Anisotropy In Tensile Properties Research Articles

Anisotropy in tensile properties of a high strength metastable β titanium alloy

High strength metastable β titanium alloys are widely employed in the aircraft industry due to their outstanding strength-to-weight ratio. While components can endure complex in-service mechanical loading, the anisotropy in tensile properties has been the subject of limited attention. In this study, its origin was investigated focusing on the role played by millimeter scale β grains as they were recently identified as a source of heterogeneous deformation. Tensile properties of Ti-10V-2Fe-3Al processed via different thermomechanical routes were assessed using multiple sampling directions. In particular, elongation values were observed to vary significantly depending on the testing direction. A combination of SEM, EBSD, µ-CT and in-situ DIC during tensile tests was employed to clarify the underlying causes of this behavior. Substantial differences in strain heterogeneity and localization were found related to features of β grains, including their crystallographic and morphologic orientations. Furthermore, multiple fracture mechanisms were observed to derive from the differences in deformation behavior, and eventually compete to trigger specimen failure. Elongation values are then determined by both the degree of strain heterogeneity and the operating fracture mechanisms. These findings provide a new understanding of the role of the microstructure in the tensile behavior of high strength metastable β titanium alloys.

Defects, microstructure and associated mechanical properties of Inconel 718 fabricated by selective electron beam melting

Comprehensive characterizations and evaluations were conducted on the relationship between process parameters and density in the Inconel 718 (IN718) alloy fabricated by selective electron beam melting (SEBM). Additionally, the microstructure and anisotropy of tensile properties were evaluated in both the as-built state and after solution treatment and aging (STA). During different stages of energy density, the primary contributors to the porosity rate transitioned from the lack of fusion (LF) pores to the inclusion of spherical and shrinkage pores, resulting in a maximum relative density of 98.95 %. The as-built parts displayed notable parallel texture to the building direction, and a significant tensile anisotropy supported by the Taylor factor. The matrix of these parts consisted of δ, γ', γ″, and carbonitrides, with a considerable presence of δ precipitation leading to a reduction in γ″ content and lower strength. After heat treatment, the δ dissolved, and γ″ precipitated extensively, while the carbonitrides and texture were retained. However, unique tensile results contradicting the predictions made by computational modeling were observed, we propose a new insight attributed to the presence of shrinkage pores along the build direction and use finite element analysis to support this view.

The effect of thermal cycle on microstructure evolution and mechanical properties of Co-free maraging steel produced by wire arc additive manufacturing

Microstructures and mechanical properties are closely related to thermal cycles during additive manufacturing. For maraging steel, the research on the effect of thermal cycles during additive manufacturing is limited. Based on the above issues, this work investigated the effect of thermal cycles in the process of wire arc additive manufacturing Co-free maraging steel on microstructure evolution and mechanical properties, and attempted to establish the relationship between thermal cycles and microstructure as well as mechanical properties of maraging steel on the basis of quantitative thermal cycle data. The results show that in the additive manufacturing process, the thermal cycles affect the cooling rate, so that the primary dendrite arm spacing and grain size gradually increase along the height direction. For maraging steel, in additive manufacturing, welding or other hot processing processes, the thermal cycles make the martensite reverse change, resulting in an increase in austenite content, resulting in grain refinement. Thermal cycles in additive manufacturing result in differences in the grain size, grain boundary ratio, dislocation density and primary dendrite arm spacing, resulting in inhomogeneity of the mechanical properties in the height direction. The difference in microstructure in different directions of additive manufacturing samples leads to anisotropy of tensile properties. The results of this work can elucidate and refine the action mechanism of thermal cycles on maraging steel. In addition, this work can be used to control thermal cycles by changing the process and cooling conditions, etc., to obtain maraging steel samples with homogeneous or gradient properties, which is highly important.

An experimental and numerical study on the evolution of pores and its effect on the tensile properties of LPBF Invar36 with different energy density

Laser powder bed fusion (LPBF) Invar 36 alloy has attracted plenty of attention in cryogenic components or precision instruments due to its low coefficient of thermal expansion in a wide range of temperature. However, the LPBF Invar 36 has a significant anisotropic mechanical property due to the unique columnar grain morphology and pore feature. In this study, the influence of scanning speed on the anisotropic tensile properties were investigated, and the melt pool dynamics were analyzed by the numerical simulation. In this respect, tensile tests of specimens parallel and perpendicular to the building direction with a scanning speed from 200 to 1400 mm/s of LPBF Invar 36 alloy were conducted, and the microstructural influential factors on the anisotropic tensile properties were analyzed. Results show that as the volumetric energy density (VED) increases, the inhomogeneity of grain morphology decreases, while the pore feature changes from the lack-of-fusion (LOF) pore perpendicular to the building direction to the bubble-shaped spherical pore, which is resulted from the collapse of the key-holes. In this case, the grain orientation and size, as well as the dislocation density in the vicinity of pores are influenced by the pore morphology due to different thermal history. On the whole, the combined morphology of dislocation and stress concentration affected by the pore morphology are the main reason of the increased level of anisotropy in tensile properties.

The role of TiN inoculant on microstructure and properties in wire arc directed energy deposition of super duplex stainless steels

The challenges in Wire Arc Directed Energy Deposition (Wire-DED) of super duplex stainless steels (SDSS) are the growth of large columnar grains and undesirably high austenite content. This results in a detrimental effect on the material properties. Here we report the additively manufactured SDSS with the addition of TiN inoculant via Wire-DED. And the effectiveness of incorporating TiN inoculant into additively manufactured SDSS was explored. The addition of trace amounts of TiN reduced the content of the austenite phase from 74.0% to 61.5%. Additionally, the introduction of TiN facilitated deoxidation and slag formation, leading to decreased inclusion content and a shift in inclusion type, which acted as nucleation sites for ferrite, promoting grain refinement. This results in weakened anisotropy of tensile properties and enhanced corrosion resistance. The anisotropy of the ultimate tensile strength and yield strength reduced from 9% to 3% and from 5% to 2%, respectively. This study demonstrates the potential of TiN inoculation in fabricating Wire-DEDed SDSS with tailored microstructures and improved performance, making them suitable for a wide range of practical applications.

The mechanical properties and microstructural evolution of 14Ni3Cr3Mo1MnTiAl high strength steel fabricated by WAAM

A self-developed high strength steel (14Ni3Cr3Mo1MnTiAl HSS) was used to manufacture a thin-walled structure by wire + arc additive manufacturing, the microstructural evolution and the anisotropy of mechanical properties were investigated. The tensile properties and impact toughness at room temperature, −20 °C and −40 °C were compared and discussed in detail. According to the results of OM and SEM, the main microstructure of WAAM-ed 14Ni3Cr3Mo1MnTiAl HSS was martensite and ferrite, and numerous Ni3Ti IMCs existed in the matrix. Reversed austenite only existed in the bottom and middle samples, and there was a K-S orientation relationship in reversed austenite and martensite/ferrite. The formation of reverted austenite could be attributed to repeated thermal cycles and high content of Ni, providing beneficial environment and condition for the formation of reverted austenite. Low temperature could increase the strength of vertical samples, while it had a negative effect on plastic and had little effect on the strength of horizontal samples. Differently, the impact toughness hardly decreased at −20 °C, while it had significant decrease at −40 °C. The anisotropy of tensile properties was attributed to the weak bonding of interlayer, phase content, the size of martensite packet/lath, and the size of Ni3Ti IMCs. Columnar crystals were one of the key factors leading to the low impact toughness of horizontal samples. The impact direction was parallel to the growth direction of columnar crystals, and the crack was prone to propagate along the boundaries of columnar crystals.

Morphology of impact polypropylene copolymer extruded cast film revealed by confocal Raman imaging.

An impact polypropylene copolymer (IPC), composed of polypropylene (PP) and ethylene-propylene copolymer (EPC), was synthesized through two-stage in-reactor polymerization. A systematic investigation of the crystalline structure, thermal behavior, morphology, and tensile properties of the IPC extruded cast film was conducted. Specifically, the morphology of EPC was obtained by confocal Raman imaging by depicting the spatial distribution of the Raman band located at 1064 cm-1. The EPC phase exhibits fibrous morphology with the long axis aligning along the machine direction (MD). A three-dimensional (3D) heterogeneous structure of the IPC cast film obtained by confocal Raman imaging confirms that the fibrous EPC phase is dispersed in a 3D framework of the PP matrix. The mesomorphic phase in the as-prepared cast film transforms to a stable α-form crystal after annealing at 130 °C, which improves the yield strength but decreases the elongation of the cast film. The WAXD and SAXS results indicate that there is no obvious orientation of the crystallites. Thus, the anisotropy of tensile properties in the MD and transverse directions is closely related to the anisotropic phase morphology at the micrometer scale. The results reveal that the mechanical performances of IPC films are determined by the crystalline structure of the PP matrix and the morphology.

Mechanical properties anisotropy of Ti−6Al−4V alloy fabricated by β forging

The anisotropy of tensile properties and fracture toughness of Ti−6Al−4V (Ti64) alloy fabricated by β forging was investigated. In order to evaluate anisotropic mechanical properties, the microstructure and crystallographic texture of billets with different orientations were characterized, and the influence of sampling direction on tensile properties and fracture toughness was studied. The results reveal that the prior β grains of Ti64 billet display pan-cake-type characteristics. At room temperature, the alloy is mainly composed of α phase, and several α phase variants produced by β→α phase transformation decrease the α texture intensity. The anisotropy of mechanical properties is affected by prior β grain morphology and the slip activation related to α texture. The initiation toughness was obtained by the J-integral resistance curve method. The initiation toughness was divided into intrinsic and extrinsic toughness by calculation. The anisotropy of intrinsic toughness is mainly attributed to the size of crack tip plastic zone, which is affected by prior β grain morphology. In contrast, the extrinsic toughness is affected by zigzag degree of crack, which is mainly influenced by α lamellae and colonies.

Through-Thickness Inhomogeneity of Microstructures and Mechanical Properties in an As-Quenched Thin Specification High Strength NM450TP Steel Plate

The inhomogeneity of microstructures and mechanical properties in an as-quenched thin specification NM450TP wear-resistant steel plate were quantitatively investigated. The results show that the microstructures exhibit inhomogeneous distribution through the thickness and the area percentage of martensite and ferrite grains varies regularly through the thickness, and the content of ferrite on the top surface of the plate is found to be two times that of ferrite at the core location and more than that of ferrite at the bottom surface. In addition, the steel plate exhibits the obvious anisotropy of tensile properties, the tensile strength paralleling to the rolling direction is lower than that along the transverse direction while the elongation paralleling to the rolling direction is better than that along the transverse direction. The result indicates that the deformation degree of prior austenite grains during hot rolling and the content of martensite after quenching dominate the mechanical properties while the ferrite content is not the main factor affecting the plasticity. The findings provide experimental evidences and lay a theoretical foundation for analyzing the subsequent processing.

Improvement on high-temperature tensile strength and ductility of selective electron beam melting TiAl alloys via small-strain forging

In the present work, a Ti–48Al–2Cr–2Nb alloy prepared by selective electron beam melting (SEBM) and post-treated by forging at a small strain was investigated. The results show that the microstructure of the as-built TiAl alloys exhibited an alternating layer of coarse single γ grains and fine γ+α2 grains configuration along the building direction. After forging at a small strain, the fraction of the coarsely grained region decreases, and substantial fine sub-grains form in the fine-grained region. Additionally, fractions of the α2 phase and α2/γ lamellar colonies increase. These factors lead to an increase in the tensile strength of alloys. Meanwhile, the low-angle grain boundaries (LAGBs) in fine sub-grains and the substantial mobile dislocations in coarse γ grains are favorable for improving the ductility of alloys. The formation of deformation twins in the γ grains facilitates the simultaneous improvement of strength and ductility. In addition, the anisotropy of tensile properties is reduced after small-strain forging, which can be attributed to the reduction in fraction of the coarse γ grains, and the increase in the fractions of the α2 phase and α2/γ lamellar structure. Therefore, small-strain forging was verified as an effective post-treatment method for the improvement of tensile strength and ductility of SEBM prepared TiAl alloys at high temperatures.

Effect of Microstructural and Textural Anisotropy on the Tensile Properties of Selective Laser Melted and Annealed Ti6Al4VE Alloy

The effect of texture on the anisotropy of tensile properties of the SLM‐formed and annealed Ti6Al4VE alloy is investigated. The component of microstructure in these alloys is dominated by the α/α′ phase. Furthermore, there is a significant difference in the intensity of X‐ray diffraction peaks between the XY plane that is perpendicular to the build direction (BD) and the XZ plane that is parallel to the BD, though alloys are annealed with the same treatment. Ultimate tensile strength and yield strength in the X direction are higher than in the Z direction and exhibit significant anisotropy in the alloy annealed with the same treatment conditions, but the plasticity presents an opposite trend. The average Schmid factor of the basal <α>, prismatic <α>, and pyramidal <c + α> slip systems in the alloy is analyzed by electron backscatter diffraction and presents that the prismatic <α> slip is the main deformation mode during the tensile testing. The <0001> fiber texture parallel to the loading direction in the XY plane is stronger than that in the XZ plane and causes a decrease in the number of the basal <α> slip system activating in the XY plane in contrast to the XZ plane.

Influence of pre-stretching on the tensile strength, fatigue properties and the in-plane anisotropy in Al-Cu-Li alloy AA2099

The influence of pre-stretching prior to artificial aging on microstructure, tensile properties, and fatigue behavior of AA2099-T8 sheets was investigated. The results showed that increased pre-strain level from 1% to 6% resulted in a more uniform distribution and increasing density of the T1 phase. The localized plastic deformation during pre-stretching led to the formation of dense precipitation bands (DPBs) and precipitation free bands (T1-PFBs) in the sheets with the 1% pre-strain. The non-uniform distribution of the T1 phase with T1-PFBs and DPBs resulted in a decrement in elongation and a strong in-plane anisotropy of tensile properties. An appropriate level of pre-strain should be executed aiming to improve elongation and reduce the tensile anisotropy by promoting a uniform distribution of the T1 phase. In-plane anisotropy of yield strength due to the presence of DPBs and T1-PFBs was quantitatively analyzed by the modified Hall-Petch formula. The results of the calculation for the modified model showed good agreement with the experiment. In addition, resistances of fatigue crack propagation (FCP) in the same orientation were gradually decreased with the increment of pre-strain due to the increment in yield strength and more uniform distribution of the T1 phase. For the in-plane anisotropy of FCP resistance, the main reasons were the development of rough fracture surfaces and the transgranular fracture modes in L-T orientation.

Effects of rolling deformation on microstructure orientations and tensile properties of TiB/(TA15-Si) composites

To explore the orientation evolution and interfacial characteristics under the β-phase region, rolling deformation with different reduction ratios of 40– 90% was carried out by hot-pressing sintering and multi-pass hot rolling on a series of TiB whiskers reinforced Ti-6.5Al-2Zr-1Mo-1 V-0.3Si (TiB/(TA15-Si)). As rolling deformation increased, the matrix exhibited slight refining and distortion within lamella α + β, and a stronger T-type α texture was gradually formed. A portion of Burgers Orientation Relation (BOR) between parent β and α phases gradually disappeared due to a decrease in rolling temperature and post deformation after phase transformation. Meanwhile, TiB rotated and presented an intenser orientation of {100} 〈010〉. Silicides heterogeneously precipitated along the α/β phase boundaries both near and away from TiB. Silicide/matrix remained fine interfaces owing to resolution and reprecipitation. Finally, the ultimate strengths at room temperature were monotonously improved with the increase of deformation due to strong work hardening. And an anisotropy of tensile properties was found under large deformation. The composite tensioned along the transverse direction (TD) can exhibit higher strength and lower ductility due to mutual competition of TD strengthening by T-type texture and RD strengthening by TiB distribution, especially at 600 °C. The fracture paths of as-rolled composites were discussed in detail. • Matrix gradually formed an intenser T-type α texture mainly inherited from β by BOR. • TiB can form an obvious orientation of {100}<010 > but have a minor proportion of OR. • Most silicide/matrix interfaces possessed orientation relations in the β-rolled composites. • The ultrahigh ultimate strength of 1505 MPa was obtained in the 90%-rolled composite. • Composites along TD exhibited higher strength and lower ductility especially at 600 °C.

Microstructural evolution and anisotropic regulation in tensile property of cold metal transfer additive manufactured Ti6Al4V alloys via ultrasonic impact treatment

Arc-based cold metal transfer additive manufacturing (CMTAM) technology presents an effective method for integrated construction of large metallic components. However, the cold metal transfer additive manufactured (CMTAMed) parts generally produce coarse columnar grains along the building direction, resulting in the anisotropy of tensile properties. In this paper, the effects of double-sided ultrasonic impact treatment (UIT) on the surface roughness, residual stress distribution, microstructure and tensile properties of the CMTAMed Ti6Al4V alloy in both horizontal and vertical directions were investigated. Results indicated that UIT significantly reduced surface roughness and realized the tensile-compressive conversion of residual stress in the surface layer. Furthermore, massive stacking faults (SFs) structures and high-density dislocations were stimulated in the original α martensite, leading to the grain refinement. Moreover, both the CMTAMed samples displayed synchronous enhancement of strength and ductility after double-sided UIT, while the growth rates of the horizontal samples are higher than that of the vertical ones, making a decline in the anisotropy of tensile properties. Consequently, the gradient microstructure and anisotropy regulation mechanism of tensile properties dominated by double-sided UIT was clarified.

Anisotropy in tensile properties and fracture behaviour of 316L stainless steel parts manufactured by fused deposition modelling and sintering

Anisotropy in tensile properties and fracture behaviour of 316L stainless steel parts manufactured by fused deposition modelling and sintering

Wire-fed electron beam directed energy deposition of Ti–6Al–2Zr–1Mo–1V alloy and the effect of annealing on the microstructure, texture, and anisotropy of tensile properties

Wire-fed electron beam directed energy deposition (EB-DED) is gaining increasing attention owing to its significant advantage of producing high-quality, large-scale metallic components. In this study, EB-DED of a near-α titanium alloy (Ti–6Al–2Zr–1Mo–1V) was conducted, and the effect of post-deposition single annealing treatment on the microstructure, texture, and anisotropy of the tensile properties was investigated. Optical microscopy, scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy were employed to study the microstructure and texture characteristics. The tensile properties along the vertical (Z axis) and horizontal (Y axis) directions were evaluated at both room temperature and 500 °C. The results indicated that the prior β grains had a strong < 001 >β texture along the grain growth direction. Although the α variant selection occurred in some micro-regions, the overall α phase texture was weak. In terms of tensile properties, the vertical specimens exhibited lower strength but higher ductility than the horizontal specimens at both room and elevated temperatures. The anisotropic elongation results from the directional columnar prior β grains and continuous grain boundary α phase, which facilitated the intergranular cracking. The tensile strength and ductility were simultaneously enhanced by annealing at 950 °C for 2 h to meet the standard requirements for wrought counterparts. Moreover, the anisotropy of the tensile properties was decreased significantly. The enhanced and isotropic mechanical properties can be attributed to the combined effect of the bi-lamellar microstructure, discontinuous grain boundary α phase, and the weak α texture.

Can we predict the microstructure of a non-woven flax/PLA composite through assessment of anisotropy in tensile properties?

Flax/Poly-(lactide) non-woven composites are an alternative to conventional glass/poly-(propylene), offering a potentially lower environmental impact solution for the automotive industry. To understand this complex material, its detailed architecture and void distribution are examined through 3D microtomography. Anisotropy in fibre orientation is also observed, further verified through off-axis tensile tests. Then micro-mechanics and laminate theory are used, taking into account fibre orientation distribution, to predict mechanical properties and compare with experimental measurements. Even though some deviation is observed at off-axis angles greater than 45°, we report that off-axis tensile tests (property) can be used to predict fibre orientation (structure) in industrial production facilities as a simple means of quality control and tailored design of new non-woven preforms.

Strengthening mechanism and anisotropy of mechanical properties of Si3N4p/Al-Mg-Si composites fabricated by sintering and extrusion

The Si3N4p reinforced Al-Mg-Si (Si3N4p/6061Al) composites with various Si3N4p contents were fabricated by hot pressing sintering and hot extrusion. The strengthening mechanism was clarified, and the effects of Si3N4p on microstructure, mechanical properties and anisotropy were investigated. The results showed that Si3N4p, Mg2Si phase, and lots of micropores distributed in as-sintered ingots, and hot extrusion effectively reduced the number of micropores. The Si3N4p was well boned with Al matrix, and no defect or reaction product existed on the interface. Both dynamic recovery and dynamic recrystallization took place during hot extrusion, and the addition of Si3N4p refined the grain structure by promoting the particle stimulated nucleation effect. The main texture of as-extruded composites was brass, while its intensity was gradually weakened with the increase of Si3N4p content. The tensile strength and wear resistance were enhanced by the addition of Si3N4p due to the strengthening mechanisms of load transfer, grain refining, and dislocation strengthening. The extruded composite without Si3N4p exhibited the strong anisotropy of tensile properties and planar anisotropy. Since the addition of Si3N4p was beneficial to refine the grain structure and weaken the intensity of brass, both the tensile properties anisotropy and planar anisotropy were significantly reduced.

Controllable mechanical anisotropy of selective laser melted Ti6Al4V: A new perspective into the effect of grain orientations and primary grain structure

This work investigated the significantly weakened anisotropy of tensile properties of selective laser melted Ti6Al4V by heat treatment and represented a new perspective into the weakening mechanism. For the as-deposited sample, the tensile strength showed an evident improvement as the oriented angle θ (the angle between the tensile axis and the substrate plane) increased from 0° to 90° while the elongation showed the opposite trend. Nevertheless, the heat treatment weakened the anisotropy of strength as the deviation was reduced from 167 to 39 MPa, while the anisotropy of ductility was also remarkably declined. An algorithm, based on the EBSD data, was newly adopted to determine the most likely active slip system of each grain and evaluate the area-weighted average Schmid factors (SFs). The statistical results presented the distinct difference in the SFs of the as-deposited sample as θ changed while the SFs of the heat-treated sample showed similar values, quantitatively explaining the anisotropy of strength and the weakening mechanism. Besides, the high dependence of SF on the primary β grains, the number of primary β grains in the tensile specimen at different θ, and the existence of equiaxed grain boundary α together explained that ductility was worst at θ = 90° and increased as θ decreased.

A multiscale investigation of deformation heterogeneity in additively manufactured 316L stainless steel

Additive manufacturing (AM) builds materials layer upon layer with the aid of an energy source. During AM processing, the complex thermal behaviors—directional heat flux, rapid solidification, and repeated thermal cycles—facilitate the formation of heterogeneous microstructures. Consequently, the mechanical behavior of AM parts can intrinsically differ from that of conventionally processed materials. This work focuses on characterizing deformation behavior in relation to AM-pertinent features and attempts to improve quantitative understanding of the process-structure-property relationships of metallic AM parts. A sample of as-deposited 316L stainless steel manufactured by the directed energy deposition technique is examined on different scales via microscope digital image correlation and electron backscatter diffraction. First, anisotropy in tensile properties is examined in the transverse plane perpendicular to the build direction. Then, it is revealed that the characteristic wavelength of strain bands formed due to deformation localization is strongly affected by tracks of deposits. Subsequently, strain localization at the grain scale is analyzed. The origin of plastic strains from the perspective of the activation of slip systems is examined, and the implications of the statistical distribution of local strain are discussed. Lastly, plastic strain accumulation and transmission in the vicinity of grain boundaries and substructure colonies are studied. The current observations and findings enrich the understanding of strain inhomogeneity in metallic AM parts and its relation to features at different length scales.