Effect of cold rolling and first precipitates on the coarsening behavior of γ″-phases in Inconel 718 alloy

The coarsening behaviors of γ″-phase particles in Inconel 718 alloy aged at 750, 800, and 850℃ were investigated by scanning electron microscopy（SEM）. Detailed observations and quantitative measurements were conducted to characterize the coarsening behavior of the γ″-phase under various aging conditions. The experimental results indicate that the existence of the δ-phase retards the formation and coarsening of the γ″-phase, without influencing its final particle size or amount. Moreover, when cold rolled with a reduction of 50%, the dimensions of the γ″ particles in Inconel 718 alloy decrease with increasing aging time. Furthermore, the coarsening behavior of the γ″-phase in the Inconel 718 alloy after a normal aging treatment（sample A） and that of the primary δ-phase（sample B） follow the Lifshitz–Slyozov–Wagner（LSW） diffusion-controlled growth theory; the thus-obtained activation energies for the γ″-phase are 292 k J·mol^-1 and 302 k J·mol^-1, respectively.

Coarsening kinetics of γ' precipitates in a Re-containing Ni-based single crystal superalloy during long-term aging

The morphological evolution and coarsening kinetics ofγ'precipitates in a Re-containing Ni-based single crystal superalloy were investigated during isothermal aging at 900,950 and 1000℃.After heat treatment,well-defined cuboidalγ'precipitates with low misfit was obtained within the experimental alloy.Then coarsening rate constants and particle size distribution(PSD)ofγ'phases were calculated and specified based on the measured precipitate sizes for va rying periods of aging times from 100 to 2000 h.After aging for 2000 h,γ'precipitates maintained cubical shape at 900℃,while exhibited sphere at 950 and 1000℃.Coarsening models based on diffusion-controlled process with a functional relationship of r^(3) vs.t(classic Lifshitz-Slyozov-Wagner coarsening model)and interface-controlled model with a function of r^(2) vs.t(trans-interface diffusion-controlled coarsening model)were investigated to fit between the experimental results and theoretical analysis.It was found that Re as the slowest diffusing solute in the alloy constituted the rate-limited step for coarsening based on LSW model,while the process limiting coarsening as governed by an interface diffusion process could possibly be related to the Al diffusion through theγ/γ'interface.The PSDs and coarsening exponent were discussed by comparing the experimental data with predictions of LSW and TIDC models.Finally,coarsening mechanism could be divided into four regimes:(i)coarsening by diffusion-controlled;(ii)coarsening by diffusion and interface cocontrolled;(iii)coarsening by interface-controlled;(iv)coarsening by interface-controlled accompanied withγ'coalescence.

Fatigue Crack Growth of Al-Li Alloy 2091 in 3.5% NaCl Solution and at 130℃

A study has been made of the fatigue crack growth in an Al-Li alloy 2091 in 3.5%NaCl solu- tion and at an elevated temperature of 130℃.The aging and environmental effects on the crack growth behaviour were examined.It is shown that the fatigue crack growth resistance decreases with aging.The specimen with underaged structures shows the highest resistance to fatigue crack growth due to more deflected crack paths and the resulting high crack closure levels.As compared to laborato- ry air,both 3.5% NaCl solution and elevated tem- perature of 130℃ cause an increase of near-thresh- old crack growth rates and a decrease of threshold levels.The elevated temperature is more effective. The effect of elevated temperature is mainly due to the precipitate coarsening which results in an enhancement of slip irreversibility.Hydrogen embritttlement is supposed to be responsible for the degradation of fatigue crack growth resistance in 3.5%NaCl solution

Influence of the Accumulative Roll Bonding Process Severity on the Microstructure and Superplastic Behaviour of 7075 Al Alloy

The 7075 Al alloy was processed by accumulative roll bonding （ARB） at 350 ℃ using 2：1, 3：1 and 4：1 thickness reductions per pass （Rp） up to 8, 6 and 3 passes, respectively. Microstructural examinations of the processed samples revealed that ARB leads to a microstructure composed of equiaxed crystallites with a mean size generally lower than 500 nm. It was found that, due to both the stored energy through- out the processing and the particle pinning effect, the alloy is affected by discontinuous recrystallisation during the inter-pass heating stages, the precise microstructural evolution being dependent on Rp. Me- chanical testing of the ARBed samples revealed that the main active deformation mechanism in the ARBed samples in the temperature range from 250 to 350 ℃ at intermediate and high strain rates is grain bound- ary sliding, the superplastic properties being determined by both the microstructure after ARB and its thermal stability.

Charting the ‘composition–strength’ space for novel austenitic,martensitic and ferritic creep resistant steels

We report results of a large computational ＇alloy by design＇ study, in which the ＇chemical composition-mechanical strength＇ space is explored for austenitic, ferritic and martensitic creep resistant steels. The approach used allows simultaneously optimization of alloy composition and processing parameters based on the integration of thermodynamic, thermo-kinetics and a genetic algorithm optimization route. The nature of the optimisation depends on both the intended matrix（ferritic, martensitic or austenitic） and the desired precipitation family. The models are validated by analysing reported strengths of existing steels. All newly designed alloys are predicted to outperform existing high end reference grades.

Effect of Alloying Method on Microstructure and Wear Resistance of Fe-Cr-V-B Based Alloy

The microstructure and wear behavior of Fe-Cr-V-B based alloy was investigated.Each of the specimens was characterized by metallographic techniques and hardness test.Phase analysis of alloy was performed by X-ray diffraction（XRD）,scanning electron microscopy（SEM）and energy dispersive spectroscopy（EDS）.The results showed that the microstructure of Fe-Cr-V-B based as-cast alloy containing 8.6mass% V was composed ofα-（Fe,Cr）solid solution,VC carbides,and V_3B_4 borides,whereas the as-cast alloy containing 24.8mass% Cr was composed ofα-（Fe,Cr）solid solution and V_3B_4borides;the eutectic V_3B_4 borides mainly comprise M2B-type borides（M presents Fe and Cr）.The existence of a large amount of V atoms affects the growth pattern of eutectic borides,which mainly display rod-like and grainy structure.The wear behavior of the experimental alloy has been investigated using pin-on-disk type friction and wear apparatus.The comparison of experimental results indicated that the relative wear resistance of the alloy containing 8.6mass% V after destabilization heat treatment in a conventional furnace at 1 373 K for 2hwas about six times higher than that of the high chromium（19mass%）cast iron.