001 ～ 70 s^? 1 are extensively investigated. The processing maps (P-maps) under these deformation conditions are constructed and the forging process parameters are then optimized based on the generated P-maps. The experimental results show that most of the deformation is located at the flow instability zone when the strain rate is high (ε? ≥ 0.6 s^? 1). At the near-β forging temperature range, the feasible deformation conditions are (990 ～ 1008 °C, 0.001 ～ 0.01 s^? 1) and its deformation mechanism is superplasticity. The optimum process parameters are (990 °C, 0.001 s^? 1). At the β forging temperature range, when the deformation strain is smaller (ε? ≤ 0.7), the suitable deformation conditions are (1035 ～ 1070 °C, 0.001 ～ 0.08 s^? 1). The optimum deformation parameters, however, are 1055 °C and 0.001 s^? 1. If the strain is increased (ε? = 0.8 ～ 1.2), there are two feasible deformation zones, viz., (1008 ～ 1025 °C, 0.001 ～ 0.02 s^? 1) and (1025 ～ 1050 °C, 0.008 ～ 0.16 s^? 1), respectively. At the β forging temperature range, the main deformation mechanism is dynamic recrystallization.     

The superplasticity and microstructure evolution of coarse-grained Ti40 alloy

The superplastic deformation characteristics of coarse-grained Ti40 alloy have been studied in the temperature and strain rate range of 760–880°C and 5?×?10^?4 to 1?×?10^?2?s^?1, respectively. The alloy exhibited good superplasticity in all test conditions except at 760°C and strain rate higher than 5?×?10^?3?s^?1, with the maximum elongation of 436% at 840°C, 1?×?10^?3?s^?1. The activation energy value was found to be close to the self-diffusion activation energy of Ti40 alloy, suggesting that the rate controlling mechanism was lattice diffusion. The coarse grain was elongated and refined which can be attributed to the occurrence of dynamic recovery and continuous dynamic recrystallisation. These processes were promoted by the subgrain formation and evolution, resulting in the good superplasticity of Ti40 alloy with coarse grains.     

Experimental study on tensile property of AZ31B magnesium alloy at different high strain rates and temperatures

As the lightest metal material, magnesium alloy is widely used in the automobile and aviation industries. Due to the crashing of the automobile is a process of complicated and highly nonlinear deformation. The material deformation behavior has changed significantly compared with quasi-static, so the deformation characteristic of magnesium alloy material under the high strain rate has great significance in the automobile industry. In this paper, the tensile deformation behavior of AZ31B magnesium alloy is studied over a large range of the strain rates, from 700 s⁻¹ to 3 × 10³ s⁻¹ and at different temperatures from 20 to 250 °C through a Split-Hopkinson Tensile Bar (SHTB) with heating equipment. Compared with the quasi-static tension, the tensile strength and fracture elongation under high strain rates is larger at room temperature, but when at the high strain rates, fracture elongation reduces with the increasing of the strain rate at room temperature, the adiabatic temperature rising can enhance the material plasticity. The morphology of fracture surfaces over wide range of strain rates and temperatures are observed by Scanning Electron Microscopy (SEM). The fracture appearance analysis indicates that the fracture pattern of AZ31B in the quasi-static tensile tests at room temperature is mainly quasi-cleavage pattern. However, the fracture morphology of AZ31B under high strain rates and high temperatures is mainly composed of the dimple pattern, which indicates ductile fracture pattern. The fracture mode is a transition from quasi-cleavage fracture to ductile fracture with the increasing of temperature, the reason for this phenomenon might be the softening effect under the high strain rates.     

Effect of processing parameters on the hot deformation behavior of as-cast TC21 titanium alloy

Isothermal compression tests of as-cast Ti–6A1–2Zr–2Sn–3Mo–1Cr–2Nb (TC21) titanium alloy are conducted in the deformation temperature ranging from 1000 to 1150 °C with an interval of 50 °C, strain rate ranging from 0.01 to 10.0 s⁻¹ and height reductions of 30%, 45%, 60% and 75% on a computer controlled Gleeble 3500 simulator. The true stress–strain curves under different deformation conditions are obtained. Based on the experimental data, the effects of deformation parameters on the hot deformation behavior of as-cast TC21 alloy were studied. The deformation mechanisms of the alloy in the whole regimes are predicted by the power dissipation efficiency and instability parameter and further investigated through the microstructure observation. It is found that at the height reductions of 30%, 45% and 60%, the softening of stress–strain curves at high strain rate (>1.0 s⁻¹) is mainly associated with flow localization, which is caused by local temperature rise, whereas at low strain rate, the softening is associated with dynamic recrystallization (DRX). However, the instability showed in flow localization occurs at low strain rate of 0.01 s⁻¹ when the height reduction reaches 75%. In addition, the effects of strain rate, deformation temperature and height reduction on microstructure evolution are discussed in detail, respectively.     

TC18钛合金的超塑性行为与变形机制

通过高温拉伸实验研究TC18钛合金在温度为720~950℃,初始应变速率为6.7×10~(-5)~3.3×10~(-1)s~(-1)时的超塑性拉伸行为和变形机制。结果表明:TC18钛合金在最佳超塑性变形条件下(890℃,3.3×10~(-4)s~(-1)),最大伸长率为470%,峰值应力为17.93MPa,晶粒大小均匀。在相变点Tβ(872℃)以下拉伸,伸长率先升高后下降,在温度为830℃,初始应变速率为3.3×10~(-4)s~(-1)时取得极大值373%,峰值应力为31.45MPa。TC18钛合金在两相区的超塑性变形机制为晶粒转动与晶界滑移,变形协调机制为晶内位错滑移与攀移;在单相区的超塑性变形机制为晶内位错运动,变形协调机制为动态回复和动态再结晶。     

Effect of fabrication conditions and Cr,Zr contents on the grain structure of 7075 and 6061 aluminum alloys

The correlation between compression conditions at temperatures in the range of 573–773 K with the strain rate range of 0.002–2 s^?1 and grain size after solution heat treatment of 7075 alloy was investigated, as contrasted with 6061 alloy. The grain coarsening occurred under specific Zener-Hollomon (Z) parameters of 5 × 10¹⁰–10¹² s^?1 for 7075 alloy, 10⁸–2 × 10¹² s^?1 for 6061 alloy, respectively. These phenomena could be explained by crystalline orientation analysis and stored deformation strain evaluation. The site of subgrains with less than 15° misorientation and stored strain after compression increased, but the site of recrystallized grains after solution heat treatment increased with Z parameter. Small Z parameter condition could get low stored strain with fine grain which is stable during SHT. Effect of Cr and Zr on the grain structure of 7075 alloy was also investigated. Cr or Zr addition could inhibit the grain coarsening. The role of Zr addition was confirmed to pinning effect of Al₃Zr dispersoids to subgrain boundaries.     

Hot tensile deformation behaviors and constitutive model of 42CrMo steel

The hot tensile deformation behaviors of 42CrMo steel are studied by uniaxial tensile tests with the temperature range of 850–1100 °C and strain rate range of 0.1–0.0001 s⁻¹. The effects of hot forming process parameters (strain rate and deformation temperature) on the elongation to fracture, strain rate sensitivity and fracture characteristics are analyzed. The constitutive equation is established to predict the peak stress under elevated temperatures. It is found that the flow stress firstly increases to a peak value and then decreases, showing a dynamic flow softening. This is mainly attributed to the dynamic recrystallization and material damage during the hot tensile deformation. The deformation temperature corresponding to the maximum elongation to fracture increases with the increase of strain rate within the studied strain rate range. Under the strain rate range of 0.1–0.001 s⁻¹, the localized necking causes the final fracture of specimens. While when the strain rate is 0.0001 s⁻¹, the gage segment of specimens maintains the uniform macroscopic deformation. The damage degree induced by cavities becomes more and more serious with the increase of the deformation temperature. Additionally, the peak stresses predicted by the proposed model well agree with the measured results.     

Effects of microstructure changes on the superplasticity of 2205 duplex stainless steel

The high temperature deformation behavior of 2205 duplex stainless steel under different conditions had been studied by tensile tests. The whole tensile test was conducted at a constant temperature 950 °C with an initial strain rate 1.5 × 10⁻³/s. Some tests were interrupted purposely and then the samples were quenched using water. Elongations of the fractured specimens were calculated. Microstructure changes just before and during the deformation were observed. Phase ratio of σ precipitate was analyzed. The results showed that the superplasticity of 2205 duplex stainless steel was directly affected by the microstructure before the deformation. The recrystallization phenomenon was distinct along with the homogenizing time and the grains became equiaxed and stable. Meanwhile, the quantity of σ phase increased when prolonged the homogenizing time. After homogenized for 7 min before the tensile test, the σ phase ratio was about 4.8% and the grain size was about 998 nm, the maximum elongation value 1260% was obtained. During the deformation progress, dynamic recrystallization was observed and quantity of σ phase increased with the increasing of deformation strain. The σ phase restricted the grain growth and kept the equiaxed duplex structure stable with a grain size of about 1 μm.     

The study of the technology parameters on the superplasticity of the new Al–Zn–Mg–Cu–Ni–Zr base alloy

Al–Zn–Mg–Cu aluminum alloy contain 0.3% Zr and 4% Ni was processed by traditional hot and cold rolling with a total reduction from 0  to  80%. The relationship between superplastic behavior and reduction of cold deformation and casting cooling rate was analyzed. It is shown that the decrease in the reduction of cold rolling do not significantly influence on flow stress and elongation. Decrease in casting cooling rate leads to insignificantly decrease in superplastic indicators. Alloy exhibits advanced superplasticity: the elongation of 400–800% at the strain rates of (5 × 10^–3–1 × 10^–1) s^–1.     

The development of internal cavitation in a superplastic zinc–aluminum alloy processed by ECAP

A Zn-22% Al eutectoid alloy was processed by Equal-Channel Angular Pressing (ECAP) to produce an ultrafine grain size and then pulled in tension at elevated temperatures to evaluate the role of internal cavitation under superplastic conditions. Tensile testing yielded a highest elongation of 2,230% at a strain rate of 1.0 × 10^?2 s^?1 at 473 K representing high strain rate superplasticity. Quantitative cavity measurements were taken to investigate the significance of the internal cavities formed during superplastic deformation. The results demonstrate that cavity nucleation occurs continuously throughout superplastic flow, and there is a transition in the cavity growth mechanism from superplastic diffusion growth at the smaller cavity sizes to plasticity-controlled growth at the larger sizes.     

Strain rate and gage length effects on tensile behavior of Kevlar 49 single yarn

In the present paper, Kevlar® 49 single yarns with different gage lengths were tested under both quasi-static loading at a strain rate of 4.2 × 10^?4 s^?1 using a MTS load frame and dynamic tensile loading over a strain rate range of 20–100 s^?1 using a servo-hydraulic high-rate testing system. The experimental results showed that the material mechanical properties are dependent on gage length and strain rate. Young’s modulus, tensile strength, maximum strain and toughness increase with increasing strain rate under dynamic loading; however the tensile strength decreases with increasing gage length under quasi-static loading. Weibull statistics were used to quantify the degree of variability in yarn strength at different gage lengths and strain rates. This data was then used to build an analytical model simulating the stress–strain response of single yarn under dynamic loading. The model predictions agree reasonably well with the experimental data.     

Effect of solute atom concentration and precipitates on serrated flow in Mg-3Nd-Zn alloy

Influence of solute atom concentration and precipitates on serrated flow, i.e., Portevin-Le Chatelier effect,was studied in Mg~(-3) Nd-Zn alloy by tensile test at 250?C with a strain rate of 1 × 10~(-3) s~(-1). Microstructure and tensile property of the Mg~(-3) Nd-Zn alloy in solution and aging conditions were also investigated.Results indicate that the serrated flow was weakened with aging time, and geometry of the serrations changed from sharp to rounded corner. Through analyzing the mechanism of the interactions between dislocations and solute atoms, it was identified that the precipitates did not only weaken the serrated flow due to the decrease in the concentration of solute atom, but also regulate the serration type by restraining the movement of dislocations during high temperature deformation.     

Superplasticity and machinability in a four-phase ceramic

Fine grained four-phase ceramic materials were fabricated to have a combination of high temperature superplasticity and room temperature machinability. The composite ceramics were made of 25 vol.% 3Y-TZP/8YSZ–25 vol.% Al₂O₃–25 vol.% MgAl₂O₄–25 vol.% LaPO₄, using LaPO₄ powders fabricated in-house. X-ray diffraction and scanning electron microscopy revealed that the grain size for the best mixed samples was in the range of 600 nm, tetragonal ZrO₂ transformed into monoclinic, and reactions in the sintered samples produced a new phase, magnetoplumbite (LaMgAl₁₁O₁₉) with lath-like grains. The formation of magnetoplumbite was facilitated by the presence of yttria and by a liquid phase generated at elevated temperatures. These four-phase ceramics had a maximum hardness of 12 GPa and fracture toughness was no more than 3 MPa√m. Deformation rates at 1400 °C under 40 MPa stress were in the superplastic range of 10^?3 s^?1 for most compositions. These four-phase ceramics were machinable as demonstrated using conventional tungsten drill bits.     

The effects of aging treatment and strain rates on damage evolution in AA 6061 aluminum alloy in compression

AA 6061 aluminum alloy in T4, T6 and T8 temper were subjected to quasi-static compressive loading at a strain-rate of 3.2 × 10⁻³ s⁻¹ and dynamic compressive loading at strain-rates between 7.0 × 10³ and 8.5 × 10³ s⁻¹. The effects of strain rates and temper condition on the deformation behavior of the alloy are discussed. Under the quasi-static loading, deformation was relatively homogeneous and controlled by strain hardening, which is more pronounced in the naturally aged than the artificially aged alloys. Thermal softening played a dominant role under impact loading leading to strain localization along narrow bands called adiabatic shear bands (ASBs). Both deformed bands consisting of aligned second phase particles and transformed bands consisting of fine recrystallized grains were observed. The average size of the recrystallized grains in the transformed bands is about 600 nm and varies slightly depending on the temper condition. The fine grains are suggested to form by dynamic recrystallization. The T4 alloy showed the highest propensity for thermal softening, strain localization and cracking under impact loading while the T8 alloy showed the least tendency. The degree of recrystallization in the transformed band is influenced by temper condition with T8 alloy having the highest fraction of unrecrystallized grains inside the transformed bands. This is related to the temperature rise in the transformed bands that was estimated to be highest in the T4 alloy and lowest in the T8 alloy. The combined effects of high temperature and severe strain inside the transformed bands caused dissolution of second phase particles and induced microstructural changes that resulted in less silicon inside the transformed bands than in the adjacent region.     

Hot workability of 00Cr13Ni5Mo2 supermartensitic stainless steel

The hot workability of 00Cr13Ni5Mo2 supermartensitic stainless steel was investigated by hot compression and hot tension tests conducted over the temperature range of 950–1200 °C and strain rates varying between 0.1 and 50 s^?1. The processing map technique was applied on the basis of dynamic materials model and Prasad instability criterion. Microstructure evolutions, Zener–Hollomon parameter as well as hot tensile ductility were examined. The results show that, as for the hot working of 00Cr13Ni5Mo2 supermartensitic stainless steel in the industrial production, the large strain deformation should be carried out in the temperature range 1140–1200 °C and strain rate range 0.1–50 s^?1, where the corresponding Zener–Hollomon parameters exhibit low values. Moreover, when deformed under high strain rate range (above 15 s^?1), the deformation temperature can be reduced reasonably.