Effects of precipitation strengthening heat treatment for Al-Mg alloy

The corrosions resulting from defects in painting layers frequently occur in Al alloys, so the application of corrosion preventing systems is also very important. Optimum conditions in terms of electrochemistry in relation to solution treatment, quenching and artificial aging treatment were established in order to optimize precipitation strengthening conditions intended to enhance the strength of Al alloys. Slow strain rate tests （SSRT） at various applied potentials were conducted in potential range from -1.8 to 0.5 V. The results show that the maximum tensile strengths, elongations and time-to-fracture are shown to be high values. After precipitation strengthening heat treatment, a tendency appear that time-to-fracture increases as elongation increases. In the potential range from -1.3 V to -0.7 V, the specimens show excellent mechanical properties, and thus this range is considered to be a corrosion prevention range.

Precipitation Softening and Precipitate Free Zones of V_(55)Ti_(30)Ni_(15) Alloys During Heat Treatment

The microstructure, hardness, and precipitate free zones（PFZ） of V55Ti30Ni15 alloys during heat treatment have been investigated in this study. The microstructure resulting from different heat treatment conditions has a great influence on hardness. The microstructure resulting from different heat treatment conditions has a great influence on hardness. Fine Ni Ti particles precipitate from the supersaturated V-matrix solid solution at 750 °C, increase in quantity until 800 °C, and then dissolve back into the V-matrix at 850 °C. The resultant hardness decreases with temperature until 800 °C, and then increases from 800 to 850 °C. The microstructure containing small Ni Ti precipitates resulting from the treatment of 18 h at800 °C has a good soft condition for workability. PFZ formed at the grain boundary of V-matrix during heat treatment was observed. Vacancies depletion in V-matrix maybe led to the formation of PFZ.

Microstructure and stress-rupture property of an experimental single crystal Ni-base superalloy with different heat treatments

The influence of heat treatment on the microstructure and stress-rupture property at 1,100 °C/140 MPa was investigated in a 5.0 wt% Re containing experimental single crystal Ni-base superalloy. The results indicate that the γmorphology is nearly cuboidal in the dendrite core after conventional heat treatment. The lattice misfit of alloy becomes more negative after modified heat treatment and results in more cuboidal γ precipitates than that after conventional heat treatment. The increased stress-rupture life after modified heat treatment is attributed to higher γ volume fraction, more negative lattice misfit, well-rafted structure, and narrower c channel width.

The microstructure of a single crystal superalloy after different aging heat treatments

To study the influence of aging heat treatments on the microstructure of single crystal superalloys with high content of refractory elements and optimal the aging heat treatment regimes, a single crystal superalloy containing 22 wt% refractory elements was investigated.Results show that for the experimental alloy, even the homogenization-solution heat treatment for 25 h cannot homogenize the alloying elements completely. During primary aging heat treatment, γ＇ phase grows larger and turns to regular cubes. Higher aging temperature induces larger γ＇ cubes. For specimens with primary aging heat treated at 1120 ℃,γ＇ morphology does not change apparently during secondary aging heat treatment. For specimens with primary aging heat treatment at 1150 ℃,γ＇phase in interdendrite grows obviously comparing with that in dendrites. By analyzing the precipitating kinetics of γ＇phase, it is found that owning to the dendrite segregation and different aging heat treatment temperatures, γ＇ phase at different regions grows under the control of different factors at different aging heat treatment stages. The two controlling factors that are driving forces of phase transformation and element diffusion rate induce obviously different growth rates of γ＇ phase. As a result, the γ＇-precipitating behaviors are variable based on different solute concentrations and aging temperatures. For advanced single crystal superalloys that are supposed to be used at relatively high temperatures, the final γ＇ size after aging heat treatment is suggested to be close to the crossing point of diffusion controlling curve and driving force controlling curve corresponding to the serving temperature. And then,high-temperature properties can be improved.

Suppressing Effect of Heat Treatment on the Portevin–Le Chatelier Phenomenon of Mg–4%Li–6%Zn–1.2%Y Alloy

Microstructural evolution and Portevin-Le Chatelier（PLC） phenomenon of the as-extruded Mg-4%Li-6%Zn-1.2%Y alloy before and after heat treatment have been investigated.It has been demonstrated that for the as-extruded and solid solution treated（T4） samples,the PLC phenomenon could be obviously observed on tensile stress-strain curves.Moreover,the PLC phenomenon in T4 samples was more salient than that in the as-extruded condition,suggesting that the occurrence of PLC phenomenon was closely related to the super-saturation degree of solute atoms in the matrix.Since most of solute atoms were consumed for the formation of Mg Zn precipitates（β1′ and a little of β2′） during the subsequent ageing treatment（T6）,the PLC phenomenon of T6 samples was eliminated.Meanwhile,due to the pinning effect of the formed Mg Zn precipitates on mobile dislocations,the tensile strength of T6 samples was relatively higher than those of the other two conditions.

σ-Phase in Lean Duplex Stainless Steel Sheets

The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels（lean DSS）were examined after aging the materials at 800 ℃.Owing to the instability of ferrite,all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 ℃ and different secondary phases may form,depending on composition and microstructure.The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS,but only information on the equilibrium microstructures were achieved.Therefore,the materials were aged at various times,in order to verify the simulations and determine the precipitation kinetics.The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements,determining phase type,composition and volumetric fraction.At 800 ℃,grade 2101 was found to be only affected by Cr_2N nitrides precipitation,whereas a significant amount of σ-phase was found to form in LDX 2404 for treatment longer than 1 h,almost totally replacing ferrite after 50 h.Up to now,the intermetallic σ-phase has been observed only in the high alloyed DSS,and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.