Experiments and modeling of microstructural and mechanical behaviors of laser-welded Ni-based superalloy at high temperatures

The performance of welded Ni-based superalloys at high temperatures is essential to be evaluated due to their particular service environment for aero-engines and high-speed aircrafts.The tensile properties and related microstructural evolutions such as the carbide precipitate and grain of a laser-welded Ni-based alloy were experimentally and numerically investigated at different temperatures(20,300,500,800℃).The results show that at room temperature,the strength of the Base Material(BM)was slightly smaller,with a difference of less than 1%,than the Welded Material(WM),which can be attributed to the more uniformly distributed needle-shaped carbide precipitates in the WM than those nonuniformly coarser spherical ones in the BM.While at 300℃ and 500℃,the strength of WM decreased more obviously compared with that of BM due to the more apparent growth of grain:13.52%loss in yield strength in WM alloys as compared with BM alloys at 300℃,and 16.57% at 500℃.At 800℃,the strength of BM and WM both decreased to a similar level due to Dynamic Recrystallization(DRX).However,a much higher elongation was observed for the BM than WM(less than 50%of BM),which can be attributed to the enhanced dislocation accumulation capability of the large spherical carbides along grain boundaries on the fracture surface in BM.Furthermore,a unified model considering the welding effects on both microstructures(dislocation,carbides,and grain)and mechanical properties evolutions at different temperatures was developed and validated.Based on this model,the key temperature ranges(20–600℃)where apparent weakening of strength and uniform plasticity occurs for welded structures were identified,providing a direct guidance for potential structure and process design.

Long-Term Performance and Microstructural Characterization of Dam Concrete in the Three Gorges Project

This study investigates the long-term performance of laboratory dam concrete in different curing environments over ten years and the microstructure of 17-year-old laboratory concrete and actual concrete cores drilled from the Three Gorges Dam.The mechanical properties of the laboratory dam concrete,whether cured in natural or standard environments,continued to improve over time.Furthermore,the laboratory dam concrete exhibited good resistance to diffusion and a refined microstructure after 17 years.However,curing and long-term exposure to the local natural environment reduced the frost resistance.Microstructural analyses of the laboratory concrete samples demonstrated that moderate-heat cement and fine fly ash(FA)particles were almost fully hydrated to form compact micro structures consisting of large quantities of homogeneous calcium(alumino)silicate hydrate(C-(A)-S-H)gels and a few crystals.No obvious interfacial transition zones were observed in the microstructure owing to the longterm pozzolanic reaction.This dense and homogenous microstructure was the crucial reason for the excellent long-term performance of the dam concrete.A high FA volume also played a significant role in the microstructural densification and performance growth of dam concrete at a later age.The concrete drilled from the dam surface exhibited a loose microstructure with higher microporosity,indicating that concrete directly exposed to the actual service environment suffered degradation caused by water and wind attacks.In this study,both macro-performance and microstructural analyses revealed that the application of moderate-heat cement and FA resulted in a dense and homogenous microstructure,which ensured the excellent long-term performance of concrete from the Three Gorges Dam after 17 years.Long-term exposure to an actual service environment may lead to microstructural degradation of the concrete surface.Therefore,the retained long-term dam concrete samples need to be further researched to better understand its microstructural evolution and development of its properties.

Alleviating the anisotropic microstructural change and boosting the lithium ions diffusion by grain orientation regulation for Ni-rich cathode materials

Generally,layered Ni-rich cathode materials exhibit the morphology of polycrystalline secondary sphere composed of numerous primary particles.While the arrangement of primary particles plays a very important role in the properties of Ni-rich cathodes.The disordered particle arrangement is harmful to the cyclic performance and structural stability,yet the fundamental understanding of disordered structure on the structural degradation behavior is unclarified.Herein,we have designed three kinds of LiNi_(0.83)Co_(0.06)Mn_(0.11)O_(2) cathode materials with different primary particle orientations by regulating the precursor coprecipitation process.Combining finite element simulation and in-situ characterization,the Li^(+)transport and structure evolution behaviors of different materials are unraveled.Specifically,the smooth Li^(+)diffusion minimizes the reaction heterogeneity,homogenizes the phase transition within grains,and mitigates the anisotropic microstructural change,thereby modulating the crack evolution behavior.Meanwhile,the optimized structure evolution ensures radial tight junctions of the primary particles,enabling enhanced Li^(+)diffusion during dynamic processes.Closed-loop bidirectional enhancement mechanism becomes critical for grain orientation regulation to stabilize the cyclic performance.This precursor engineering with particle orientation regulation provides the useful guidance for the structural design and feature enhancement of Ni-rich layered cathodes.

Influence of Non-Natural Ageing Temperature on the Microstructural Characteristics and Mechanical Properties of Cast Aluminum 6063 Alloy

This research considered the effect of non-natural aging on the microstructural characteristics and mechanical properties of as-cast aluminum 6063 alloys. The samples were developed through a sand casting process and machined into tensile and impact test samples before carrying out solution heat treatment at 550?C (0.83 Tm) on two parts of the samples while retaining one part as the control. The two parts were further divided into sets denoted A and B and were aged at 180?C (0.27 Tm) and 160?C (0.24 Tm), respectively, for 12 hours. The results showed that sample A has the optimal yield strength and ultimate tensile strength of 192 and 206 MPa, respectively. Likewise, the sample gave the highest impact strength value of about 9.63 J/mm2. The observed results were supported by the optical micrograph, which revealed that the sample has evenly dispersed precipitates in its microstructure. This is deemed responsible for the observed increase in strength of the sample.

Enhanced in-vitro degradation resistance and cytocompatibility of a thermomechanically processed novel Mg alloy:Insights into the role of microstructural attributes

The role of microstructural features on in-vitro degradation and surface film development of a thermomechanically processed Mg-4Zn-0.5Ca-0.8Mn alloy has been investigated employing electrochemical studies,scanning electron microscopy and X-ray photoelectron spectroscopy.The specimen forged at 523 K temperature developed a coarse unimodal microstructure consisting of basal oriented grains,whereas the specimens forged at 623 K and 723 K temperatures exhibited bimodal microstructures containing randomly oriented fine grains and basal oriented coarse grains.The bimodal microstructures exerted higher resistance to corrosion compared to the unimodal microstructure in presence of a protective surface film.The optimum size distribution of fine and coarse grains as well as the prevalence of basal oriented grains led to the lowest anodic current density in the specimen forged at 623 K.The morphology of Ca_(2)Mg_(6)Zn_(3)precipitates governed the cathodic kinetics by controlling the anode to cathode surface area ratio.Despite the specimen forged at 723 K comprised comparatively lower fraction of precipitates than at 623 K,the mesh-like precipitate morphology increased the effective cathodic surface area,leading to enhanced localised corrosion in the former specimen.Optimal microstructural features developed at 623 K forging temperature formed a well-protective surface film with lower Mg(OH)_(2)to MgO ratio,exhibiting distinctly high polarization resistance and superior cytocompatibility in terms of cell-proliferation and cell-differentiation.

Microstructural evolution and dynamic recrystallization mechanisms of additively manufactured TiAl alloy with heterogeneous microstructure during hot compression

Microstructural evolution and dynamic recrystallization(DRX)mechanisms of a Ti-48Al-2Cr-2Nb(at.%)alloy prepared by selective electron beam melting(SEBM)during hot deformation at 1150℃and 0.1 s^(-1)were investigated by hot compression tests,optical microscope(OM),scanning electron microscope(SEM),electron back-scattered diffraction(EBSD)and transmission electron microscope(TEM).The results show that the initial microstructure of the as-SEBMed alloy exhibits layers of coarseγgrains and fineγ+α_(2)+(α_(2)/γ)lamellar mixture grains alternately along the building direction.During the early stage of hot deformation,deformation twins tend to form within the coarse grains,facilitating subsequent deformation,and a small number of DRX grains appear in the fine-grained regions.With the increase of strain,extensive DRX grains are formed through different DRX mechanisms in both coarse and fine-grained regions,involving discontinuous dynamic recrystallization mechanism(DDRX)in the fine-grained regions and a coexistence of DDRX and continuous dynamic recrystallization(CDRX)in the coarsegrained regions.

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.

Microstructural characterization,tribological and corrosion behavior of AA7075-TiC composites

Aluminum alloys are the potential materials in the automobile and aerospace sectors due to their lower density,easy forming and excellent corrosion resistance.The demand of high strength-to-weight ratio materials in structural applications needs the engineering industries to seek aluminum alloy with new versions of hard and brittle ceramic particles.The microstructure,hardness,wear and corrosion behaviors of AA7075 composites with 2.5wt.%and 5wt.%TiC particles were studied.Microscopic analysis is evident that the transformation of the strong dendritic morphology to non-dendritic morphology on the incorporation of TiC into AA7075.Furthermore,the precipitation of the second-phase compounds such as Al_(2)CuMg,Al_(2)Cu andFe-rich Al_6(Cu,Fe)/Al_(7)Cu_(2)Fe)is promoted by TiC particles at inter-and intra-dendritic regions.Accordingly,the hardness of composites is improved by grain boundary strengthening and particulate strengthening mechanisms.Both coefficient of friction and wear rate have an inverse relation with TiC concentration.The base alloy without TiC shows adhesive-type wear-induced deformation due to the formation of an oxide film,while composite samples exhibit a mechanically mixed layer and abrasive-type wear behavior.Composite samples shows a higher corrosion rate due to the presence of numerous precipitates which promote pitting corrosion.

Microstructural evolution and strengthening mechanism of aligned steel fiber cement-based tail backfills exposed to electromagnetic induction

Cemented tailings backfill(CTB)not only boosts mining safety and cuts surface environmental pollution but also recovers ores previously retained as pillars,thereby improving resource utilization.The use of alternative reinforcing products,such as steel fiber(SF),has continuously strengthened CTB into SFCTB.This approach prevents strength decreases over time and reinforces its long-term durability,especially when mining ore in adjacent underground stopes.In this study,various microstructure and strength tests were performed on SFCTB,considering steel fiber ratio and electromagnetic induction strength effects.Lab findings show that combining steel fibers and their distribution dominantly influences the improvement of the fill’s strength.Fill’s strength rises by fiber insertion and has an evident correlation with fiber insertion and magnetic induction strength.When magnetic induction strength is 3×10^(-4) T,peak uniaxial compressive stress reaches 5.73 MPa for a fiber ratio of 2.0vol%.The cracks’expansion mainly started from the specimen’s upper part,which steadily expanded downward by increasing the load until damage occurred.The doping of steel fiber and its directional distribution delayed crack development.When the doping of steel fiber was 2.0vol%,SFCTBs showed excellent ductility characteristics.The energy required for fills to reach destruction increases when steel-fiber insertion and magnetic induction strength increase.This study provides notional references for steel fibers as underground filling additives to enhance the fill’s durability in the course of mining operations.

Effect of Co on Solidification Characteristics and Microstructural Transformation of Non-equilibrium Solidified Cu-Ni Alloys

Non-equilibrium solidification structures of Cu55Ni45 and Cu55Ni43Co2 alloys were prepared by the molten glass purification cycle superheating method.The variation of the recalescence phenomenon with the degree of undercooling in the rapid solidification process was investigated using an infrared thermometer.The addition of the Co element affected the evolution of the recalescence phenomenon in Cu-Ni alloys.The images of the solid-liquid interface migration during the rapid solidification of supercooled melts were captured by using a high-speed camera.The solidification rate of Cu-Ni alloys,with the addition of Co elements,was explored.Finally,the grain refinement structure with low supercooling was characterised using electron backscatter diffraction(EBSD).The effect of Co on the microstructural evolution during nonequilibrium solidification of Cu-Ni alloys under conditions of small supercooling is investigated by comparing the microstructures of Cu55Ni45 and Cu55Ni43Co2 alloys.The experimental results show that the addition of a small amount of Co weakens the recalescence behaviour of the Cu55Ni45 alloy and significantly reduces the thermal strain in the rapid solidification phase.In the rapid solidification phase,the thermal strain is greatly reduced,and there is a significant increase in the characteristic undercooling degree.Furthermore,the addition of Co and the reduction of Cu not only result in a lower solidification rate of the alloy,but also contribute to the homogenisation of the grain size.

Microstructural evolution and mechanical properties of AZ31 Mg alloy fabricated by a novel bifurcation-equal channel angular pressing

In this work,a novel type of short-process deformation technology of Mg alloys,bifurcation-equal channel angular pressing(B-ECAP),was proposed to refine grain and improve the basal texture.The cylindrical billets were first compressed into the die cavity,then sequentially flowed downward through a 90°corner and two 120°shear steps.The total strain of B-ECAP process could reach 3.924 in a single pass.The results of microstructure observation showed that DRX occurred at upsetting process in the die cavity and completed at position D.The grains were refined to 6.3μm at being extruded at 300℃ and grew obviously with the extrusion temperature increase.The shear tress induced by 900 corner and two 120°shear steps resulted in the basal poles of most grains tilted to extrusion direction(ED)by±25°.Compared with the original billets,the extruded sheets exhibited higher yield strengths(YS),which was mainly attributed to the grain refinement.The higher Schmid factor caused by ED-tilt texture resulted in a fracture elongation(FE)more than that of the original bar in ED,while was equivalent to that in transverse direction(TD).As the extrusion temperature increased,the variation of UTS and YS in ED and TD decreased gradually without ductility obviously decrease.

Microstructural analysis of marl stabilized with municipal solid waste and nano-MgO

Municipal solid waste(MSW)is accumulating over elapsed time across the world,and it is observed in many projects associated with weak soils,such as marl.Therefore,effective solutions to the environmental problem are essential.Conventional techniques for stabilizing marl generally use substances such as lime and cement,which could exacerbate pollution.For this,some new stabilizers,e.g.nano-MgO,are used.There are large quantities of marls and MSW in Shiraz City,Iran.The present study aims to evaluate the feasibility of using nano-MgO as a green low-carbon binder to remove MSW from the environment and make construction projects more cost-effective.Consolidated drained shear tests were conducted to evaluate the mechanical behaviors of the nano-MgO treated marl specimens at high normal stresses.The marl specimens containing MSW percentages of 15%,25%,35%,and 45%and nano-MgO percentages of 0.25%,0.5%,0.75%,and 1%,were used.It is found that the marl containing 15%and 25%MSW and 0.5%nano-MgO at 28-d curing can perform cation exchange and form new cementitious products.The soils with merely MSW show good performance due to the removal of the kaolinite and the formation of brucite.However,the soil with 25%MSW and 0.5%nano-MgO shows the same strength enhancement as the specimen with the optimal nano-MgO(0.75%)through the formation of dolomite,with a 20.59%increase in strain energy(SE).

Deformation mechanisms and microstructural characteristics of AZ61 magnesium alloys processed by a continuous expansion extrusion approach

The unique continuous extrusion-based severe plastic deformation approaches were proposed recently to process high-performance magnesium (Mg) alloys,while the in-depth deformation mechanisms under such complicated thermomechanical conditions were not well understood In the present work,the fundamental deformation behaviors of AZ61 Mg alloy from 25 to 400°C were firstly examined under uniaxial compression deformation.Then the deformation mechanisms and microstructural characteristics of AZ61 Mg alloy during continuous expansion extrusion forming (CEEF) were systematically investigated by microstructural observations,finite element and cellular automata simulations The results showed that the continuous evolutions of temperature,larger strain level and complex stress state with strain rate range of 0~5.98 s-1during CEEF brought the distinctive dynamic recrystallization behaviors and texture development in AZ61 Mg alloy,which were different to that of uniaxial compression deformation.In details,a remarkable grain refinement was achieved via CEEF processing due to the simultaneous actions of continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX).Gradually enhanced CDRX were observed from center to edge region,which had significant effects on the texture distribution and texture strength.The c-axis of most grains rotated under distinctive shear strain following parabolic metal flow,resulting in stable fiber texture.In addition,the evolution of the internal texture of the alloy led to an obvious increase in the Schmid factor for the activation of basal(c+a)slip system.©2022 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of Ke Ai Communications Co.Ltd.

A critical review on solid-state welding of high entropy alloys-processing,microstructural characteristics and mechanical properties of joints

The high entropy alloys(HEAs)are the newly developed high-performance materials that have gained significant importance in defence,nuclear and aerospace sector due to their superior mechanical properties,heat resistance,high temperature strength and corrosion resistance.These alloys are manufactured by the equal mixing or larger proportions of five or more alloying elements.HEAs exhibit superior mechanical performance compared to traditional engineering alloys because of the extensive alloying composition and higher entropy of mixing.Solid state welding(SSW)techniques such as friction stir welding(FSW),rotary friction welding(RFW),diffusion bonding(DB)and explosive welding(EW)have been efficiently deployed for improving the microstructural integrity and mechanical properties of welded HEA joints.The HEA interlayers revealed greater potential in supressing the formation of deleterious intermetallic phases and maximizing the mechanical properties of HEAs joints.The similar and dissimilar joining of HEAs has been manifested to be viable for HEA systems which further expands their industrial applications.Thus,the main objective of this review paper is to present a critical review of current state of research,challenges and opportunities and main directions in SSW of HEAs mainly CoCrFeNiMn and Al_xCoCrFeNi alloys.The state of the art of problems,progress and future outlook in SSW of HEAs are critically reviewed by considering the formation of phases,microstructural evolution and mechanical properties of HEAs joints.

Microstructural evolution and hot tensile behavior of Mg−3Zn−0.5Zr alloy subjected to multi-pass friction stir processing

The microstructures and hot tensile behaviors of ZK30 alloys subjected to single-and multi-pass friction stir processing(FSP)were systematically investigated.Following single-pass FSP(S-FSP),coarse grains underwent refinement to 1−2μm,with a distinct basal texture emerging in the stir zone(SZ).Additionally,second-phase particles were fragmented,dispersed,and partially dissolved.Multi-pass FSP(M-FSP)further enhanced the homogeneity of the microstructure,reduced texture intensity differences,and decreased the fraction of second-phase particles by 50%.Both S-FSP and M-FSP SZs demonstrated superplasticity at strain rates below 1×10^(−3)s^(−1)and at temperatures of 250−350℃.The S-FSP SZ exhibited an elongation of 390%at 250℃and 1×10^(−4)s^(−1),while the M-FSP SZ achieved an elongation of 406%at 350℃and 1×10^(−3)s^(−1).The superplastic deformation of SZ was co-dominated by grain boundary sliding(GBS)and the solute-drag mechanism in S-FSP and mainly by GBS in M-FSP.

Microstructural evolution of ultra-high strength Al-Zn-Cu-Mg-Zr alloy containing Sc during homogenization

The microstructural evolution and composition distribution of an Al-Zn-Cu-Mg-Sc-Zr alloy during homogenization were investigated by optical microscopy（OM）,scanning electron microscopy（SEM）,energy dispersive spectrometry（EDS）,X-ray diffraction（XRD） and differential scanning calorimetry（DSC）.The results show that severe dendritic segregation exists in Al-Zn-Cu-Mg-Sc-Zr alloy ingot.There are a lot of eutectic phases at grain boundary and the distribution of the main elements varies periodically along interdendritic region.The main eutectic phases at grain boundary are Al7Cu2Fe phase and T（Al2Mg3Zn3）.The residual phases are dissolved into the matrix gradually during homogenization with increasing temperature and prolonging holding time,which can be described by a constitutive equation in exponential function.The overburnt temperature of the alloy is 473.9 ℃.The optimum parameters of homogenization are 470 ℃ and 24 h,which is consistent with the result of homogenization kinetic analysis.

Microstructural evolution and mechanical properties of duplex-phase Ti6242 alloy treated by laser shock peening

The effects of laser shock peening(LSP)on the microstructural evolution and mechanical properties of the Ti6242 alloy,including the residual stress,surface roughness,Vickers microhardness,tensile mechanical response,and high-cycle fatigue properties,were studied.The results showed that the LSP induced residual compressive stresses on the surface and near surface of the material.The maximum surface residual compressive stress was−661 MPa,and the compressive-stress-affected depth was greater than 1000μm.The roughness and Vickers micro-hardness increased with the number of shocks,and the maximum hardness-affected depth was about 700μm after three LSP treatments.LSP enhanced the fraction of low-angle grain boundaries,changed the grain preferred orientations,and notably increased the pole density ofαphase on the near surface from 2.41 to 3.46.The surface hardness values of the LSP samples increased with the increase of the number of shocks due to work hardening,while the LSP had a limited effect on the tensile properties.The high-cycle fatigue life of the LSP-treated sample was significantly enhanced by more than 20%compared with that of the untreated sample,which was caused by the suppression of the initiation and propagation of fatigue cracks.

Microstructural evolution of Mg, Ag and Zn micro-alloyed Al-Cu-Li alloy during homogenization

The microstructural evolution of a Mg, Ag and Zn micro-alloyed Al?3.8Cu?1.28Li (mass fraction, %) alloy ingot during two-step homogenization was examined in detail by optical microscopy (OM), differential scanning calorimetry (DSC), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) methods. The results show that severe dendritic segregation exists in the as-cast ingot. There are many secondary phases, includingTB(Al7Cu4Li),θ(Al2Cu),R(Al5CuLi3) andS(Al2CuMg) phases, and a small amount of (Mg+Ag+Zn)-containing and AlCuFeMn phases. The fractions of intermetallic phases decrease sharply after 2 h of second-step homogenization. By prolonging the second-step homogenization time, theTB,θ,R,S and (Mg+Ag+Zn)-containing phases completely dissolve into the matrix. The dendritic segregation is eliminated, and the homogenization kinetics can be described by a constitutive equation in exponential function. However, it seems that the AlCuFeMn phase is separated into Al7Cu2Fe and AlCuMn phases, and the size of Al7Cu2Fe phase exhibits nearly no change when the second-step homogenization time is longer than 2 h.

Microstructural evolution of AZ61 magnesium alloy predeformed by ECAE during semisolid isothermal treatment

The microstructural evolution of AZ61 magnesium alloy predeformed by equal channel angular extrusion（ECAE） during semisolid isothermal treatment（SSIT） was investigated by means of optical metalloscopy and image analysis equipment.The process involved application of ECAE to as-cast alloy at 310 ℃ to induce strain prior to heating in the semisolid region for different time lengths.The results show that extrusion pass,isothermal temperature and processing route have an influence on microstructural evolution of predeformed AZ61 magnesium alloy during SSIT.With the increase of extrusion pass,the solid particle size is reduced gradually.When isothermal temperature increases from 530 ℃ to 560 ℃,the average particle size increases from 22 μm to 35 μm.When isothermal temperature is 575 ℃,the average particle size decreases.The particle size of microstructure of AZ61 magnesium alloy predeformed by ECAE at BC during SSIT is the finest.

Effect of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi shape memory alloy

As-received nickel-titanium （NiTi） shape memory alloy with a nominal composition of Ni50.9Ti49.1 （mole fraction,%） was subjected to solution treatment at 1123 K for 2 h and subsequent aging for 2 h at 573 K, 723 K and 873 K, respectively. The influence of solution treatment and aging on microstructural evolution and mechanical behavior of NiTi alloy was systematically investigated by transmission electron microscopy （TEM）, high resolution transmission electron microscopy （HRTEM）, scanning electron microscopy （SEM） and compression test. Solution treatment contributes to eliminating the Ti2Ni phase in the as-received NiTi sample, in which the TiC phase is unable to be removed. Solution treatment leads to ordered domain of atomic arrangement in NiTi alloy. In all the aged NiTi samples, the Ni4Ti3 precipitates, the R phase and the B2 austenite coexist in the NiTi matrix at room temperature, while the martensitic twins can be observed in the NiTi samples aged at 873 K. In the NiTi samples aged at 573 and 723 K, the fine and dense Ni4Ti3 precipitates distribute uniformly in the NiTi matrix, and thus they are coherent with the B2 matrix. However, in the NiTi sample aged at 873 K, the Ni4Ti3 precipitates exhibit the very inhomogeneous size, and they are coherent, semi-coherent and incoherent with the B2 matrix. In the case of aging at 723 K, the NiTi sample exhibits the maximum yield strength, where the fine and homogeneous Ni4Ti3 precipitates act as the effective obstacles against the dislocation motion, which results in the maximum critical resolved shear stress for dislocation slip.