Microstructure instability of fully lamellar TiAl alloy containing high content of Nb after long-term thermal cycling

Microstructure instabilities of the fully lamellar Ti-45Al-8.5Nb-（W,B,Y） alloy were investigated by SEM and TEM after long-term thermal cycling（500 and 1000 thermal cycles） at 900 °C. Two major categories of microstructure instability were produced in the alloy after the thermal cycling： 1） The discontinuous coarsening implies that grain boundary migrations are inclined to occur in the Al-segregation region after the long-term thermal cycling, especially after 1000 thermal cycles. Al-segregation can be reduced during the process of long-term thermal cycling as a result of element diffusion; 2） The α2 lamellae become thinner and are broken after 1000 thermal cycles caused by the dissolution of α2 lamellae through phase transformation of α2→γ. The γ grains nucleate within the α2 lamellae or（α2＋γ） lamellae in a random direction.

Effect of Thermal Cycling on Properties and Microstructure of SnAgCuCe Soldered Joints in QFP Devices

Increasing global concern about the environment is bringing regulatory （European directives） and consumer （＂green products＂） pressure on the electronics industry in Europe and Japan to reduce or completely eliminate the use of lead （Pb） in products. Among all lead-free solder alloys, SnAgCu solder system, which has better thermo-mechanical properties compared with those of SnPb solder, is proven to be one of the promising candidates for electronic assembly. Previous work also revealed that adding a small amount of rare earth Ce into SnAgCu solder can visibly improve the properties and inhibit the excessive growth of the intermetallic compound layer. Thermal fatigue properties of SnAgCuCe soldered joints in QFP devices under thermal conditions have been investigated by finite element method and experiments. Based on creep model of low stress and high stress, corresponding creep subroutine was established for simulating the stress and strain response of SnAgCuCe soldered joint from -55 ℃ to 125 ℃, and fatigue life was calculated using creep fatigue life prediction equation. Moreover, thermal cycling experiments were conducted, the experimental results were found to be close to the simulated results. In addition, the tensile force of SnAgCuCe soldered joints decreased with increasing number of thermal cycles, and the fracture mechanism transformed from toughness fracture to brittle intergranular fracture. Moreover the tensile force changes and fracture microstructure evolution could benefit the quantitative evaluations of the mechanical performances of lead-free soldered joints under thermal cycling loadings.

Effects of Shot Peening Process on Thermal Cycling Lifetime of TBCs Prepared by EB-PVD

Conventional two-layered thermal barrier coatings （TBCs） are prepared by electron beam physical vapor deposition （EB-PVD） with ZrO2-8 wt% Y2O3 （8YSZ） as top coat and CoCrAlY as bond coat on disk-shaped Ni based super-alloy. In this paper, three kinds of shot peening process with different lengths of operating time were adopted for bond coating. As a result, changes took place in its surface roughness and the surface micro-hardness. A thermal cycling test at 1 273 Kx55 rain and another at room temperature for 5 min were performed to study the effects of shot peening process on the thermal cycling lifetime of TBCs. It is found that a moderate shot peening process will be able to prolong the life time. The oxidation dynamic of the as-processed TBCs basically accords with the parabolic rule, and the oxidation test also attests to the spallation between YSZ and thermal growth oxide （TGO） responsible mainly for the failure of TBCs.

Study on the Thermal Expansion and Thermal Cycling of AlNp/Al Composites

The AIN particle reinforced aluminum matrix composites with 50% volume fraction were fabricated by squeeze-casting technology. The thermal expansion behavior and its response to thermal cycling were studied between 20℃ and 400℃. Compared with four theoretical models, the measured CTEs of the composite lie within the elastic bounds derived by Schapery’s analysis. Schapery’s model and Kerner’s model agree well with the CTEs of the composites at lower temperature and elevated temperature, respectively. Strain hysteresis was observed between heating and cooling curves during cycling. This was attributed primarily to the anelastic behavior of the matrix induced by matrix residual stresses.

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, pet Ni alloy. It is shown that M(s), and M(f) temperature increase with increasing the number of cycles, while A(s) and A(f) temperature decrease during thermal cycling. The total strain at and permanent strain epsilon (p) 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.

Microstructural Evolution and Cracking of Pb-free Ball Grid Array Assemblies under Thermal Cycling

Two kinds of CBGA （ceramic ball grid array） assemblies were made by reflow soldering process using two different Pb-free solders. Microstructural evolution and cracks induced by thermal cycling in CBGA assemblies were examined by scanning electron microscopy （SEM） and finite element method （FEM）. Before thermal cycling, intermetallic compounds （IMCs） Cu6Sn5 and Ag3Sn were observed at the solder interface between Cu and Ag metallizations, respectively. After thermal cycling, another IMC Cu3Sn was observed near the Cu pad in both two assemblies and the layers of Cu6Sn5 and Ag3Sn became thicker. As a result of thermal cycling, cyclic stress and strain were accumulated in the solder joint leading to fatigue cracking. Both experiments and FEM revealed that cracks preferred to initiate at the corner of each solder joint. Multi-modes of the crack propagation were found in the two assemblies. Based on Coffin-Manson equation, the thermal fatigue life was calculated and the predicted life showed good agreement with the experimental results.

Thermal cycling behavior of alumina-graphite brazed joints in electron tube applications

Alumina was joined with graphite by active metal brazing technique at 895,900,905,and 910 ℃ for 10 min in vacuum of0.67 mPa using Ti-Cu-Ag（68.8Ag-26.7Cu-4.5Ti;mass fraction,%） as filler material.The brazed samples were thermal cycled between 30 and 600 ℃ and characterized.X-ray diffraction results show strong reaction between titanium and carbon as well as titanium and alumina.Scanning electron microscopy and helium leak tests show that the initial and thermal cycled brazed samples are devoid of cracks or anv other defects and hermeticity in nature.Brazing strength of the joints is found to be satisfactory.

Influence of Moderate Temperature Treatment on Thermal Cycling Behaviour and Mechanical Properties of the CuAINiMnTi Alloys

The influence of moderate temperature treatment after quenching on thermal cycling behaviour and mechanical properties of the CuAlNiMnTi shape memory alloys have been investigated.It is shown that there will be α phase precipitation in the alloy when it is subjected to quenching followed by moderate temperature treatment at 600℃.The precipitation of α phase can suppress NiAl-base phase precipitation during subsequent thermal cycling,therefore the stabilization of transformation temperature during thermal cycling can be achieved.The suitable amount of α phase precipitation contributes to the improvement of mechanical properties of the alloy by changing the fracture morphology.

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.

Micro-macro evolution of mechanical behaviors of thermally damaged rock:A state-of-the-art review

The influence of thermal damage on macroscopic and microscopic characteristics of different rocks has received much attention in the field of rock engineering.When the rocks are subjected to thermal treatment,the change of macroscopic characteristics and evolution of micro-structure would be induced,ultimately resulting in different degrees of thermal damage in rocks.To better understand the thermal damage mechanism of different rocks and its effect on the rock performance,this study reviews a large number of test results of rock specimens experiencing heating and cooling treatment in the laboratory.Firstly,the variations of macroscopic behaviors,including physical parameters,mechanical parameters,thermal conductivity and permeability,are examined.The variations of mechanical parameters with thermal treatment variables(i.e.temperature or the number of thermal cycles)are divided into four types.Secondly,several measuring methods for microstructure,such as polarizing microscopy,fluorescent method,scanning electron microscopy(SEM),X-ray computerized tomography(CT),acoustic emission(AE)and ultrasonic technique,are introduced.Furthermore,the effect of thermal damage on the mechanical parameters of rocks in response to different thermal treatments,involving temperature magnitude,cooling method and thermal cycle,are discussed.Finally,the limitations and prospects for the research of rock thermal damage are proposed.

Effect of thermal cycling on the mechanical properties of Zr_(41)Ti_(14)Cu_(12.5)Ni_(10)Be_(22.5) alloy

The effect of thermal cycling treatment on mechanical properties and thermal stability of a Zr41Ti14Cu12.5Ni10Be22.5 bulk metallic glass is investigated.The metallic glassy samples are sealed into quartz tubes under high vacuum condition,and liquid nitrogen together with electric furnace are used to control a periodical temperature variation between 196℃ and 150℃.The structure and properties of the tested samples for different thermal cycles have been examined by X-ray diffraction analysis,mechanical properties measurement and thermal analysis.It has been found that the structure and properties of the samples do not show a significant change even after 200 cycles,which suggests Zr41Ti14Cu12.5Ni10Be22.5 alloy as having potential in aerospace environment.

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.

Oxidation and Thermal Fatigue Behaviors of Two Type Hot Work Steels During Thermal Cycling

Thermal fatigue test has been carried out on widely used hot work steel 4Cr5MoSiV1 and a low alloyed steel 3Cr3MoV in temperature range of 200 to 700 ℃. Tempering resistance, as well as high temperature hardness/ strength of steel specimens, works as a dominating material parameter on thermal fatigue resistance. During the heating period, high hardness can depress the inelastic deformation. This deformation is the origination of tensile stress, which acts as the driving [orce o{ heat checking during the cooling period. The cyclic strain-oxidation interac- tion can speed up the damage on surface defects, which plays an obvious role in initiation of thermal cracks. On 4CrSMoSiV1 steel specimens, borders between the matrix and inclusions such as titanium compounds, or lager car- bides such as primary carbides, are focused by strain and attacked by oxidation, and are main initiating places of cracks. While on 3Cr3MoV steel specimens, larger strain causes plastic deformation concentrating around grain boundaries. Then the following oxidation accelerates this grain boundary damage and creates cracks.

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.

Influence of cryogenic thermal cycling treatment on the thermophysical properties of carbon/carbon composites between room temperature and 1900℃

Influence of cryogenic thermal cycling treatment （from -120 ℃ to 120 ℃ at 1.3 × 10^-3 Pa） on the thermo- physical properties including thermal conductivity （TC）, thermal diffusivity （TD）, specific heat （SH） and coefficient of thermal expansion （CTE） ranging from room temperature to 1900 ℃ of carbon/carbon （C/C） composites in x-y and z directions were studied. Test results showed that fiber/matrix interracial debonding, fiber pull-out and microcracks occurred after the cryogenic thermal treatment and they increased significantly with the cycle number increasing, while cycled more than 30 times, the space ofmicrodefects reduced obviously due to the accumulation of cyclic thermal stress. TC, TD, SH and CTE of the cryogenic thermal cycling treated C/C composites were first decreased and then increased in both directions （x-y and z directions） with the increase of thermal cycles. A model regarding the heat conduction in cryogenic thermal cycling treated C/C composites was proposed.

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.

IN SITU METALLOGRAPHIC OBSERVAION OF THERMALCYCLING EFFECTS ON PHASE TRANSFORMATION IN Cu-Zn-Al SHAPE MEMORY ALLOY

In this paper, an in situ metallographic video system was used to study the morphology in Cu-Zn-Al shape memory alloy (SMA) during thermal cycling phase tmnsforma-tion. There are seven different types of martensite morphology in Cu-Zn-Al SMA,i.e. plate, spean bamboo, noose, round-spot, line and dot-like martensites. During transformation and inverse-transformation, the thermoelastic martensites were rising and falling, growing and shrinking, splitting and merping. The growth patterns of thermoelastic martensite were divided into three types: fast growing, very slow ex-pansion and uniform automatic growth. Automatic growth appeared when ageing at a temperature below Ms. The result, for the first time, coofirmed Olson and Cohen,s suggestion that thermoelastic martensite may be automatically grown when ageing be-low Ms. With increasing numbers of thermal cycling, both the memory recovery ratio (% )and memory recoverp degradation (%) degraded logarithmically. The degrada-tion was steep in the initial stage and then became more and more gradual in the middle and the final stages caused by the stabilisation of martensite. In the initial stage, vacancies assumed the controlling role, while dislocations took the major role in other stages. The martensite transformation caused the intedeces to become bent and blurmd, but with increasing cycling numbers, the movements of interface had the same degradation effects as the memory recovery ratio curves. Under these conditions, we also found the martensites on one side of the intedece became reorientated and par-allel to the intedece. It was therefore concluded that the movement degradation of the interface and the appearance of bamboo like martensite induced the degradation of two way shape memory effect (TWMP).

Effect of Thermal Cycling Treatment on Microyield Behavior of Al_2O_3p/Al Composite

An aluminum alloy (6061) matrix composite reinforced with 35% (vol.) Al 2O 3 particles was fabricated by squeeze casting method. The Al 2O 3 particles were spherical. The microyield behavior of the composite and the effect of different thermal cycling treatment on the microyield behaviors of the composite were studied. Based on TEM and HREM observation of microstructure, the mechanism of microyield behavior in the Al 2O 3p/6061 composite was analyzed. The results indicate that the microyield behavior of the Al 2O 3p/6061 composite can be described by Brown Lukens theory, which was used satisfactorily for aluminum alloys and other light alloys, and is affected greatly by the different thermal cycling treatment. The more the cycles of thermal cycling treatment, the higher to microyield strength at small strains. Thermal cycling treatment affects mainly the thermal mismatch stress and the density of movable dislocations in the matrix.