Microstructure and high-temperature mechanical properties of laser beam welded TC4/TA15 dissimilar titanium alloy joints

The microstructure evolution and high-temperature mechanical properties of laser beam welded TC4/TA15 dissimilar titanium alloy joints under different welding parameters were studied.The results show that the weld fusion zone of TC4/TA15 dissimilar welded joints consists of coarsenedβcolumnar crystals that contain mainly acicularα’martensite.The heat affected zone is composed of the initialαphase and the transformedβstructure,and the width of heat affected zone on the TA15 side is narrower than that on the TC4 side.With increasing temperature,the yield strength and ultimate tensile strength of the TC4/TA15 dissimilar welded joints decrease and the highest plastic deformation is obtained at 800°C.The tensile strength of the dissimilar joints with different welding parameters and base material satisfies the following relation(from high to low):TA15 base material>dissimilar joints>TC4 base material.The microhardness of a cross-section of the TC4/TA15 dissimilar joints reaches a maximum at the centre of the weld and is reduced globally after heat treatment,but the microhardness distribution is not changed.An elevated temperature tensile fracture of the dissimilar joints is located on the side of the TC4 base material.Necking occurs during the tensile tests and the fracture characteristics are typical when ductility is present in the material.

Microstructure and high-temperature mechanical properties of near net shaped Ti−45Al−7Nb−0.3W alloy by hot isostatic pressing process

Near net shaped Ti−45Al−7Nb−0.3W alloy(at.%)parts were manufactured by hot isostatic pressing(HIP).The microstructure and high-temperature mechanical properties of the alloy were investigated by X-ray diffractometry(XRD),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).The results show that at a temperature of 700℃,the peak yield stress(YS)and ultimate tensile stress(UTS)of alloy are 534 and 575 MPa,respectively,and the alloy shows satisfactory comprehensive mechanical properties at 850℃.The alloy exhibits superplastic characteristics at 1000℃ with an initial strain rate of 5×10^−5 s^−1.When the tensile temperature is below 750℃,the deformation mechanisms are dislocation movements and mechanical twinning.Increasing the tensile temperature above 800℃,grain boundary sliding and grain rotation occur more frequently due to the accumulation of dislocations at grain boundary.

High-temperature mechanical properties of aluminium alloys reinforced with titanium diboride (TiB_2) particles

The physical and mechanical properties of metal matrix composites were improved by the addition of reinforcements. The mechanical properties of particulate-reinforced metal-matrix composites based on aluminium alloys （6061 and 7015） at high temperatures were studied. Titanium diboride （TiB2） particles were used as the reinforcement. All the composites were produced by hot extrusion. The tensile properties and fracture characteristics of these materials were investigated at room temperature and at high temperatures to determine their ultimate strength and strain to failure. The fracture surface was analysed by scanning electron microscopy. TiB2 particles provide high stability of the alumin- ium alloys （6061 and 7015） in the fabrication process. An improvement in the mechanical behaviour was achieved by adding TiB2 particles as reinforcement in both the aluminium alloys. Adding TiB2 particles reduces the ductility of the aluminium alloys but does not change the microscopic mode of failure, and the fracture surface exhibits a ductile appearance with dimples formed by coalescence.

High-temperature mechanical properties and deformation behavior of high Nb containing TiAl alloys fabricated by spark plasma sintering

A high Nb containing TiA1 alloy was prepared from the pre-alloyed powder of Ti-45Al-8.5Nb-0.2B-0.2W-0.02Y （at%） by spark plasma sintering （SPS）. Its high-temperature mechanical properties and compressive deformation behavior were investigated in a temperature range of 700 to 1050℃ and a strain rate range of 0.002 to 0.2 s 1. The results show that the high-temperature mechanical properties of the high Nb containing TiA1 alloy are sensitive to deformation temperature and strain rate, and the sensitivity to strain rate tends to rise with the deformation temperature increasing. The hot workability of the alloy is good at temperatures higher than 900℃, while fracture occurs at lower temperatures. The flow curves of the samples compressed at or above 900℃ exhibit obvious flow softening after the peak stress. Un- der the deformation condition of 900-1050℃ and 0.002-0.2 s 1, the interrelations of peak flow stress, strain rate, and deformation tempera- ture follow the Arrhenius＇ equation modified by a hyperbolic sine function with a stress exponent of 5.99 and an apparent activation energy of 441.2 kJ.mol-1.

High-temperature mechanical properties of near-eutectoid steel

The high-temperature mechanical properties of near-eutectoid steel were studied with a Cleeble-1500 simu- lation machine. Zero strength temperature （ZST）, zero ductility temperature （ZDT）, hot ductility curves, and strength curves were measured. Two brittle zones and one plastic zone were found in the temperature range from the melting point to 600℃. Embrittlement in zone I is caused by the existence of liquid film along dendritic interfaces. Ductility loss in zone Ⅲ mainly results from precipitates and inclusions as well as S segregation along grain boundaries. Pearlite transformation also accounts for ductility deterioration in the temperature range of 700-600℃. Moreover, the straightening temperature of the test steel should be higher than 925℃ for avoiding the initiation and propagation of surface cracks in billets.

A novel high-Cr CoNi-based superalloy with superior high-temperature microstructural stability, oxidation resistance and mechanical properties

A novel multicomponent high-Cr CoNi-based superalloy with superior comprehensive performance was prepared,and the evaluation of its high-temperature microstructural stability,oxidation resistance,and mechanical properties was conducted mainly using its cast polycrystalline alloy.The results disclosed that the morphology of theγ′phase remained stable,and the coarsening rate was slow during the long-term aging at 900–1000℃.The activation energy forγ′precipitate coarsening of alloy 9CoNi-Cr was(402±51)kJ/mol,which is higher compared with those of CMSX-4 and some other Ni-based and Co-based superalloys.Importantly,there was no indica-tion of the formation of topologically close-packed phases during this process.All these factors demonstrated the superior microstructural stability of the alloy.The mass gain of alloy 9CoNi-Cr was 0.6 mg/cm^(2) after oxidation at 1000℃ for 100 h,and the oxidation resistance was comparable to advanced Ni-based superalloys CMSX-4,which can be attributed to the formation of a continuous Al_(2)O_(3) protective layer.Moreover,the compressive yield strength of this cast polycrystalline alloy at high temperatures is clearly higher than that of the conventional Ni-based cast superalloy and the compressive minimum creep rate at 950℃ is comparable to that of the conventional Ni-based cast superalloy,demonstrating the alloy’s good mechanical properties at high temperature.This is partially because high Cr is bene-ficial in improving theγandγ′phase strengths of alloy 9CoNi-Cr.

Improving mechanical properties and high-temperature oxidation of press hardened steel by adding Cr and Si

This work investigated the effect of Cr and Si on the mechanical properties and oxidation resistance of press hardened steel.Results indicated that the microstructure of the Cr-Si micro-alloyed press hardened steel consisted of lath martensite,M_(23)C_(6)carbides,and retained austenite.The retained austenite and carbides are responsible for the increase in elongation of the micro-alloyed steel.In addition,after oxidation at 930℃for 5 min,the thickness of the oxide scales on the Cr-Si micro-alloyed press hardened steel is less than 5μm,much thinner than 45.50μm-thick oxide scales on 22MnB5.The oxide scales of the Cr-Si micro-alloyed steel are composed of Fe_(2)O_(3),Fe_(3)O_(4),mixed spinel oxide(FeCr_(2)O_(4)and Fe_(2)SiO_(4)),and amorphous SiO_(2).Adding Cr and Si significantly reduces the thickness of the oxide scales and prevents the generation of the FeO phase.Due to the increase of spinel FeCr_(2)O_(4)and Fe_(2)SiO_(4)phase in the inner oxide scale and the amorphous SiO_(2)close to the substrate,the oxidation resistance of the Cr-Si micro-alloyed press hardened steel is improved.

Effect of V on high-temperature mechanical properties of Al-5Cu-1.5Ni alloy

The effects of V on microstructure,room temperature,and high-temperature(350℃)mechanical properties of Al-5Cu-1.5Ni alloy were investigated.The results show that Al-5Cu-1.5Ni-xV alloys mainly compriseα-Al,Al_(2)Cu,Al_(3)(Ni,Cu)_(2),Al_(3)Ni,Al7Cu4Ni,and Al_(10)V phases.The addition of V can signally refineα-Al dendritic structure,which improves the mechanical properties of the as-cast alloys at room and high temperatures.After T6 heat treatment(540℃/12 h/WC+170℃/8 h/AC),α-Al,network skeleton structure Al_(3)(Ni,Cu)_(2),and Al_(10)V phases all show coarsening,while the fine nano-θ'-Al_(2)Cu precipitated from the matrix and dispersed distributed.The introduction of V promotes the quantity of precipitatedθ'-Al_(2)Cu particles.The ultimate tensile strength of the heat-treated alloys at room and high temperatures is greatly improved compared to that of the as-cast alloy.The highest high-temperature ultimate tensile strength of the alloys with V is 111.8 MPa,21.5%higher than that of the base alloy.The analysis shows that the improvement of high-temperature mechanical properties after heat treatment is mainly due to the introduction of V,increasing the precipitated quantity of nano-θ'-Al_(2)Cu particles and improving its thermal stability.The dispersion strengthening effect ofθ'-Al_(2)Cu particles exceeds the weakening effect of other adverse microstructures on the mechanical properties.

Effect of extrusion process on microstructure and mechanical properties of Ni_3Al-B-Cr alloy during self-propagation high-temperature synthesis

The well-densified Ni3Al-0.5B-5Cr alloy was fabricated by self-propagation high-temperature synthesis and extrusion technique. Microstructure examination shows that the synthesized alloy has fine microstructure and contains Ni3Al, Al2O3, Ni3B and Cr3Ni2 phases. Moreover, the self-propagation high-temperature synthesis and extrusion lead to great deformation and recrystallization in the alloy, which helps to refine the microstructure and weaken the misorientation. In addition, the subsequent extrusion procedure redistributes the Al2O3 particles and eliminates the γ-Ni phase. Compared with the alloy synthesized without extrusion, the Ni3Al-0.5B-5Cr alloy fabricated by self-propagation high-temperature synthesis and extrusion has better room temperature mechanical properties, which should be ascribed to the microstructure evolution.

Enhanced thermal stability and mechanical properties of high-temperature resistant Al-Cu alloy with Zr and Mn micro-alloying

The high temperature(HT)thermal stability and mechanical properties of Al-5%Cu(AC)and Al-5%Cu-0.2%Mn-0.2 Zr%(ACMZ)alloys from 573 to 673 K were systematically studied.The results displayed that micro-alloying additions of Zr and Mn elements have presented a significant role in stabilizing the main strengthening metastableθ′precipitates at a temperature as high as 573 K.Simultaneously,the HT tensile test demonstrated that ACMZ alloy retained their strength of(88.6±8.8)MPa,which was much higher than that of AC alloy((32.5±0.8)MPa)after the thermal exposure at 573 K for 200 h.Finally,the underlying mechanisms of strength and ductility enhancement mechanism of the ACMZ alloy at HT were discussed in detail.

Extraordinary high-temperature mechanical properties in binder-free nanopolycrystalline WC ceramic

From the perspective of high-temperature applications,materials with excellent high-temperature mechanical properties are always desirable.The present work demonstrates that the binder-free nanopolycrystalline WC ceramic with an average grain size of 103 nm obtained by high-pressure and hightemperature sintering exhibits excellent mechanical properties at both room temperature and high temperature up to 1000℃.Specifically,the binder-free nanopolycrystalline WC ceramic still maintains a considerably high Vicker hardness H_(V)of 23.4 GPa at 1000℃,which is only 22%lower than the room temperature H_(V).This outstanding thermo-mechanical stability is superior to that of typical technical ceramics,e.g.SiC,Si_(3)N_(4),Al_(2)O_(3),etc.Nanocrystalline grains with many dislocations,numerous low-energy,highly stableΣ2 grain boundaries,and a relatively low thermal expansion coefficient,are responsible for the observed outstanding high-temperature mechanical properties.

Improvement of High-Temperature Mechanical Properties of Low-Carbon RAFM Steel by MX Precipitates

To investigate the influence of tantalum content on high-temperature mechanical properties of low-carbon reduced acti- vation ferritic/martensitic （RAFM） steels, RAFM steels containing different tantalum contents （0 and 0.073%） were fabricated, and the tensile tests at room temperature and high temperature were performed, as well as the creep tests were conducted at 550 ~C with the applied stress of 180 and 220 MPa. It was found that 0.073% tantalum addition results in the increase in amount of stable carbonitrides （MX）, and the creep rupture time of the steel under 180 MPa is obviously increased. In addition, the increase in MX caused by tantalum addition also leads to the improvement of high-temperature tensile strength. The improvement of high-temperature mechanical properties of RAFM steels is primarily related to the evolution of precipitates.

Effects of nano-ceramic additives on high-temperature mechanical properties and corrosion behavior of 310S austenitic stainless steel

A novel approach to reduce Ni content for the 310S austenitic stainless steel was proposed.The nano-ceramic additive(L)was applied to 310S steel to replace part of Ni element and reduce the cost.By means of thermal simulation,X-ray diffraction,field emission scanning electron microscopy,and electron backscattered diffraction,the effects of nanoceramic additives on high-temperature mechanical properties and corrosion behavior of the 310S steel were studied.The results indicate that the morphology and density of the(Fe,Cr)_(23)C_(6)carbides are varied,which play an important role in the high-temperature mechanical properties and corrosion behavior.After adding nano-ceramic additives,the high-temperature tensile strength and yield strength are improved simultaneously,in spite of a slight decrease in the total elongation.During high-temperature corrosion process,the mass gain of all the samples is parabolic with time.The mass gain is increased in the 310S steel with nano-ceramic additive,while the substrate thickness is significantly larger than 310S steel.The more stable and adherent FeCr_(2)O_(4)spinel form is the reason why the high-temperature corrosion resistance was increased.The(Fe,Cr)_(23)C_(6)carbides distribution along grain boundaries is detrimental to the high-temperature corrosion resistance.

High-Temperature Mechanical Properties of Shipbuilding Steel BC in Continuous Casting

The mechanical property of shipbuilding steel BC has been studied by means of tensile test at various temperatures from 700 ℃ to 1000℃ with theGleeble-1500D thermo-mechanical simulator.The results indicate that the yield strength and tensile strength of steel have an analogous change pattern as temperature decreasing,and the transition in variation rate of strength occurs at 800℃ in both of them;the hot ductility trough of steel BC is a temperature range from 725℃ to 800℃,while the best hot ductility ranges from 875℃ to 1000℃ with the ductility value over 80%.For the sake of understanding the fracture mechanism of the steel,fracture surface and microstructure of the specimens have been examined by scanning electron microscope and metalloscope correspondingly.The results show that both the second phase particles and the pro-eutectoid ferrite surrounding the austenite boundaries play a significant role to the variation of hot ductility of steel BC.Deservedly,the research is important to the improvement and the further studies on the quality of steel during slab continuous casting process.

Enhanced mechanical properties of lamellar Cu/Al composites processed via high-temperature accumulative roll bonding

Cu/Al multilayers were produced by high-temperature accumulative roll bonding(ARB)methods up to three passes.To achieve a high bonding strength,prior to ARB processing,the Cu and Al sheets were heated to 350,400,450 and 500 ℃,respectively.The mechanical properties were evaluated by tensile tests.The microstructure was examined by optical microscopy and scanning electron microscopy equipped with energy dispersive spectrometry.The ultimate tensile stress,the grain size and the thickness of diffusion layer of lamellar composites increase with rolling temperature.When the rolling temperature is 400 ℃,the laminates show the highest ductility,but the yield stress is the lowest.As the rolling temperature further increases,both the yield stress and the ultimate tensile stress increase and the ductility decreases slightly.The mechanical properties of lamellar composites processed by low and high temperature ARB are determined by grain size and the thickness of diffusion layer,respectively.

Mechanical Properties of DS NiAl/Cr(Mo) Alloys with Low Addition of Hf for High-temperature Applications

A multiphase NiAl-28Cr-5.85Mo-0.15Hf alloy, which was directionally solidified （DS） in an Al2O3-SiO2 mold by standard Bridgman method and then underwent prolonged solution and aging treatment was prepared. The microstructure, tensile properties as well as tensile creep of the heat-treated alloy at different temperatures were studied. The alloy was composed of NiAI, Cr（Mo） and Hf-rich phase and small amount of fine Heusler phase （Ni2AlHf）. Although the present alloy exhibited high tensile strength at low temperature, it was weaker than that of system with high content Hf but still stronger than that of many NiAl-based alloys at high temperatures. The fracture toughness is lower than that of DS NiAl-28Cr-6Mo alloy. Nevertheless, advantageous effects on the mechanical properties, i.e. the decrease in brittle-to-ductile transition temperature （BDTT） were obtained for the low content of Hf. The obtained creep curves exhibit conventional shape： a short primary creep and long accelerated creep stages. The rupture properties of the heat-treated alloy follow the Monkman-Grant relationship, which exhibits similar creep behavior to that of NiAl/Cr（Mo） system with high Hf content.

Effects of Cooling Rate on High-temperature Mechanical Properties of A356 Alloys

High temperature mechanical properties of A356 alloy castings under different solidification cooling rates have been studied and the influence of cooling rates on secondary dendrite arm spacing (SDAS) and mechanical properties has been discussed. To get different cooling rates, three different types of mold—green sand, green sand with chill and permanent mold, were used to pour castings which would subsequently be machined into tensile test and metallographic specimens. The temperature curves of castings’ solidification in three different mold were recorded using thermal couples, which would be used to calculate their corresponding cooling rates. Tensile tests were carried out at 20, 200, 300, 400 and 500 ℃ and then mechanical properties, such as tensile strength, yield strength, elongation, of specimens from different mold types at different test temperatures were obtained. And SDAS of different specimens were measured using optical metallographic photos. From integrated analysis of all those results, following conclusions could be reached. The relationship between SDAS and cooling rates is negative, and the quantitative relationship has been obtained through data fitting analyzing. Generally speaking, tensile strength and yield strength decease as the temperature elevates while elongation behaves in the contrary trend. Through the regression analysis of SDAS, mechanical properties and temperature, the relationship among them is obtained, which makes quantitative prediction of A356 alloy’s mechanical properties at different temperatures with different solidification cooling rates be possible.

Evolution laws of microstructures and mechanical properties during heat treatments for near-αhigh-temperature titanium alloys

Evolution laws of microstructures,mechanical properties,and fractographs after different solution temperatures were investigated through various analysis methods.With the increasing solution temperatures,contents of the primaryαphase decreased,and contents of transformedβstructures increased.Lamellarαgrains dominated the characteristics of transformedβstructures,and widths of secondaryαlamellas increased monotonously.For as-forged alloy,large silicides with equiaxed and rod-like morphologies,and nano-scale silicides were found.Silicides with large sizes might be(Ti,Zr,Nb)_(5)Si_(3) and(Ti,Zr,Nb)_(6)Si_(3).Rod-like silicides with small sizes precipitated in retainedβphase,exhibiting near 45°angles withα/βboundaries.Retainedβphases in as-heat treated alloys were incontinuous.980STA exhibited an excellent combination of room temperature(RT)and 650°C mechanical properties.Characteristics of fracture surfaces largely depended on the evolutions of microstructures.Meanwhile,silicides promoted the formation of mico-voids.

Experimental Evaluation of the Mechanical Properties of Cement Sheath under High-Temperature Conditions

The sealing integrity of cement sheath in offshore wells is seriously threatened under high-temperature conditions,resulting in gas channeling and other problems.Given the lack of experimental results,in this study relevant samples of a cement slurry sealing section of a typical offshore high-temperature well have been prepared and analyzed.In particular,the mechanical properties have been assessed with a triaxial pressure servo instrument and a high-temperature curing kettle.The density and the Poisson’s ratio of the samples have also been tested.The stress-strain curve has been drawn to obtain the elastic modulus and the compressive strength.The rock brittleness index has been calculated according to the measured elastic modulus and the Poisson’s ratio,together with the brittleness and the compressibility of the cement samples.The test results show that the mechanical properties and bonding strength of the cement samples are optimal at 130°C,medium at 150°C,and poor at 180°C.

High-temperature Mechanical Properties and Dynamic Recrystallization Mechanism of in situ Silicide-reinforced MoNbTaTiVSi Refractory High-entropy Alloy Composite

The hypoeutectic composite material composed of BCC phase and in situ precipitated Ti_(5)Si_(3) was prepared by adding Si into MoNbTaTiV high-entropy alloy.The obvious oriented in situ Ti_(5)Si_(3) phase formed eutectic phase with BCC phase in the inter-dendritic area,which leads to excellent properties of the composite.The alloy exhibits ultra-high yield stress of 718 MPa at 1200℃ and obvious compression plasticity.After reaching the maximum strength,dynamic recovery(DRV)and dynamic recrystallization(DRX)caused soften phenomena.The DRX mechanism of the dual-phase eutectic structure is analyzed by electron backscatter diffraction.The DRX of the BCC phase conforms to the discontinuous DRX and continuous DRX mechanisms,while the Ti_(5)Si_(3) phase has a geometric DRX mechanism in addition to the above two mechanisms.The high performance of this composite has enough potential high-temperature applications such as nuclear and aero engine.