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.

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.

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.

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.

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).

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.

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.

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.

Effects of cold rolling on the microstructure and mechanical properties of Fe-Ni-Mn-Mo-Ti-Cr maraging steels

Effects of cold rolling on the microstructure and mechanical properties of Fe-Ni-Mn-Mo-Ti-Cr maraging steels were studied.To investigate the microstructure and mechanical properties,optical microscopy,scanning electron microscopy,X-ray diffraction,tensile test,and hardness test were used.The results show that the solution-annealing treatment in the cold-rolled steel redounds to the formation of sub-microcrystalline Fe2（Mo,Ti） Laves phase particles,which are stable at high temperatures.These secondary Laves phase particles prevent from recrystallization at high temperatures and correspond to semi-brittle fracture in the subsequent aging treatment.

PRECIPITATION AND MICROSTRUCTURE OF Cr MODIFIED Al_3Ti

The microstructures of Cr modified Al3Ti containing Al2Ti and L10-AlTi precipitates have been investigated in terms of transmission electron microscopy(TEM). Fine precipitation of Al2Ti (Ga2Hf type structure) and Ll0-AlTi(Cr) occurs in Ll2-Al3Ti(Cr) by aging around 973K. The aging behavior was investigated by microhardness measurements. TEM observations reveal that plate-like Al2Ti precipitates lie on{100} planes of the Ll2-Al3Ti(Cr) matrix with the c axis of the tetragonal phase perpendicular to the thin plate. As contrast with Al2Ti precipitates, Ll0-AlTi(Cr) precipitates form thin plate and lie on {100} planes of Ll2-Al3Ti(Cr) matrix at the initial aging time and for long time aging the habit plane of the thin plate deviates from {100} plane and finally forms a kinked plate. The coherency stresses across the precipitate/matrix interface are considered to be the main factors controlling the precipitate morphology.

Electrical Conductivity Behavior of the Aluminum Alloy 2024 during Artificial Aging

Precipitation-hardened aluminum alloys need to be heat treated to achieve the mechanical properties required for their application.The production of these materials can be optimized to make them more attractive and competitive comparing to other materials such as composites that have a growing and large market share in aeronautics field.One way to do this is by controlling the artificial aging of precipitation-hardenable aluminum alloys,such as the 2000 series Al-Cu alloys.These alloys can be monitored in real time by analyzing their conductivity behavior inside the furnace.The objective of this work is to evaluate the electrical conductivity behavior in real time of the 2024 alloy during the artificial aging at 190°C.For this,analyses were made in order to assess the behavior of the microhardness curve by aging time and its microstructural characterization with thermal treatments in the times of 1 h to 9 h interrupted every 1 h.The results of the electrical conductivity versus hardness curve showed a significant correlation and indicate that this measure has great potential to be used as a tool to control the thermal treatment of aging.

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).

Improving the Mechanical Properties of Mg–Gd–Y–Ag–Zr Alloy via Pre‑Strain and Two‑Stage Ageing

In this study,we investigated the effects of single-stage ageing(SSA),two-stage ageing(TSA),2%pre-strain+single-stage ageing(P2%SSA)and 2%pre-strain-I-two-stage ageing(P2%TSA)on the mechanical properties of as-extruded Mg-8Gd-3Y-0.5Ag-0.5Zr alloy(E alloy).Compared with the SSA treatment,the TSA treatment increased the number density of/β'phase.The P2%SSA and P2%TSA treatments generated theγ'phase and chain-like precipitates in addition to the/β'phase.The contributions of these ageing treatments to the alloy strengthening can be ranked as P2%TSA>P2%SSA>TSA>SSA,because the increments in the tensile yield strength were estimated to be 199 MPa>148 MPa>144 MPa>110 MPa.Dif­ferent from the traditional strengthening of/β'phase in the E+SSA and E+TSA alloys,the composite precipitates compris­ing the/β'phase,γ'phase and chain-like precipitates in the E+P2%SSA and E+P2%TSA alloys provided better combined strengthening effect.The/β'phase was still dominated in the strengthening effect of the composite precipitates.Owing to the higher number density of/β'phase in the composite precipitates,the E+P2%TSA alloy exhibited the better mechanical performance as compared with the E+P2%SSA alloy.Finally,the E+P2%TSA alloy had the ultimate tensile strength of 452 MPa,the tensile yield strength of 401 MPa and elongation to failure of 3.3%.

DEVELOPMENT OF MATERIAL USED FOR AVIATION APPARATUS AXLE JOURNALS

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Microstructure evolutions of Mg-8Gd-2Er(wt.%) alloy during isothermal ageing at 200℃

The precipitation of the Mg-8Gd-2Er （wt.%） alloy at 200 ℃ was investigated by transmission electron microscopy （TEM） and X-ray diffraction （XRD）. The precipitation sequence was described as supersaturated solid solution （S.S.S.S., cph）→β（D019）→β（bco）→ β-phase（fcc）, which was different from previously reported four stages sequence： （S.S.S.S., cph）→β（DO19）→β（bco）→β（fcc）→β-phase（fcc）. The morphologies and crystal structures of both β＇ and,β precipitates in the alloy were studied in the present investigation. The orientation relationship between the precipitate phases and the α-matrix was observed. The ultimate tensile strength （UTS） and tensile yield strength （TYS） of the as-cast alloy were 189 and 110 MPa, respectively, and the UTS and the YTS were improved greatly after ageing treatment. The values of UTS and YTS after peak ageing process were 308 and 246 MPa, respectively. The improved mechanical properties were mainly ascribed to the formation of β phase. The age hardening response decreased with increasing ageing time because the β phase got coarse as ageing time increased.

Effects of Pre-Ageing Treatment on Subsequent Artificial Ageing Characteristics of an Al-1.01Mg-0.68Si-1.78Cu Alloy

The influences of pre-ageing temperature and natural ageing time on subsequent artificial age hardening behavior and precipitation sequence of new type Al-1.01Mg-0.O8Si-l.78Cu alloy were investigated by hardness test, differential scanning calorimetry （DSC） test and transmission electronic microscopy （TEM） observations. When pre-ageing temperature is 20℃ （natural ageing）, the peak hardness of subsequent artificial aged alloy is lower than that of T6 treated alloy （negative effect）, while a positive effect appears when pre-ageing temperature is above 80℃. The size of needlelike /3-precipitate in subsequent artificial aged alloy is much coarser when pre-ageing temperature is 20℃, which causes a decrease in peak-hardness. The positive effect occurs again when natural ageing time is longer than 3 weeks. There are seven exothermic peaks in DSC curve of as-quenched alloy, while the number and height of exothermic peak decrease with increasing pre-ageing temperature and natural ageing time.

Development of high mechanical properties and moderate thermal conductivity cast Mg alloy with multiple RE via heat treatment

A new cast Mg-2Gd-2Nd-2Y-1Ho-1Er-0.5Zn-0.4Zr （wt%） alloy was prepared by direct-chill semicontinuous casting technology. The microstructure, mechanical properties and thermal conductivity of the alloy in as-cast, solid-solution treated and especially peak-aged conditions were investigated. The as-cast alloy mainly consists of α-Mg matrix, （Mg, Zn）3 RE phase and basal plane stacking faults. After proper solid-solution treatment, the microstructure becomes almost Mg-based single phase solid solution except just very few RE-riched particles. The as-cast and solid-solution treated alloys exhibit moderate tensile properties and thermal conductivity. It is noteworthy that the Mg alloy with 8 wt% multiple RE exhibits remarkable age-hardening response （AHV=35.7）, which demonstrates that the multiple RE （RE = Gd, Nd, Y, Ho, Er） alloying instead of single Gd can effectively improve the age-hardening response. The peak-aged alloy has a relatively good combination of high strength/hardness （UTS （ultimate tensile strength） 〉 300 MPa; TYS （tensile yield strength）〉210 MPa; 115.3 HV）, proper ductility （ε≈ 6%） and moderate thermal conductivity （52.5 W/（m K））. The relative mechanisms mainly involving aging precipitation of β″ and β＂ phases were discussed. The results provide a basis for development of high performance cast Mg alloys.2017 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science ＆ Technology.

Development of Low Carbon Microalloyed High Strength Ultra-Thin Cast Strip Products Produced by the Castrip~ Process With Excellent Strength-Ductility Properties

The CASTRIP* process produces a range of high strength Ultra-thin Cast Strip (UCS) products (380-550 MPa) in thicknesses between 0.9 mm and 1.5 mm, which is very challenging to produce via conventional hot and cold rolled processing routes. The twin roll CASTRIP process fully exploits the hardenability and strengthening potential of Nb in a low C-Mn-Nb-V microalloyed steel type. Significant microstructural strengthening from solute Nb was obtained, even at low microalloying levels, as well as modest Mn additions, through enhancing the hardenability and further strengthening was obtained in coated products by exploiting age hardening during processing on a continuous hot dip galvanising line. Atom probe tomography and TEM determined that Nb was retained in solid solution and subsequent age hardening resulted from the formation of Nb and V rich nanosized particles. Age hardening was achieved without loss of ductility producing galvanised strip with an excellent strength-ductility combination (Y.S. 】600 MPa, T.E. 】10%).

Microstructure and Mechanical Properties of a CoFeNi_2V_(0.5)Nb_(0.75) Eutectic High Entropy Alloy in As-cast and Heat-treated Conditions

The eutectic CoFeNi2V0.5Nb0.75 high entropy alloys （HEAs） were heated at 500, 600, 700, 800 and 1000 ℃, respectively for 6 h and subsequently quenched in the water to investigate their thermal stability and phase transformation at high temperature. The microstructure and mechanical properties of the samples were investigated by scanning electron microscopy, X-ray diffraction, compressive and hardness tests. It was found that the as-cast CoFeNi2V0.5Nb0.75 HEAs showed a eutectic microstructure with alternating fcc solid solution phase and Fe2Nb-type Laves phase. The NbNi4-type intermetallic phase appeared when the heat-treated temperature was higher than 600 ℃. With increasing quenching temperature, the volume fraction of the NbNi4-type intermetallic phase increased while that of the eutectic regions decreased. The sample quenched at 800 ℃ showed the most excellent comprehensive mechanical properties; its fracture strength, yield strength and plastic strain were as high as 2586.76 MPa, 2075.18 MPa and 16.73%, respectively. Moreover, the eutectic CoFeNi2V0.5Nb0.75 HEAs exhibited apparent age hardening, especially quenched at 700 ℃, the hardness reached up to the maximum value of HV 727.52.

Microstructure and mechanical properties of Mg-xY-1.5MM-0.4Zr alloys

Age hardening,microstructure and mechanical properties of Mg-xY-1.5MM-0.4Zr （x=0,2,4,6 wt.%） alloys （MM represents Ce-based misch-metal） were investigated by scanning electron microscopy （SEM） and transmission electron microscopy （TEM）. The results showed that the formed precipitates being responsible for age hardening changed from fine hexagonal-shaped equilibrium Mg12MM phase to metastable β＇ phase with bco crystal structure when Y was added into Mg-1.5MM-0.4Zr alloy,and the volume fraction of precipitate phases also increased. With the increase of Y content in Mg-Y-1.5MM-0.4Zr alloys,it was found that the age hardening was enhanced,the grain sizes became finer and the tensile strength was improved. The cubic-shaped β-Mg24Y5 precipitate phases were observed at grain boundaries in Mg-6Y-1.5MM-0.4Zr alloy. It was suggested that the distribution of prismatic shaped β＇ phases and cubic shaped β-Mg24Y5 precipitate phases in Mg matrix might account for the remarkable enhancement of tensile strength of Mg-Y-MM-Zr alloy. It was shown that the Mg-6Y-1.5MM-0.4Zr alloy was with maximum tensile strength at aged-peak hardness,UTS of 280 MPa at room temperature and 223 MPa at 250 oC,respectively.