Effect of Thermal Cycling under Load on Martensite Transformation and Two-way Shape Memory Effect in a TiNi Alloy

The effect of thermal cycling under loading on martensitic transformation and two-way shape memory effect was investigated for Ti-49.8 at. pci Ni alloy. It is shown that Ms and Mf temperature increase with increasing the number of cycles, while As and Af temperature decrease during thermal cycling. The total strain et and permanent strain ep increase with increasing applied stress and number of cycles. The two-way shape memory effect can be improved by proper thermal cycling training under loading, while excessively high applied stress results in the deterioration of TWSME. The reason for the changes in martensitic transformation characteristics and two-way shape memory effect during thermal cycling under loading is discussed based on the analysis of microstructure by TEM observations.

Thermal cycling behavior of nanostructured and conventional yttria-stabilized zirconia thermal barrier coatings via air plasma spray

Two thermal barrier coating(TBC)systems comprising NiCoCrAlY bond coat onto a second-generation Ni-based single crystal superalloy and nano structured4 mol%Y_(2)O_(3)-stabilized ZrO_(2)(4YSZ)and conventional yttria-stabilized zirconia(YSZ)top coats upwardly were deposited by approaches of arc ion plating(AIP)and air plasma spray(APS).As indicted by the experimental results,the 4YSZ TBCs exhibited superior thermal cycling resistance compared with conventional YSZ TBCs at 1100℃.The 4YSZ top coat exhibited higher toughness due to its intrinsic property of nanocrystalline structure,homogeneously distributed and diverse directions of pores and preexisted cracks.The cracks and spallation in 4YSZ TBCs occurred at the interface of top coat and thermally grown oxide(TGO)layer.Instead,the crack initiation and propagation started along the lamellar interface in the top coat of conventional YSZ TBCs,leading to the rapid crack bridging and subsequent spalling of top coat.Additionally,before and after oxidation,the 4YSZ top coat showed higher hardness compared to conventional YSZ top coat.Degradation mechanism and distribution of residual stress in TGO for the 4YSZ TBCs were investigated in the current study.

Thermal cycling creep properties of a directionally solidified superalloy DZ125

Aero-engine turbine blades may suffer overheating during service,which can result in severe microstructural and mechanical degradation within tens of seconds.In this study,the thermal cycling creep under(950℃/15 min+1100℃/1 min)-100 MPa was performed on a directionally solidified superalloy,DZ125.The effects of overheating and thermal cycling on the creep properties were evaluated in terms of creep behavior and microstructural evolution against isothermally crept specimens under 950℃/100 MPa,950℃/270 MPa,and 1100℃/100 MPa.The results indicated that the thermal cycling creep life was reduced dramatically compared to the isothermal creep under 950℃/100 MPa.The plastic creep deformation mainly occurred during the overheating stage during the thermal cycling creep.The thermal cycling creep curve exhibited three stages,similar to the 1100℃isothermal creep,but its minimum creep rate occurred at a lower creep strain.The overheating events caused severe microstructural degradation,such as substantial dissolution ofγ'phase,earlier formation of raftedγ'microstructure,widening of theγchannels,and instability of the interfacial dislocation networks.This microstructural degradation was the main reason for the dramatic decrease in thermal cycling creep life,as the thermal cycling promoted more dislocations to cut intoγ'phase and more cracks to initiate at grain boundaries,carbides,and residual eutectic pools.This study underlines the importance of evaluating the thermal cycling creep properties of superalloys to be used as turbine blades and provides insights into the effect of thermal cycling on directionally solidified superalloys for component design.

Microstructure,wetting property of Sn-Ag-Cu-Bi-xCe solder and IMC growth at solder/Cu interface during thermal cycling

The effects of adding small amounts of cerium(Ce) to Sn-3.3 Ag-0.2 Cu-4.7 Bi solder on microstructure,wettability characteristic,interfacial morphology and the growth of interfacial intermetallic compound(IMC) during thermal cycling were investigated by optical microscopy(OM),scanning electron microscopy(SEM) and solderability tester.It is found that the p-Sn phase,Ag_(3)Sn phase and Cu6 Sn5 phase in the solder are refined and the wetting force increases.Ce is an active element;it more easily accumulates at the solder/flux interface in the molten state,which decreases the interfacial surface energy and reduces the driving force for IMC formation on Cu substrate;therefore,the thickness of IMC at the solder/Cu interface decreases when appropriate Ce was added into the solder.Moreover,the Ce-containing solders have lower growth rate of interfacial IMC than solders without Ce during the thermal cycling between-55 and 125℃.When the Ce content is 0.03 wt% in the Sn-3.3 Ag-0.2 Cu-4.7 Bi solder,the solder has the best wettability and the minimum growth rate of interfacial IMC layer.

Thermal cycling performance of La_(2)Ce_(2)O_(7)/YSZ TBCs with Pt/Dy co-doped NiAl bond coat on single crystal superalloy

Thermal barrier coatings(TBCs) consisting ofLa_(2)Ce_(2)O_(7)(LCO) and Y_(2)O_(3)-stabilized-ZrO_(2)(YSZ) doubleceramic layer and Dy/Pt co-doped NiAl bond coat were produced by electron beam physical vapor deposition(EBPVD). Thermal cyclic performance of the TBCs was evaluated by flame shock testing at 1300 ℃. For comparison, the TBCs with a undoped NiAl bond coat were also studied. The microstructural evolution and failure mechanisms of the above TBCs during thermal cycling were investigated. Spallation failure of the TBCs with the undoped bond coat occurs after around 500 cycles by cracking at the interface between YSZ ceramic layer and thermally grown oxides(TGO) layer. The TBCs with Pt/Dy modified bond coat reveal improved interface bonding even after 1200 thermal cycles, whereas some delamination cracks are presented in the LCO layer. On the other hand,the Pt/Dy modified bond coat effectively suppresses the formation of the needle-like topologically closed packed phases(TCP) in the single crystal superalloy.

Composition optimization, high-temperature stability, and thermal cycling performance of Sc-doped Gd_(2)Zr_(2)O_(7) thermal barrier coatings: Theoretical and experimental studies

Sc was doped into Gd_(2)Zr_(2)O_(7) for expanding the potential for thermal barrier coating (TBC) applications. The solid solution mechanism of Sc in the Gd_(2)Zr_(2)O_(7) lattice, and the mechanical and thermophysical properties of the doped Gd_(2)Zr_(2)O_(7) were systematically studied by the first-principles method, based on which the Sc doping content was optimized. Additionally, Sc-doped Gd_(2)Zr_(2)O_(7) TBCs with the optimized composition were prepared by air plasma spraying using YSZ as a bottom ceramic coating (Gd-Sc/YSZ TBCs), and their sintering behavior and thermal cycling performance were examined. Results revealed that at low Sc doping levels, Sc has a large tendency to occupy the lattice interstitial sites, and when the doping content is above 11.11 at%, Sc substituting for Gd in the lattice becomes dominant. Among the doped Gd_(2)Zr_(2)O_(7), the composition with 16.67 at% Sc content has the lowest Pugh’s indicator (G/B) and the highest Poisson ratio (σ) indicative of the highest toughness, and the decreasing trends of Debye temperature and thermal conductivity slow down at this composition. By considering the mechanical and thermophysical properties comprehensively, the Sc doping content was optimized to be 16.67 at%. The fabricated Gd-Sc coatings remain phase and structural stability after sintering at 1400 ℃ for 100 h. Gd-Sc/YSZ TBCs exhibit excellent thermal shock resistance, which is related to the good thermal match between Gd-Sc and YSZ coatings, and the buffering effect of the YSZ coating during thermal cycling. These results revealed that Sc-doped Gd_(2)Zr_(2)O_(7) has a high potential for TBC applications, especially for the composition with 16.67 at% Sc content.

Mechanical property and structural changes by thermal cycling in phase-separated metallic glasses

Nondestructive cryogenically thermal cycling has been a simple but effective treatment to enhance mechanical properties of glassy materials.However,how the structural heterogeneities on nanometer scales are affected by thermal cycling is still an issue.Here,we report the response of spatial heterogeneities in three selected Ti_(41)Zr_(25)Be_(28)Fe_(6),Zr_(56)Co_(14)Cu_(14)Al_(16)and Zr_(42)Y_(14)Co_(22)Al_(22)(at.%)metallic glasses(MGs)with different compositions to the thermal cycling,which show significantly different structure and properties after the same treatments and could be ascribed to the joint contribution of relaxation and rejuvenation induced by thermal cycling.The rejuvenation is initially prevailed in a Zr-Y-containing MG,whereas the relaxation is dominant in a Cu-Co-containing MG,both eventually entering into a dynamic equilibrium state.By employing nanometer-scale structural models,the intrinsic correlation between the spatial heterogeneity and thermal cycling is proposed.The discovery could provide the fundamental understanding of the role of spatial heterogeneity in influencing the macroscopic properties of MGs via thermal cycling and help design high-performance glassy materials by tailoring their atomic structures with suitable thermal treatments.

Finite element simulation for mechanical response of surface mounted solder joints under different temperature cycling

Nonlinear finite element simulation for mechanical response of surface mounted solder joint under different temperature cycling was carried out. Seven sets of parameters were used in order to evaluate the influence of temperature cycling profile parameters. The results show that temperature cycling history has significant effect on the stress response of the solder joint. Based on the concept of relative damage stress proposed by the authors, it is found that enough high temperature holding time is necessary for designing the temperature cycling profile in accelerated thermal fatigue test.

Serrated magnetic properties in metallic glass by thermal cycle

Fe-based metallic glasses(MGs) with excellent soft magnetic properties are applicable in a wide range of electronic industry. We show that the cryogenic thermal cycle has a sensitive effect on soft magnetic properties of Fe_(78)Si_9B_(13) glassy ribbon. The values of magnetic induction(or magnetic flux density) B and H_c coercivity c show fluctuation with increasing number of thermal cycles. This phenomenon is explained as thermal-cycle-induced stochastically structural aging or rejuvenation which randomly fluctuates magnetic anisotropy and, consequently, the magnetic induction and coercivity. Overall,increasing the number of thermal cycles improves the soft magnetic properties of the ribbon. The results could help understand the relationship between relaxation and magnetic property, and the thermal cycle could provide an effective approach to improving performances of metallic glasses in industry.

Finite Element Simulation of Thermal Field in Double Wire Welding

In this paper, the thermal field of double wire welding is simulated by using ANSYS software. Simulation results were shown that the total heat input (E) is the most significant parameters to change the value of t8/5;By the mean of rationally controlling the proportion of the front arc heat input (E1) in the total heat input (E) and appropriately selecting double wire spacing (L), It is effective means to get the double wire welding thermal cycle. By the way of simulation, it is possible to manage the thermal input in the double welding wires and to control the temperature field and cooling rate that are fundamental for the final joint quality, it is great importance guidance to optimize the double wire welding process parameters.

Thermal cycle and its influence on the microstructure of laser welded butt joint of 8 mm thick Ti-6Al-4V alloy

Ti-6Al-4V alloy is extensively used in the manufacture of components in aviation.In the current study,the laser welding process is adopted to joint the Ti-6Al-4V alloy plate which has the thick of 8 mm.A three-dimensional finite element model is established to simulate the temperature distribution of laser welding process.The thermal cycle curves are produced on the strength of the simulation results.Meanwhile,the microstructure characteristics of the welded joint are investigated combined with simulation results.The results show that weld zone,heat affected zone and based metal experience similar thermal cycles process and the cooling rate has an important influence on the formation of microstructure.Moreover,the simulation results are well matched with experiment results.

Effects of supersonic fine particle bombarding on thermal cyclic failure lifetime of thermal barrier coating

Thermal barrier coating ( TBC) consisting of a NiCoCrAlY bond coat ( BC) and a ZrO2-8 wt. % Y2O3 topcoat ( TC) was fabricated on the nickel-base superalloy by air plasma spray ( APS) . The BC was treated by supersonic fine particle bombarding ( SFPB) . Thermal cyclic failure and residual stress in thermally grown oxide ( TGO) scale were studied by SEM with EDS and ruby fluorescence spectroscopy ( RFS) . As shown in the results,after treated by SFPB,thickening of TGO was relatively slow,which reduced the level of growth stress. The TBC with SFPB treatment was still remained well undergoing 350 times of thermal cycle. However,after thermal cycle with the same times,the separation of TC was observed in TBC without SFPB treatment. The residual stress analysis by RFS showed that the residual stress of SFPB-treated TBC increased with the increasing number of thermal cycle. The residual stress of conventional TBC reached a value of 650 MPa at 350 times of cycle and that of SFPB-treated TBC only reached 532 MPa at 400 times of cycle. The BC with SFPB treatment after 400 times of cycle was analyzed by RFS,the high stress value was not observed in local thickened region of TGO. Thermal cycling resistance of TBC can be improved by the SFPB technology.

Tailoring microstructure evolution and mechanical properties of a high-performance alloy steel through controlled thermal cycles of a direct laser depositing process

Direct laser deposition(DLD),as a popular metal additive manufacturing process,shows advantages of technical flexibility and high efficiency to gain a high-performance alloy steel component.However,during the processing of DLD,the deposited steel layer is affected by the subsequent layer depositing.The DLD block shows different microstructure and mechanical properties at the bottom,middle and top of the deposited parts.To date,there are few research works about the effects of inter-layer interval time and laser power on the microstructure evolution and mechanical properties of the deposited layers.In this study,the idle time and laser power layer by layer during DLD of 12CrNi2 steel were controlled to cause the deposited layers to maintain a high cooling rate,while the bottom deposited layer was subjected to a weak tempering effect.Results show that a high proportion of martensite is produced,which improves the strength of the deposited layer.Under the laser scanning strategy of laser power 2,500 W,scanning velocity 5 mm·s^(-1),powder feeding rate 11 g·min^(-1),overlap rate 50%,and a laser power difference of 50 W and a 2 min interval,the tensile strength of the deposited layer of 12CrNi2 steel is in the range of 873-1,022 MPa,and the elongation is in the range of 16.2%-18.9%.This study provides a method to reduce the tempering effect of the subsequent deposition layers on the bottom layers,which can increase the proportion of martensite in the low-alloy high-strength steel,so as to improve the yield strength of the alloy steel.

Thermal simulation of single thermal cycle for high strength steel pipe

Single thermal cycle simulation tests were carried out for X80 high strength steel pipes from three steel mills by a Gleeble 3500HS thermal simulation test machine,and coincidence degree of the thermal simulation curve with the set curve under heat inputs of 6–30 kJ/cm was observed;The relationship between different heat inputs and microstructure,impact toughness and hardness of steel pipe CGHAZ(coarse grain heat affected zone)was studied by metallographic examination,impact test and hardness test.The results show that with the increase of heat input,original austenite grain size increases gradually,the lath bainite ratio decreases and the granular bainite ratio increases.The impact toughness of C steel pipe is lower than those of A and B steel pipe,and the impact toughness of CGHAZ from the three steel pipes show different trends:for A steel pipe CGHAZ,impact toughness increases first and then decreases,with the highest value of 270–320 J under 20–25 kJ/cm;for B steel pipe CGHAZ,impact toughness decreases slightly;for C steel pipe CGHAZ,impact toughness increases,with the highest value of 260–300 J under 25 kJ/cm.As the heat input increases,the hardness of three X80 steel pipes CGHAZ shows a decreasing trhighend,and C steel pipe has the largest decreasing range.

Performance Evaluation of a Molten Carbonate Fuel Cell-Graphene Thermionic Converter-Thermally Regenerative Electrochemical Cycles Hybrid System

A combined system model is proposed including a molten carbonate fuel cell(MCFC),a graphene thermionic converter(GTIC)and thermally regenerative electrochemical cycles(TRECs).The expressions for power output,energy efficiency of the subsystems and the couple system are formulated by considering several irreversible losses.Energy conservation equations between the subsystems are achieved leaned on the first law of thermodynamics.The optimum operating ranges for the combined system are determined compared with the MCFC system.Results reveal that the peak power output density(POD)and the corresponding energy efficiency are 28.22%and 10.76%higher than that of the single MCFC system,respectively.The effects of five designing parameters on the power density and energy efficiency of the MCFC/GTIC/TRECs model are also investigated and discussed.

Formability,microstructure evolution and mechanical properties of wire arc additively manufactured AZ80M magnesium alloy using gas tungsten arc welding

Wire arc additive manufacturing(WAAM)technology has been used to fabricate the multi-layer single-pass deposited wall of AZ80M magnesium(Mg)alloy by gas tungsten arc welding.The formability,thermal cycles,microstructural evolution and mechanical properties of the WAAM AZ80M Mg alloy were investigated.The results show that there was significant difference in the temperature variation and the geometries between the original several layers and the subsequent deposited layers.Owing to the arc energy input,the interpass temperature rised rapidly and then stabilized at 150℃.As a result,the width of the deposited wall increased and then kept stable.There were obvious differences in the microstructure of the WAAM AZ80M Mg alloy among the top zone,intermediate zone and bottom zone of deposited wall.During the arc deposition process,theβphase of the WAAM AZ80M Mg alloy redissolved due to the cyclic heat accumulation,and then precipitated in the grain boundary.The cyclic heat accumulation also led to weakening of dendrite segregation.From the substrate to the top zone,the hardness of the deposited wall decreased gradually,and the intermediate zone which was the main body of deposited wall had relatively uniform hardness.The tensile properties of the WAAM AZ80M Mg alloy were different between the vertical direction and the horizontal direction.And the maximum ultimate tensile strength of the WAAM AZ80M Mg alloy was 308 MPa which was close to that of the as-extruded AZ80M Mg alloy.

Mixed graphite cast iron for automotive exhaust component applications

Both spheroidal graphite iron and compacted graphite iron are used in the automotive industry. A recently proposed mixed graphite iron exhibits a microstructure between the conventional spheroidal graphite iron and compacted graphite iron. Evaluation results clearly indicate the suitability and benefits of mixed graphite iron for exhaust component applications with respect to casting, machining, mechanical, thermophysical, oxidation, and thermal fatigue properties. A new ASTM standard specification(A1095) has been created for compacted, mixed, and spheroidal graphite silicon-molybdenum iron castings. This paper attempts to outline the latest progress in mixed graphite iron published.

Exploitation of Waste Heat from a Solid Oxide Fuel Cell via an Alkali Metal Thermoelectric Converter and Electrochemical Cycles

In order to employ the waste heat effectively,a novel three-stage integrated system based upon a solid oxide fuel cell(SOFC),an alkali metal thermoelectric converter(AMTEC)and thermally regenerative electrochemical cycles(TRECs)is put forward.Considering the main electrochemically and thermodynamically irreversible losses,the power output and the efficiency of the subsystems and the integrated system are compared,and optimally operating regions for the current density,the power output,and the efficiency of the integrated system are explored.Calculations demonstrate that the maximum power density of the considered system is up to 7466 W/m2,which allows 18%and 74%higher than that of the conventional SOFC-AMTEC device and the stand-alone fuel cell model,respectively.It is proved that the considered system is an efficient approach to boost energy efficiency.Moreover,the influence of several significant parameters on the comprehensive performance of the integrated system is expounded in detail,including the electrolyte thickness of the SOFC,the leakage resistance of the SOFC,and the area ratio between the SOFC electrode and the AMTEC subsystem.

Modeling effect of cooling conditions on solidification process during thermal cycle of rollers in twin-roll strip casting

In the twin-roll strip casting process,molten steel solidifies by losing heat through its interface with the casting rollers.The heat extraction along this interface has an effect on the quality of the strips and should be affected by coating,rolls’material,and cooling water flow rate.It is necessary to understand the effect of these casting parameters on the solidification structure of twin-roll strip casting.A three-dimensional computational domain is set up to simulate the solidification process of molten steel and heat exchange between steel strip/air,coating,rolls,and cooling water in the channel of roll sleeves.The effect of the cooling water intensity and flow intensity of molten steel in the pool on the solidification structures is studied during the thermal cycle of rolls in the twin-roll strip casting.These predicted results are helpful to optimize casting parameters and improve the strip quality in the twin-roll strip casting process.

Charging-free thermally regenerative electrochemical cycle for electricity generation from daytime solar heat and nighttime darkness

The extensive exploration of energy conversion harvested from the environment into electricity is recently driven by the significant demand to power off-grid electronics,particularly Internet-of-Things(IoT)sensors.This highlight previews the latest advance of a charging-free thermally regenerative electrochemical cycle(TREC)for continuous electricity generation from solar heat and darkness with the aid of dual-mode thermal regulations.Such a spontaneous all-day electricity generation with high power and efficiency shows great potential for powering a wide range of distributed electronics for IoT and other applications.