Creep anisotropy behavior,deformation mechanism,and its efficient suppression method in Inconel 625 superalloy

Mill products,such as sheets,usually show obvious anisotropy in their mechanical properties,which greatly affects both their applications and workability.In this study,the orientation-dependent tensile and creep behaviors of Inconel 625 alloy sheets with weak local textures were systematically investigated at 650°C.The results showed that Inconel 625 superalloy exhibits nearly isotropic tensile properties;how-ever,obvious creep anisotropy appears when loading along different directions.Creep life in the rolling direction(150±5 h)was approximately 4.5 times longer than in the transverse direction(33±1 h).Se-vere creep anisotropy was found to be determined by two aspects:(i)the change in deformation mecha-nisms along the rolling direction(a combination of boundary sliding,dislocation slipping,and twinning)and the transverse direction(dislocation slipping).Stronger impingement of slip bands on grain bound-aries accelerated intergranular crack initiation and propagation during loading along the transverse direc-tion,which resulted in a short creep life.(ii)The differences in strain compatibility at grain boundaries(i.e.,creep loading along the transverse direction resulted in the initial grains forming strong Brass,Cu,and S textures).Creep cracks preferentially nucleated at the junctions between Cu/Brass texture grains,as these interfaces exhibit the worst strain compatibility(Luster-Morris parameter m≤0.22).Furthermore,pre-stress aging(PSA)treatment is proposed as an efficient method to suppress creep anisotropy.The discrepancy rate of creep life was found to be reduced by nearly 50%after PSA treatment.

Microstructural evolution and FCC twinning behavior during hot deformation of high temperature titanium alloy Ti65

Although the development of titanium alloys with working temperatures above 600?C faces enormous difficulties and challenges,the related research has not stopped.In the present work,detailed analyses on microstructure evolution and hot deformation behavior of a new temperature resistant 650?C titanium alloy Ti65 were investigated from micrometer scale to nanometer scale.The results revealed that lamellarαgrains gradually fragmentized and spheroidized during theα+βphase region compression and the orientation of the c-axis ofαgrains gradually aligned to radial directions,forming two high Schmid factors(SFs)value texture eventually with the increase of strain to 0.7.Moreover,there were some strengthening characters in theα+βphase region such as lenticularαsand nano silicide(TiZr)6 Si3.In theβphase region,fine equiaxed dynamic recrystallized(DRX)βgrains were formed.Besides,the variant selection ofαm′artensite followed Burgers orientation relationship during the compression process.The main deformation mechanisms of theα+βphase region were dislocation slip and orientation dependent spheroidization.Whereas,the deformation process in theβphase region was controlled byβgrain DRX.Interestingly,many nano scale FCC twins were generated at the interface ofαl′ath during deforming in theβphase region,which was firstly observed in Ti65 alloy.

Orientation dependent behavior of tensile-creep deformation of hot rolled Ti65 titanium alloy sheet

The mill products like sheet always have one or more severe textures inevitably,and its effect on mechanical properties is not a negligible issue.The orientation dependent tensile-creep behavior induced by rolling texture of Ti65 titanium alloy sheet has been systematically investigated at 650℃.There are some anisotropic characteristics between TD and RD of Ti65 sheet.The UTS and TYS of TD are higher than RD at 650℃.Besides,the creep endurance time of TD(172.6–174.5 h)is about three times longer than RD(55.6–65.1 h)at 650℃and 240 MPa.Moreover,the grains are inclined to form Texture III■after creep along with TD,but to form Texture I■after creep along with RD.Finally,the crack initiation site is different during creep in TD and RD.The reason for anisotropic properties of tensile and creep has been summarized in two aspects:(i)the change of the SFs(Schmid factors)value between TD and RD;(ii)the difference of creep mechanism between TD(grain boundary sliding)and RD(dislocation slip).Anisotropy of Ti65 sheet should be fully considered to increase structural efficiency in the engineering design and application.

Dependence of mechanical properties on the microstructure characteristics of a near β titanium alloy Ti-7333

In this work, the microstructure and the corresponding tensile properties of the rolled Ti-7Mo-3Nb-3Cr-3Al(Ti-7333) alloy before and after the thermal treatments were investigated. The results show that a strong α-fiber texture is developed in the rolled Ti-7333 alloy. The deformed matrix and the texture significantly induce the variant selection of β phase. The high strength of the rolled Ti-7333 alloy is attributed to the <110> texture parallel to the tensile direction and the dispersed α phase within the matrix. After the solution treatment followed by the aging treatment, the texture decreases and the microstructure consists of the equiaxed β grains, the spheroidal α_p phase and various needle-like α variants. Eventually, the alloy could achieve an optimal combination with the strength of about 1450 MPa,the ductility of about 10.5% and a considerable shear strength of about 775 MPa. This balance can be ascribed to the performance of the spheroidal α_p phase and various needle-like α_s variants. The results indicate that the Ti-7333 alloy could be a promising candidate material for the high-strength fastener.

ω-Assisted refinement of α phase and its effect on the tensile properties of a near β titanium alloy

In this work, the phase transformation sequence during the continuous heating process(3℃/min) was investigated in a near β titanium alloy. The results show that the staring formation of ω phase is about267℃, and the ending precipitation temperature about 386℃ during the heating process. When the heating temperature is greater than 485℃, there are no ω phase detected within the β matrix. Combined with the microstructural characterization, it is found that ω phase facilitates the nucleation of αphase nearby the ω/β interface and has a great effect on the refinement for α phase. As compared with the specimens directly aged, the specimens with ω-assisted refinement of α phase possess high tensile strength, but there is no yield stage detected on their stress-strain curve. Combined with the analyses of the fracture morphology, the specimens with ω-assisted refinement of α phase present a brittle fracture.This is mainly ascribed to its relatively lager width of grain boundaries and the absence of widmanst?ttenα precipitates.

On the nature of a peculiar initial yield behavior in metastable β titanium alloy Ti-5Al-5Mo-5V-3Cr-0.5Fe with different initial microstructures

The compressive yielding phenomenon of titanium alloys is not as focused and sufficiently ascertain as the tensile yielding phenomenon.In the present work,the peculiar compressive yielding behavior and the different dynamic responses of three different initial microstructures(singleβ,clavateβand lamellarβ)were investigated in an attractive metastableβtitanium alloy Ti-5553 using electron microscopes/crystallographic calculation/crystal plastic finite element simulation.Results reveal that the distinct compressive yielding behavior,steep peaks of sudden drop in the initial stage(very small true strain 0.03)of stress loading have appeared in the compression stress-strain curves except for the lamellarβinitial microstructure.Dislocation slip is the essential mechanism of the initial yielding behavior.Interlaced multiple-slip bands formed in the singleβinitial microstructure during the warm deformation process.A small quantity of single slip bands was observed in the deformed clavateβinitial microstructure.The abundant varied nano/ultrafineβsprecipitates were nucleated dynamically and dispersedly in all the three deformed initial microstructures.The multiple-slip bands formation and substantial nanoscaleβsresult in the highest peak of flow stress for singleβinitial microstructure.The compressive slip bands are formed early in the elastic–plastic deformation stage.As the increasing strain,the sample showed a significant compressive bulge,or eventually forming a strong adiabatic shear band or crack.These results are expected to provide a reference for the study of deformation behavior and mechanical properties of metastableβtitanium alloys.

Microstructural influences on the high cycle fatigue life dispersion and damage mechanism in a metastable β titanium alloy

In this work,the effect of microstructure features on the high-cycle fatigue behavior of Ti-7Mo-3Nb-3Cr-3Al(Ti-7333)alloy is investigated.Fatigue tests were carried out at room temperature in lab air atmosphere using a sinusoidal wave at a frequency of 120 Hz and a stress ratio of 0.1.Results show that the fatigue strength is closely related to the microstructure features,especially theα_(p) percentage.The Ti-7333 alloy with a lowerα_(p) percentage exhibits a higher scatter in fatigue data.The bimodal fatigue behavior and the duality of the S-N curve are reported in the Ti-7333 alloy with relatively lowerα_(p) percentage.Crack initiation region shows the compoundα_(p)/βfacets.Facetedα_(p) particles show crystallographic orientation and morphology dependence characteristics.Crack-initiation was accompanied by faceting process across elongatedα_(p) particles or multiple adjacentα_(p) particles.These particles generally oriented for basal slip result in near basal facets.Fatigue crack can also initiate at elongatedα_(p) particle well oriented for prismatic slip.Theβfacet is in close correspondence to{110}or{112}plane with high Schmid factor.Based on the fracture observation and FIB-CS analysis,three classes of fatigue-critical microstructural configurations are deduced.A phenomenological model for the formation ofα_(p) facet in the bimodal microstructure is proposed.This work provides an insight into the fatigue damage process of theβprecipitate strengthened metastableβtitanium alloys.

Duality of the fatigue behavior and failure mechanism in notched specimens of Ti-7Mo-3Nb-3Cr-3Al alloy

An interesting phenomenon of dual S-N fatigue behavior is investigated in a metastable β titanium alloy,Ti-7 Mo-3 Nb-3 Cr-3 Al notched cylindrical specimens with an elastic stress concentration factor of Kt=3.Fractographic studies revealed all specimens,and irrespective of lifetime,failed from the specimen surface because of stress concentration occurs at the notch root.Typically,the short-life-distribution is usually associated with surface-failure-without-facets and the long-life-distribution generally occurs due to surface-failure-with-facets.This competing failure leads to increasing the variability in fatigue lifetime and further facilitates the difficulty in prediction of fatigue lifetime.Crack-initiation area characterization was conducted by using mechanical grinding,focused ion beam milling and subsequent electron backscattered diffraction(EBSD) analysis of the 2 D section across faceted grains.Results show that the α_p particles(especially the elongated α_p particles) well-oriented for basal slip activation is a preferential fatigue-critical microstructural configuration.Additionally,the β+α_s matrix has a higher KAM value than the α_p particles in fatigued microstructures and significant dislocation activity in the form of dislocation tangles is observed in α_p boundaries.