Dongliang Lin

Superplasticity in Fe3Al-Ti alloy with large grains

In this paper the authors report the superplasticity behavior in Fe[sub 3]Al with large grains. The tensile behavior of one Fe[sub 3]Al based alloy, Fe-28Al-2Ti (in atomic percent), under different strain rates at high temperatures was examined. The microstructure before and after deformation was observed by optical microscopy and transmission electron microscopy (TEM). The results revealed that the Fe[sub 3]Al alloy with a large grain size of 100[mu]m exhibited a large elongation of more than 150% from 800 C to 900 C with a strain rate range of 10[sup [minus]4] [approximately] 10[sup [minus]3]/s. The maximum elongation is 332.8% at 850 C under a strain rate of 1 [times] 10[sup [minus]3]/s. A tensile elongation of up to 507% was obtained by slightly changing the gauge section and/or modifying the composition. The reason for the large elongation is ascribed to the dynamic recovery and recrystallization in this alloy at high temperature.

Superplasticity in large-grained Fe3Al alloys

The superplasticity behavior of Fe-28Al, Fe-28Al-2Ti and Fe-28Al-4Ti (all compositions reported in this paper are in atomic percent) alloys has been investigated by tensile testing, optical microscopy and transmission electron microscopy. Tensile tests were performed at 700–900 °C under a strain rate range of about 10−5 − 10−2/s.The maximum strain rate sensitivity index m was found to be 0.5 and the largest elongation reached 620%. The flow activation energy was measured to be 263 kJmol for Fe-28Al and 191 KJmol for Fe-28Al-2Ti, which is much lower than the creep activation energy generally observed in Fe3Al alloys. After deformation, the grain size became much finer; from about 100 μm to 20–30 μm. As combined with TEM observations, we suggested that a continuous recrystallization process took place and superplasticity may arise from this process.

Thermal activation processes of tensile deformation in γ-TiAl alloy

Abstract Thermal activation parameters: activation volume V , activation enthalpy Δ H , activation free enthalpy Δ G and activation entropy Δ S of tensile deformation at yield point of a near-lamellar γ -titanium aluminide of Ti–47a/oAl–2a/oMn–2a/oNb–0.8v/oTiB 2 have been measured in a wide temperature range from 77 to 1273 K. From the values and their temperature dependence of the measured activation parameters, as well as the temperature dependence of the yield stress, the dislocation mechanisms of tensile deformation of γ -titanium aluminide have been speculated. It is found that there exist three temperature regions, which correspond to different possible thermal activation mechanisms of dislocation motion. In low temperature region (77–398 K), the mechanism is mainly characterized by the overcoming of Peierls–Nabarro friction. In intermediate temperature region (523–973 K), the mechanism is a weak thermal activated process although the plastic flow stress is neither sensitive to temperature nor to the strain rate. In the high temperature region (973–1273 K), the rate controlling mechanism is dislocation climbing. In addition, it is found that, activation entropy Δ S , whose variation with temperature is similar to that of activation volume V , also reflects the thermal activation mechanism of dislocation movement in some degree.

Energy and structural simulations of the interaction between the grain boundary and dislocations in Ni3Al alloys

Abstract The embedded-atom-type potentials and static relaxation method have been nused to simulate the interaction between the grain boundary (GB) and dislocations in Ni3Al alloys. The focus has been placed on the energy of the interaction, the distortion of GB structural units and the dislocation core structure near the GB. The effects of various factors, such as the GB chemistry, the boron segregation and the applied stress, on the interaction have been studied and discussed. It is found that, when analysed from the both an energy and a structural viewpoint, slip transfer will facilitated by boron segregation, which is helpful for the understanding of the mechanisms responsible for boron-enhanced ductility.

A theoretical investigation on non-equlibrium grain boundary segregation

Abstract Based on the concept that mobile solute-vacancy complexes migrating to grain boundaries (vacancy sink) is responsible for the non-equilibrium grain boundary segregation of solute atoms in an alloy, analytical expressions describing such a segregation process are presented. The driving force for the segregation is a decrease in the free energy of the system caused by the annihilation of vacancies at grain boundaries during cooling from an initial temperature. From the theoretical expressions, a sufficient condition for the non-equilibrium segregation to occur is determined to be that D s f /D p s f and D p are diffusion coefficients for free solute atoms and solute-vacancy complexes, respectively). Besides, the expressions predict that the grain boundary segregation enrichment increases with a decrease in the ratio D s f /D p , the vacancy formation energy or the cooling rate (above a critical cooling rate) and an increase in the initial temperature or the binding energy of complexes, while the relative enrichment of solute atoms at grain boundaries decreases with an increase in the bulk concentration of solutes. Furthermore, an analytical relationship for the critical cooling rate giving the maximum grain boundary enrichment is also presented. The relationship exhibits that the critical cooling rate increases with an increase in the diffusion coefficient for solutes or complexes, the initial temperature or the binding energy of complexes and a decrease in vacancy formation energy, while it is independent of the bulk concentration of solute. On the other hand, the calculated results are compared with the computer simulations done by Karlsson in boron-doped Austenite. Although good agreement between the two studies on the characteristics of the non-equilibrium grain boundary segregation behavior is achieved, real segregation profiles of solutes can only be calculated quantitatively by the functional expression without spending much computation time.

Strain rate sensitivity of ductility and fracture behaviors in a Fe–28Al alloy

Abstract The mechanical properties of Fe–28Al under a strain rate ranging from 10 −4 to 1300 s −1 were tested in air and water at room temperature. The ductility, yield stress and ultimate stress of Fe–28Al are sensitive to the strain rate and gradually increase with strain rate in all test ranges. The fracture surface observations indicated that the fracture mode of Fe–28Al varies from complete transgranular fracture to a mixture of transgranular cleavage and intergranular fracture with strain rate increasing from 10 −4 to 1300 s −1 . It is proved that environmental embrittlement is a main factor controlling the ductility of Fe–28Al, even at high strain rate, and grain boundaries play different roles in the ductility of Fe–28Al at low and high strain rate.

MÖssbauer Study on B2 Intermetallic Compound Fe-40Al and its Mn or Ti Containing Alloys

The Mossbauer measurements at room temperature have been carried out on the B2 intermetallic compound Fe-40Al and its Mn or Ti containing alloys. The results show that all the Mossbauer spectra of Fe-40Al and its Mn or Ti containing alloys are approximately one-peak singlets. The Fe atoms in these alloys show no significant magnetic moment. The spectra of Mn or Ti containing alloys have been fitted with two Lorentzian lines by the method of least squares. By studying the isomer shifts and other parameters of these fitted lines, it have been shown that Mn atoms occupy both Fe and A1 sublattices, Ti atoms occupy preferentially Fe sublattice, however, a few of Ti atoms occupy A1 sublattice only as Ti concentration is over certain amount (∼5-at%).

Deformation and fracture behavior of a directionally solidified Ni3Al alloy

Abstract The {111} slip behavior of a directionally solidified (DS) Ni3Alue5f8Zrue5f8B alloy has been investigated in tension in the temperature range of 773–1373 K, and in-situ transmission electron microscope (TEM) observation of the fracture behavior of the same alloy has also been carried out. The dependence of deformation rate on yield stress (at 0.2% plastic strain) is found to be exponential above the peak temperature. The dependence of deformation rate on flow stress (at 3.8% plastic strain) obeys a power-law relation above the peak temperature, with the stress exponent n = 4.89 in the temperature range of 1073–1173 K, suggesting a deformation mechanism of edge dislocation climb, and n = 2.70 in the temperature range of 1273–1373 K, suggesting a deformation mechanism of viscous screw dislocation glide. The dislocation structure develops with increasing stress and strain, the dislocation density ρ is related to the applied stress σ as ρ σ σ1.7, and no steady-state dislocation density is achieved up to 3.8% strain. With increasing temperature, the activation volume decreases below the peak temperature, and stays constant at a low level above the peak temperature, reflecting two different processes of thermal activation or unpinning of the Kear-Wilsdorf (KW) locks. Room-temperature in-situ TEM straining tests revealed that, near a crack tip edge, dislocations glide away quickly and leave behind many long straight screw dislocations pinned by the KW locks. The large number of pinned screw dislocations reduce the mobility of dislocations and induce cleavage fracture. Additionally, for the first time, an athermal unpinning process has been observed for screw dislocations around a crack tip.

High Temperature {111} Slip Behavior of a Directionally Solidified Ni3Al Alloy

The deformation behavior of {111} slip in a directionally solidified (DS) Ni 3 Al-Zr-B alloy has been investigated in tension in the temperature range of 773-1373K. At 0.2% strain, deformation rate was found to be exponential with yield stress above the peak temperature (1023K). However, at a larger strain of 3.8% strain, deformation rate on flow stress obeyed a power law relation above the peak temperature, with the stress exponent n=4.89 in the temperature range of 1073-1173K, suggesting a deformation mechanism of edge dislocation climb, and n=2.70 in the temperature range of 1273-1373K, suggesting a deformation mechanism of viscous screw dislocation glide. The dislocation structure developed with increasing stress and strain, the dislocation density, ρ, was related to the applied stress, σ, as ρσ 1.7 , and no steady state dislocation density was achieved up to 3.8% strain. With increasing temperature, the activation volume decreased below the peak temperature, and stayed constant at a low level above the peak temperature, reflecting two different processes of thermal activation or unpinning of the Kear-Wilsdorf locks.

Creep Behavior of a Directionally Solidified Ni 3 Al Alloy

The creep behavior of a directionally solidified multicomponent Ni 3 Al alloy was investigated in a temperature range from 923 to 1173 K. The dislocation structure during secondary stage creep has been examined by transmission electron microscopy. At lower temperatures from 923 to 1023K under a stress of 500 MPa, there exist a number of dense three – dimensional dislocation networks in the Ni 3 Al creep specimen, while at a higher temperature of 1173 K under a stress of 200 MPa the dislocation structure degenerates to regular two – dimensional dislocation networks. Climb of dislocations occurs in the overall test temperture range. The stress dependence and temperature dependence of creep rates for the Ni 3 Al alloy were also determined. It was found that power law creep is obeyed with the stress exponent equal to 4.7 and the activation energy equal to 326.6 kJ/mol. The climb of dislocations was suggested to be the rate controlling factor for the secondary stage creep rate.