In situ thermomechanical processing to avoid grain boundary precipitation and strength-ductility loss of age hardening alloys

To avoid grain boundary(GB) precipitation during aging, a new strategy of in situ thermomechanical processing for age hardening alloys was proposed. Specifically, high-density nanoscale precipitates were introduced into ultrafine grain(UFG) interiors of 7075 Al alloy by equal-channel-angular(ECAP) processing at 250 ℃ for 8 passes, thus avoiding GB precipitation. Tensile test results indicated that the UFG 7075 Al alloy exhibits superior mechanical properties(yield strength of 350 MPa, ultimate tensile strength of 500 MPa, uniform elongation of 18% and tensile ductility of 19%) compared with the UFG 1050 Al counterpart(yield strength of 170 MPa, ultimate tensile strength of 180 MPa, uniform elongation of 2.5% and tensile ductility of 7%). Fracture surface morphology studies revealed numerous homogeneous micro shear bands in necking shrinkage areas of both UFG 7075 Al and 1050 Al alloys, which are controlled by cooperative GB sliding. Moreover, the introduction of nanoscale precipitates in UFG 7075 Al matrix weakened the tendency of shear fracture, resulting in a higher tensile ductility and more homogeneous deformation. Different from the GB precipitation during postmortem aging, in situ thermomechanical treatment dynamically formed GBs after precipitation, thus avoiding precipitation on GBs.

Effect of aging treatment on the precipitation transformation and age hardening of Mg-Gd-Y-Zn-Zr alloy

In this study, the precipitation transformation and age hardening of solution-treated Mg-9Gd-4Y-2Zn-0.5Zr(wt.%) alloy were investigated at different aging treatment parameters. The precipitation sequences of the alloy aged at 200℃, 250℃ and 300℃ are β’’(DO19) → β’(BCO) → β(FCC), β’’(DO19) → β’(BCO) → β_(1)(FCC) → β(FCC) and β(FCC), respectively. The streaks sequences of the alloy aged at 200℃, 250℃ and 300℃ are SF, SF → 14H-LPSO and SF → 14H-LPSO, respectively. For the alloy aged at 200℃ and 250℃, the increase in hardness with increasing aging time is contributed from the increase in precipitate volume fraction and the transformation from β’’ to β’ phase with basal → prismatic and spherical → spindle-like precipitate changes. The decrease in hardness after the peak-aging stage is attributed to the appearance of micro-sized β precipitates. Because of the smaller size of precipitates and the triangular arrangement of β’ precipitate, the hardness of the alloy aged at 200℃ is higher than that aged at 250℃. For the alloy aged at 300℃, the appearance of only micro-sized β precipitate and its coarsening with increasing aging time leads to the lowest hardness and an overall decrease in hardness with the aging time.

Effect of Y on age hardening response and mechanical properties of Mg-xY-1.5LPC-0.4Zr alloys

Ageing hardening,microstructure and mechanical properties of Mg-xY-1.5LPC-0.4Zr（x=0,2,4,6）alloys（LPC represents La-based rare earth metal）were investigated.It was found that the age hardening was enhanced,the grains became finer and the tensile strength was improved with the increase of Y content in Mg-Y-1.5LPC-0.4Zr alloy.The results show that the formed precipitates responsible for age hardening change from fine hexagonal-shaped equilibrium Mg12RE phase to metastableβ′phase with orthorhombic-bc crystal structure when Y is added into Mg-1.5LPC-0.4Zr alloy,and the volume fraction of precipitate phases also increases.The cubic-shapedβ-Mg24Y5 precipitate phases were also observed at grain boundaries in Mg-6Y-1.5LPC-0.4Zr alloy. The distribution of prismatic shapedβ′phases and cubic shapedβ-Mg24Y5 precipitate phases in Mg matrix may account for the remarkable enhancement of tensile strength of Mg-Y-LPC-Zr alloy.The Mg-6Y-1.5LPC-0.4Zr alloy exhibits maximum tensile strength at peak-aged hardness,and the values are 250 MPa at room temperature and 210 MPa at 250°C.

Age hardening,fracture behavior and mechanical properties of QE22 Mg alloy

The microstructure,mechanical properties and fracture behavior of an as-received QE22 alloy have been investigated under different thermal conditions,including solution treated(ST),under aged(UA),peak aged(PA)and over aged(OA)conditions.A significant increase in hardness of 27%,yield strength of 60%and ultimate tensile strength of 19%was observed in peak aged sample as compared to solution treated sample.The improvements of mechanical strength properties are mainly associated with the metastable λ and β′precipitates.Grain growth was not observed in the ST samples after subjecting to UA and PA treatments due to the presence of eutectic Mg_(12)Nd particles along the grain boundaries.In over aged sample,significant grain growth occurred because of dissolution of eutectic phase particles.Different natures of crack initiation and propagation were observed under different thermal conditions during tensile testing at room temperature.The mode of failure of solution treated sample is transgranular,cleavage and twin boundary fractures.A mixed mode of transgranular,intergranular,cleavage and twin boundary failure is observed in both peak aged and over aged samples.

Age-hardening of surface aged hardening alloy

Steel T8 treated by plasma surface decarburizing was alloyed by the Xu Tec process with Co,W and Mo. An alloyed layer of Fe Co W Mo with low carbon content was formed on the surface of the high carbon steel, thus an advanced gradient composite was produced. The specimens then were treated by the solution and aging treatments. The characteristics of age hardening of the alloying layer were studied. The hardness of the surface layer increases from HV200 to HV1 200 after the solution treatment at 1 190 ℃ and aging at 400 ℃ for 30 min. The results show that the surface aged high speed steel possesses not only high surface hardness, but also enough bulk strength. [

Evaluation of Cracking Resistance of Copper-Bearing Age Hardening Steel Weldment

The weldability of a low-carbon copper-bearing age hardening steel was evaluated using cracking suscepti- bility calculation, HAZ maximum hardness measurement, and Y-groove cracking evaluation test. The results show that the hardenability characteristics and cold cracking susceptibility of the steel are very low. The results also indicate that a crack-free weldment can be obtained during the welding of this type of steel even at an ambient temperature as low as -5 ℃ as well as in an absolute humidity lower than 4 000 Pa without any preheat treatment. A slight preheat treatment can prevent the joint from cracking when welding is carried out at lower ambient temperature or higher absolute humidity.

METASTABLE PHASE AND AGE HARDENING IN RAPIDLY－SOLIDIFIED SILVER－RICH Ag－Gd ALLOYS

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Age hardening process modeling and optimization of aluminum alloy A356/Cow horn particulate composite for brake drum application using RSM,ANN and simulated annealing

Most conventional ceramic based aluminum metal matrix composites(MMCs) are either heavy,costly or combination of both. In order to reduce cost and weight,while at the same time maintaining quality,cow horn particles(CHp) was used with aluminum alloy A356 to produce MMC for brake drum application and other engineering uses. The aim of this research is to model the age hardening process of the produced composite using response surface methodology(RSM) and artificial neural network(ANN),and to use the developed ANN as fitness function for a simulated annealing optimization algorithm(SA-NN system) for optimization of age hardening process parameters. The results show that ANN modeled the age hardening data excellently and better than RSM with a correlation coefficient of experimental response with ANN predictions being 0.9921 as against 0.9583 for the RSM. The SA-NN system optimized process parameters were in very close agreement with the experimental values with the maximum relative error of 1.2%,minimum of 0.35% and average of 0.71%.

Age Hardening in Rapidly Solidified Al-V-Zr Alloy

Rapidly solidified Al-3.9V-0.1 Zr alloy is a single sol- id solution.The metastable limited solid solubility of solute V in Al reaches 3.9 wt-% under about 10~6 K/s cooling rate.A strong age hardening response have been observed in this alloy.A large amount of dislocation lines and loops were observed in the alloy aged at 100-150℃: and an age peak occurs at 450℃,the hardness value in- creases by a factor of 2.The precipitate which is responsi- ble for age hardening is identified to be Al_V(Al).

Solid Solubility and Age Hardening of Rapidly Solidified Al-Er Alloys

The metastable extension of solid solubility and age hardening of rapidly solidified Al-Er alloy were inves- tigated.The splat foils(0.04～0.06 mm in thickness)and spun ribbons(0.02～0.04 mm in thickness)of Al-Er alloys were prepared by melt hammer-anvil and melt spinning techniques,respectively.The cooling rate was about 10~6～10~7 K/s.The metastable extended solid solubility of Er in Al evaluated by measurements of lat- tice parameters of rapidly solidified alloys was 0.75 at%.An age hardening response was observed in the quenched and aged specimen.The precipitate responsible for age hardening was identified to be Cu_3Au type cubic Al_3Er.

Effect of Sc on Precipitation Hardening of AlSi6Mg Alloy

The effect of Sc on precipitation hardening of AlSi6Mg was studied.Zr was previously reported that it increased the effectiveness of Sc in wrought aluminum in many areas so Zr was also used together with Sc in this study. Different levels of Sc and Zr additions were added to AlSi6Mg before casting in the permanent mold.The samples were precipitation hardened at different aging temperatures and for various aging time before testing for tensile strength and hardness.It was found that Sc addition into Al6SiMg can change its response to age hardening.Additions of Sc and Sc with Zr increased both yield strength and hardness for both aging temperatures.In addition,Sc was found to modify eutectic Si to obtain fibrous morphology.This effect of Sc on eutectic silicon modification has never been reported before.

Properties improvement and microstructure changes during thermomechanical treatment in sintered Cu-Au alloy

The changes in hardness, microhardness, electrical conductivity and microstructure of the sintered Cu-4%Au （mole fraction） alloy during thermomechanical treatment were studied. Following the primary strain hardening, an annealing of rolled alloy in the temperature range of 60-350 ℃ provided additional strengthening due to the anneal hardening effect. An increase in properties took place in two stages, and the best combination of properties was achieved in the alloy pre-rolled with 40% reduction after annealing at 260 ℃. Significant microstructural changes followed the changes of properties in the course of the thermomechanical treatment.

EFFECT OF PRECIPITATION HARDENING ON SERRATED FLOW CHARACTERISTICS IN AN Al－Li ALLOY

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Effects of process parameters on mechanical properties and microstructures of creep aged 2124 aluminum alloy

A series of tests were carried microstructures of 2124 aluminum alloy in increase of aging time, temperature and low-to-peak-to-low manner. No significant out to investigate the effects of process parameters on mechanical properties and creep aging process. The results show that creep strain and creep rate increase with the applied stress. The hardness of specimen varies with aging time and stress in a effect of temperature on hardness of material is seen in the range of 185-195 ℃. The optimum mechanical properties are obtained at the conditions of （200 MPa, 185 ℃, 8 h） as the result of the coexistence of strengthening S＂ and S＇ phases in the matrix by transmission electron microscopy （TEM）. TEM observation shows that applied stress promotes the formation and growth of precioitates and no obvious stress orientation effect is observed in the matrix.

Effects of deformation temperatures on microstructures,aging behaviors and mechanical properties of Mg-Gd-Er-Zr alloys fabricated by hard-plate rolling

In this investigation,a high-strength Mg-12Gd-1.0Er-0.5Zr(wt.%)alloy sheet was produced by hot extrusion(HE)and subsequent hard-plate rolling(HPR)at different temperatures.The results indicate that the microstructures of these final-rolled sheets are inhomogeneous,mainly including coarse deformed grains and dynamic recrystallized(DRXed)grains,and the volume fraction of these coarse deformed grains increases as the rolling temperature increases.Thus,more DRXed grains can be found in R-385℃sheet,resulting in a smaller average grain size and weaker basal texture,while the biggest grains and the highest strong basal texture are present in R-450℃sheet.Amounts of dynamic precipitation ofβphases which are mainly determined by the rolling temperature are present in these sheets,and its precipitation can consume the content of Gd solutes in the matrix.As a result,the lowest number density ofβphase in R-450℃sheet is beneficial to modify the age hardening response.Thus,the R-450℃sheet displays the best age hardening response because of a severe traditional precipitation ofβ’(more)andβH/βM(less)precipitates,resulting in a sharp improvement in strength,i.e.ultimate tensile strength(UTS)of∼518±17 MPa and yield strength(YS)of∼438±18 MPa.However,the elongation(EL)of this sheet reduces greatly,and its value is∼2.7±0.3%.By contrasting,the EL of the peak-aging R-385℃sheet keeps better,changing from∼4.9±1.2%to∼4.8±1.4%due to a novel dislocation-induced chain-like precipitate which is helpful to keep good balance between strength and ductility.

Microstructure of Mg-8Zn-4Al-1Ca aged alloy

The microstructure of Mg-8Zn-4Al-1Ca aged alloy was investigated by TEM and HRTEM. The results show that the hardening produced in the Mg-8Zn-4Al-1Ca alloy is considerably higher than that in the Mg-8Zn-4A1 alloy. A dense dispersion of disc-like Ca2Mg6Zn3 precipitates are formed in Mg-8Zn-4Al-1Ca alloy aged at 160 ℃ for 16 h. In addition, the lattice distortions, honeycomb-looking Moir＆#233; fringes, edge dislocations and dislocation loop also exist in the microstructure. The precipitates of alloy aged at 160 ℃ for 48 h are coarse disc-like and fine dispersed grainy. When the alloy is subjected to aging at 160 ℃ for 227 h, the microstructure consists of numerous MgZn2 precipitates and Ca2Mg6Zn3 precipitates. All the analyses show that Ca is a particularly effective trace addition in improving the age-hardening and postponing the formation of MgZn2 precipitates in Mg-8Zn-4Al alloy aged at 160 ℃.

PLASMA SURFACE ALLOYING OF AGE-HIGH SPEED STEEL

It, ageing high speed steel (AHSS) with high cobalt, has a strong ability of age hardening. However, it has two disadvantages, One is short of toughness, the other is its high price because of its too much alloy content especially cobalt. All of these effect on its widespread using. Now there is a new method without the two flows to form this steel in the surface of ingot iron by use of double glow plasma surface alloying technique.In this paper, the following three parts are introduced: the metallurgical process of this surface ageing high speed steel (SAHSS), the micro structure of the alloying layer, the alloy distribution in the layer. By using the designed source pole, the special alloying process, the required surface alloying content, just like the AHSS—Co23W11Mo7 can be achieved in the surface layer. This new method has wide prospect in the fields of tools, moulds and other wear resisting fields.

Consequence of reinforced SiC particles and post process artificial ageing on microstructure and mechanical properties of friction stir processed AA7075

Friction stir processing and post process artificial ageing was successfully carried out on AA7075 with and without reinforcement of SiC particles producing defect free processed zone with uniform distribution of filler material.Effect of SiC particle reinforcement and artificial ageing times on the microstructural modifications was characterized using optical and electron microscopy,electron backscattered diffraction and X-Ray diffraction.Hardness,impact and wear tests were carried out to investigate mechanical behaviour before and after processing.Reinforcement of SiC particles during FSP and subsequent age hardening treatment brought about nearly twofold increase in hardness and impact toughness values by the combined effect of grain refinement,Zener pinning,dispersion strengthening and precipitation hardening.Significant improvement in wear resistance in terms of wear loss was also observed after processing compared to the reference material AA7075-T6.Fractured surface of post FSP age hardened AA7075 alloy exhibited features of ductile fracture during Charpy impact test.

Age hardening responses of as-extruded Mg-2.5Sn-1.5Ca alloys with a wide range of Al concentration

This article aims to explore the age hardening responses of both as-extruded and as-aged Mg-2.5 Sn-1.5 Ca-x Al alloys(x=2.0,4.0 and 9.0 wt%,termed TXA322,TXA324 and TXA329,respectively)through microstructural and mechanical characterization.Results indicate that grain size of as-extruded TXA322,TXA324 and TXA329 alloys were^16μm,~10μm and^12μm,respectively.A number of<a>and<c+a>dislocations were observed in all the as-extruded samples.Guinier–Preston(GP)zones were evidently identified in TXA322 alloy,while only a small number of Mg17 Al12 phases existed in both TXA324 and TXA329 alloys.An aging treatment facilitated the precipitation of a high number density of GP zones within the matrix of TXA322 alloy.In contrast,no obvious nano-precipitates were in as-aged TXA324 alloy.Numerous nano-Mg17 Al12 phases were formed through a following aging treatment in TXA329 alloy.In terms of mechanical properties,it is apparent that an increment in ultimate tensile strength of^46 MPa and^40 MPa was yielded in peak-aged TXA322 and TXA329 alloys,while no obvious variations in UTS were present in peak-aged TXA324 alloy,in comparison with the as-extruded counterparts.

Novel bake hardening mechanism for bainite-strengthened complex phase steel

In the automobile industry,stamping and paint baking processes are used to strengthen components,and this not only saves costs,but also further reduces the bodyweight of automobiles.In this work,the bake hardening mechanism of the complex phase(CP)steel CP980 was explored by comparing it with that of DP1180,which has a clear bake hardening mechanism and a carbon content similar to that of CP980.By analyzing the bake hardening response and the microstructural changes of the two steels,we found that the bake hardening process of CP980 was divided into three stages.In the first two stages,the carbon atoms diffused into dislocations to form Cottrell atmosphere pinning dislocations,and excess car-bon atoms formed carbon clusters or low-temperature carbide pinning dislocations that were similar to DP1180.In the third stage,the dislocation acted as rapid channels for carbon diffusion,and fine cementite gradually formed when the carbon clusters gathered at the dislocations as precursors,resulting in pre-cipitation hardening.This novel bake-hardening(BH)mechanism is crucial for obtaining a comprehensive understanding of the strain-baking behavior of advanced high-strength steals(AHSS).