Alloy Grain Research Articles

High-Temperature corrosion of P91/T91, 304L, Sanicro 28 and Inconel 625 exposed at 600 °C under continuous KCl deposition

This study investigates the corrosion attack after breakaway oxidation on four commercial alloys (T91/P91, 304L, Sanicro 28 and Inconel 625) in the presence of KCl(g)/KCl(s) at 600 °C. The study suggests that an increase in corrosion resistant alloying elements (mainly nickel and chromium) results in the decrease of the thickness of the general oxide scale and not equally deep alloy grain boundary attack for the austenitic alloys. The corrosion attack in the presence of KCl is suggested to proceed by chromate formation and chlorine-induced acceleration of the diffusion of ions through the scale.

Interaction of dynamic precipitation and dynamic recrystallization of a Mg-4Sn-3Al-1Zn alloy during hot compression

The intrinsic correlation between dynamic precipitation and Dynamic Recrystallization (DRX) in a rare earth (RE)-free engineering Mg-4Sn-3Al-1Zn (TAZ431, wt.%) magnesium alloy has been systematically investigated. Hot compression tests were performed on the TAZ431 alloy, in comparison to Mg-3Al-1Zn (AZ31, wt.%) as a reference alloy, with varying temperatures, strain rates, and strain levels (T = 300 ‐ 460 °C; ε̇ = 0.001 - 1 s‐1; ε = 0 to -0.9). Correlative microstructural characterizations including Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), and Electron Backscatter Diffraction (EBSD), were employed to explore the behaviors of precipitation, DRX and intragranular deformation as subjected to hot compression. The results indicate a significant interaction between Mg2Sn dynamic precipitation and the formation of DRXed grains. The TAZ431 alloy with Sn alloying exhibits profuse precipitation and larger DRX fraction within the microstructure, resulting in the increase of modulus and hardness. Nucleation of DRXed grains in the TAZ431 alloy was stimulated by relatively large (over 1 micrometer) undissolved Mg2Sn intermetallic particles. Meanwhile, the precipitation of nano-sized Mg2Sn occurred within DRXed grains and was facilitated by DRX process, consequently hindering the further growth of DRXed grains. The findings here may have implications for alloy design and thermal-mechanical processing of magnesium alloy structural components.

Anisotropy in flow behaviour of directionally solidified Ni-based superalloy under compression at an elevated temperature

Directionally solidified (DS) superalloys consist of coarse columnar grains with <001> preferred orientation parallel to the maximum stress axis; but, grain orientations in the transverse direction are not controlled. However, during service, substantial loading could occur in the transverse direction as well, leading to early intergranular failure of components such as DS turbine blades. The present study investigates the effect of crystallographic orientation as well as specimen orientation (transverse and longitudinal with respect to grain growth direction) on the mechanical response of DS Ni-based superalloy in compression. Compression tests were performed at 700 ºC on transverse (with predominantly <101> or <001> oriented grains) and longitudinal (with <001> oriented grain) samples. Though there were negligible changes in yield stress, variations in hardening behaviour and ductility were observed in all the tested samples. Unlike transverse samples, longitudinal samples showed significant strain hardening with no failure even up to 0.3 true strain. Deformation mechanisms that led to these differences are identified using advance characterization techniques. γ' shearing with antiphase boundary formation was observed in the sample containing <101> grains, whereas γ' shearing with the formation of superlattice intrinsic stacking faults occurred in the sample having <001> grains. In addition, substantial lattice rotation and deformation twinning also contribute to strain hardening, especially in longitudinal samples. The effect of orientation is explained in terms of effective stacking fault energy. The study suggests that grains with <001> orientation undergo significant strain hardening and thereby accommodate large strain as compared to <101> oriented grains in DS alloys.

Revealing the effect of misorientation on the deformation behavior of a Mg-Y alloy from the perspective of local strain

The addition of rare earth (RE) elements in magnesium alloys may significantly enhance the ductility of magnesium alloys, which is generally attributed to the decrease of stacking fault energy, the reduced critical resolved shear stress (CRSS) differences among different deformation modes and the weakening of basal texture. Rare earth elements also change the misorientation between adjacent grains of magnesium alloy, whose effects on the deformation behavior and thus the ductility of magnesium alloy were not emphasized before. In this work, ex-situ high-resolution digital image correlation (HR-DIC) coupled with electron backscatter diffraction (EBSD) was used to reveal the effect of the misorientation on the deformation behavior of a Mg-Y binary alloy from the perspective of local strain. It is shown that the heterogeneity of local strain is largely dependent on the misorientation between adjacent grains. For the intergranular deformation, three scenarios are observed, namely strain transfer, strain concentration and suppressed deformation, which mainly occurs at low misorientation, moderate misorientation and high misorientation, respectively. In addition, much more non-basal slip is found to be activated near the grain boundaries with moderate misorientation angles due to the high geometric compatibility factor (m’B-Pr/Py), which is favorable for the intergranular deformation compatibility, thus reducing the strain concentration near grain boundaries and achieving high ductility. For the intragranular deformation, a wide range of misorientation between adjacent grains in the Mg-Y alloy causes multiple basal slip systems to be activated within a grain, which also facilitates work hardening, thus contributing to improving ductility. Therefore, tailoring the misorientation between adjacent grains may be an effective way to improve the ductility of magnesium alloys.

The Development of a High-Strength Mg-10.3Gd-4.4Y-0.9Zn-0.7Mn Alloy Subjected to Large Differential-Thermal Extrusion and Isothermal Aging.

The large differential-thermal extrusion (LDTE) process, a novel approach for efficiently fabricating a high-strength Mg-10.3Gd-4.4Y-0.9Zn-0.7Mn (wt.%) alloy, is introduced in this work. Unlike typical isothermal extrusion processes, where the ingot and die temperatures are kept the same, LDTE involves significantly higher ingot temperatures (~120 °C) compared to the die temperature. For high-strength Mg-RE alloys, the maximum isothermal extrusion ram speed is normally limited to 1 mm/s. This research uses the LDTE process to significantly increase the ram speed to 2.0 mm/s. The LPTE-processed alloy possesses a phase composition that is similar to that of isothermal extruded alloys, including α-Mg, 14H-type long-period stacking ordered (LPSO) and β-Mg5(Gd, Y) phases. The weakly preferentially oriented α-Mg grains in the LDTE-processed alloy have <101¯0>Mg//ED fibrous and <0001>Mg//ED anomalous textures as their two main constituents. After isothermal aging, high quantitative densities of prismatic β' and basal γ' precipitates are produced, which have the beneficial effect of precipitation hardening. With a yield tensile strength of 344 MPa, an ultimate tensile strength of 488 MPa, and an elongation of 9.7%, the alloy produced by the LDTE process exhibits an exceptional strength-ductility balance, further demonstrating the potential of this method for efficiently producing high-strength Mg alloys.

Comparison of the microstructure and mechanical properties of WC-Ti(C, N)-Ni gradient cemented carbides prepared by SPS pre-sintering and vacuum pre-sintering

Two-step sintering to prepare gradient cemented carbides tends to the grain growth of hard phases, which adversely affects the mechanical properties. Spark plasma sintering is a promising pre-sintering process to mitigate grain growth by rapid prototyping and densification. In the present work, gradient cemented carbides with Ni binder instead of traditional Co binder were fabricated by both SPS pre-sintering and vacuum pre-sintering, followed by gradient sintering. The microstructure and elemental distributions of the prepared gradient cemented carbides were thoroughly investigated by XRD, SEM, EDS, EPMA and EBSD, focusing on the gradient layer formation and WC grain growth. The influence of WC grain size on the gradient layer formation was discussed with the aid of the DICTRA simulation. Finer WC grains in the SPS pre-sintered alloy show a smaller tortuosity factor, resulting in a thicker thickness of the gradient surface layer. The variations of hardness from the surface to the interior and the fracture toughness of the bulk for the gradient cemented carbides with different pre-sintering methods were measured through the micro-indentation method. Higher hardness and fracture toughness were obtained by SPS pre-sintered gradient cemented carbide.

Solidification Crack Free Welding Strategy via Ultra High Precision Laser Scanning at a Single Grain of Directionally Solidified and Single Crystal Superalloys

In this study, possibility of solidification crack-free welding of directionally solidified CM247LC and single crystal CMSX-4 superalloys was metallurgically investigated using single-mode fiber laser scanning. The key metallurgical factors of solidification cracking have been identified as the formation of highly misoriented solidification grain boundaries (low level of epitaxial growth behavior) at the fusion zone that affects the repulsive force between two adjacent dendrites and dendrite coalescence undercooling at the terminal stage of weld solidification. In particular, under extremely low heat input and ultra-high-speed laser beam scan conditions (welding speed: 1,000 mm/s, heat input: 2 J/mm, and energy density: 65 J/mm&lt;sup&gt;2&lt;/sup&gt;), an effective fusion zone could be achieved within a single directionally solidified grain of CM247LC alloy owing to the characteristics of single-mode fiber laser. The fusion zone successfully showed a 99.9% of epitaxial growth achievement ratio (that is, low fraction of highly misoriented solidification grain boundaries at the fusion zone) without solidification cracking. Based on these results on CM247LC alloy, solidification crack-free welds could be successfully achieved using quadruple-pass and square-type weld geometries.

Hot Working of an Fe-25Al-1.5Ta Alloy Produced by Laser Powder Bed Fusion

In the present work, hot working was used as a post-processing method for Fe-25Al-1.5Ta (at.%) alloy built using laser powder bed fusion (LPBF) to refine the undesirable columnar microstructure with heterogeneous grain sizes and strong textures in the build direction. The hot deformation behavior and workability were investigated using constitutive modeling and the concept of processing maps. Uniaxial compression tests were conducted up to a true strain of 0.8 at 900 °C, 1000 °C, and 1100 °C with strain rates of 0.0013 s−1, 0.01 s−1, and 0.1 s−1. The constitutive equations were derived to describe the flow stress–strain behavior in relation to the Zener–Hollomon parameter. Processing maps based on a dynamic materials model were plotted to evaluate the hot workability and to determine the optimal processing window as well as the active deformation mechanisms. The microstructure of the deformed specimens was characterized by scanning electron microscopy equipped with an electron backscatter diffraction detector. The results indicated a high degree of hot workability of the LPBF builds without flow instabilities over the entire deformation range tested. The epitaxially elongated grains of the as-built alloys were significantly refined after deformation through dynamic softening processes, and the porosity was reduced due to compressive deformation. The current study revealed a well-suited parameter range of 1000–1080 °C/0.004–0.012 s−1 for the safe and efficient deformation of the LPBF-fabricated Fe-25Al-1.5Ta alloys. The effectiveness of the process combination of LPBF with subsequent hot forming could be verified with regard to microstructure refinement and porosity reduction.

Effect of Cu Addition on Properties of an Al-La Alloy

The effect of copper content on the microstructure and properties of an aluminum copper alloy containing lanthanum was studied by adding 0.5 wt% lanthanum and different contents of copper to the 2A12 alloy. We used a universal testing machine, Brinell hardness tester, and friction and wear testing machine to test the mechanical properties of the alloy. The microstructure and phase composition of the alloy were analyzed using scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The results show that, after adding La and Cu, the alloy grain is refined, and the fracture mode changes from dissociation fracture to quasi dissociation fracture. A new phase containing La accumulates at the grain boundary. If the amount of Cu added is too large, the tendency for hot cracking increases, which can easily lead to the formation of voids or even cracks. The tensile strength and hardness of the cast alloy significantly increase with the increase in Cu addition. In the range of adding 2 wt% to 5 wt% Cu, the tensile strength and hardness of the cast alloy increases by approximately 12% and 17.84%, respectively, compared with the original alloy. With the increase in Cu addition, the elongation of the alloy first increases and then decreases, reaching its maximum value when Cu addition is 3 wt%. Adding Cu is beneficial for improving the wear resistance of the alloy. When the addition of Cu is 3 wt%, the alloy has the best wear resistance and minimum wear amount. This is due to the enrichment of La and Cu at the grain boundaries, forming new La phases or other phases with higher hardness, which changes the properties of the alloy. It shows that the content of Cu in the 2A12 alloy can be increased to 7-8% with the increasing of additional rare-earth elements, and a new Al-Cu (7 wt% to 8 wt%)-Mg (1.2 wt% to 1.8 wt%)-Mn (0.3 wt% to 0.9 wt%)-La (0.5 wt%) alloy, in which tensile strength, hardness, and elongation of the alloy are increased by 8.1%, 16%, and 79.1%, respectively, can be formed. In addition, the wear resistance of Al-La alloys also improves significantly with the addition of copper content. The coefficient of friction is reduced by 68% compared with no copper addition.

Effect of Y and Gd solutes on grain refinement of the as-extruded Mg-Gd(-Y)-Zn-Mn alloys

Mg alloys exhibit high Hall-Petch coefficients, so obtaining refined grains in wrought alloys is important for their high performance. This study explored the roles of Y and Gd solutes on grain refinement of the Mg-(1.25–2.5)Gd-(0–1.25)Y-0.5Zn-0.4Mn(at%) extruded alloys, and the mechanical properties were tested. Microstructural analyses show that incomplete dynamic recrystallization (DRX) occurred after hot extrusion of Mg-Gd(-Y)-Zn-Mn alloys and DRX behaviors varied with Y and Gd contents. The addition of Y or an increase in Y/Gd atomic ratio (by partially substituting Gd with Y) resulted in a reduction of the average DRXed grain size, while adding Gd (or Y) or decreasing the Y/Gd atomic ratio facilitated DRX nucleation. Mechanical properties demonstrate that the joint alloying of two RE elements, Y and Gd, enhanced the experimented alloys’ strength surpassing the effect of using Gd alone. The Mg-2.5Gd-0.75Y-0.5Zn-0.4Mn(at%) extruded alloy displayed the optimal mechanical characteristics having a 402 MPa ultimate tensile strength, a 345 MPa yield strength, and 5.2% moderate ductility.

Strengthening of wire arc additive manufactured aluminium alloy through interlayer rotary friction processing

To eliminate porosity and refine the grains in aluminium alloys fabricated via wire arc additive manufacturing (WAAM), a novel strengthening method – robotic rotary friction processing (RFP) – was introduced. The feasibility of RFP was assessed by processing the deposited beads under various average peak loads. During RFP, a rotating tool pressed the deposited beads to induce sufficient plastic deformation. After 3.5 kN-RFP, the WAAM densification ratio was improved to >99.5%, and pores were completely closed and eliminated owing to compression deformation and sub-grains bonding. The processed deposits exhibited effective grain refinement and high-density dislocation entanglement, which led to 22.8% and 34.2% improvements in the horizontal and vertical tensile strengths, respectively. Furthermore, the strengthening mechanism of RFP on deposited metals was clarified.

Optimizing microstructure, magnetic properties and mechanical properties of sintered NdFeB magnet by double alloy method and grain boundary diffusion

This paper presents improvements in the coercivity, mechanical strength, and corrosion resistance of NdFeB magnets by modifying their microstructure using the double alloy method and grain boundary diffusion. The coercivity of magnet increases significantly from 16.66 kOe to 23.78 kOe with 0.64 wt% Tb usage. The overall magnetic performance (OMP) (BH)max (MGOe) + Hcj (kOe) of diffusion magnet improves approximately 7.09%. Furthermore, the mechanical properties and corrosion resistance are enhanced through the diffusion process. The microstructure forms thin intergranular phases of RE-Fe-O and a double core-shell structure consisting of Dy and Tb. These modifications contributed to the improved properties of NdFeB magnets. The formation mechanism of double core-shell is analyzed in detail. This work proposes a viable method for preparing high comprehensive performance magnet with a low Dy/Tb content.

Performance evaluation of additive TiO2, MWCNT and GNP reinforced particles on Mg AZ31 based matrix composites by friction stir processing

Magnesium alloy-based matrix composites are extensively used in transportation industries because of their high specific strength and low weight. Several existing technologies were utilized for preparing these composites in any proportion of constituent elements. However, friction stir processing (FSP), which employs plastic deformation with high-energy input, has proven as the most successful with proper homogenization. This study examines the microstructure and mechanical characteristics evolved by FSP of a novel magnesium alloy grade (AZ31) with Titanium dioxide (TiO2), Multi-walled carbon nanotubes (MWCNT) and Research grade Graphene nanoplatelets (GNP) that have considerable electrical, thermal and mechanical performance. At a parametric combination, tool rotation of 1200 rpm, traverse speed of 80 mm per minute, plunge depth with 0.25 mm, AZ31 alloy was processed on a specifically engineered fixture with subzero coolant circulation through it. The average size of grains of the base alloy reduced from 52.96 µm to 2.07 µm (AZ31/TiO2), 3.19 µm (AZ31/MWCNT) and 2.54 µm (AZ31/GNP) composites at this combination. The hardness and tensile strength were enhanced from 115 to 130% and 25 to 38%, respectively. Micrographs shows that the shortened grain elongation occurred due to coolant circulation resulted in restricted strengthening precipitate dissolution, which usually occur at high temperature.

Study on the Strengthening Effect of Interlayer Micro-rolling on Additively Manufactured Al–4.7Si Alloy

Al-Si alloy is one of the most important and commonly used materials in the manufacture of lightweight components for automobiles. However, Its application is limited by low-strength properties, which are usually caused by micropores and columnar grains of the as-deposited WAAM alloy. To effectively address these issues, a special method of inter-layer micro rolling was used to fabricate the additively manufactured Al–4.7Si alloy in this paper. The strengthening mechanism of micro-rolling on this alloy was systematically studied by analyzing microstructure and properties. The experimental results showed that the number of micropores was substantially reduced, and the growth of columnar grains was significantly interrupted by inter-layer micro rolling, which had a clear effect on improving the mechanical properties of 4043 Al-Si alloy. The average ultimate tensile strength (UTS) and yield strength (YS) of the micro-rolled sample with 4 kN load achieved 159 MPa and 72 MPa, respectively, which were 18.6% and 38.5% higher than that without rolling, and elongation (EL) increased from 12.3% to 16.2%. This study is expected to provide an experimental and theoretical basis for improving the mechanical properties of additively manufactured aluminum alloys.

Exploring the trace rare earth Sm on the microstructure and mechanical performance of brazed diamonds by Ni–Cr filler alloy

In this study, the properties of Ni-based filler alloy are improved by adding rare earth Sm to them and brazing them at a temperature of 1080 °C. Relevant tests are carried out on filler alloy and brazed joints by means of electron microscopy (SEM), energy spectroscopy (EDS) and X-ray diffractometry (XRD). The results show that the addition of rare earth Sm can refine the grain of the filler alloy, improve the hardness of the filler alloy and reduce the thermal damage of the diamond after welding. The addition of rare earth Sm affects the diffusion of Cr, which reacts with the diamond at the interface to form Cr3C2 and Cr7C3, and the formation of these two carbides improves the bonding strength between the filler alloy and the diamond. When the addition of rare earth Sm is 1.5 wt%, the melting point of the filler alloy is the lowest, the grain refinement is the best, the hardness is the highest, and the thermal damage of the brazed sample is the lowest. In addition, the filler alloy of 1.5 wt%Sm has the greatest effect on the diffusion of the Cr element, the bonding strength between the filler alloy and diamond is the highest, and the mechanical properties of the brazed specimens are the best.

Co-doping of La/Ce and La/Er induced precipitation strengthening for designing high strength Al-Mg-Si electrical conductive alloys

In order to break through the trade-off between mechanical strength and electrical conductivity of all aluminum alloy conductors, two rare earth elements (REs) including La/Ce and La/Er were simultaneously introduced to modify the Al-Mg-Si conductive alloys by resistance melting. Doping La/Ce triggered the occurrence of co-precipitation and the promotion of dislocation density around co-precipitated phases, and the ultimate tensile strength reached 223 MPa which was 28.4% higher as compared to the La single-doped alloy. Adding La/Er induced the precipitation of ErFeSi phases along alloy grain boundaries which captured Si solute atoms and lacerated the continuous distribution of original β-Al3FeSi phases, and the electrical conductivity of 52.35%IACS approached the conductive level of the La single-doped alloy and significantly higher than that of the La/Ce co-doped alloy. It is anticipated that co-doping of La/Ce/Er could provide better mechanical and electrical properties.

Preparation of ultrafine-grained Nb-1Zr alloy by increasing the time of large plastic deformation during friction stir processing

The grain evolution mechanism of Nb-1Zr alloy during friction stir processing (FSP) was studied under three large plastic deformation conditions: high-temperature-short-time, high-temperature-long-time, and low-temperature-long-time. The niobium alloy grains can be continuously refined during processing. By increasing the time for large plastic deformation during friction stir processing, ultrafine-grained niobium alloys can be prepared with only a single pass of processing. To the best of the authors' knowledge, this phenomenon has been confirmed for the first time and can be used as an efficient processing strategy for grain refinement by FSP.

Microstructures, mechanical properties, and oxidation behavior of in-situ ultra-fine duplex carbides reinforced Ni-based superalloy composites

A Ni-based superalloy composite (FTSC) was successfully prepared by spark plasma sintering with Ti3SiC2 (15 vol%) and FGH97 alloy powders as precursors. During sintering, the weakly bonded Si layers de-intercalate from Ti3SiC2 and the remaining TiC is eroded by the elements of the matrix, which promoted in-situ precipitation of MC and M6C carbides. MC with core-shell structure builds up a semi-coherent interface with the alloy matrix: (1¯11¯)γ/γ'‖(11¯1)MC with [110]γ/γ'‖[110]MC. The Zener pinning effect of ultra-fine duplex carbides results in refinement of alloy grains and notably improvement in mechanical strength. Comparing to the alloy FGH97, the composite FTSC possesses 55% higher microhardness, 100% higher compressive yield strength and 92% higher flexural strength. On the other hand, the refined grains ensured fast formation of a protective alumina scale at surface, which provides high oxidation resistance for the composite.

Effects of hydrogen on the dynamic mechanical properties and microstructure of 7055 and 7A52 aluminum alloys

In the present work, the effect of hydrogen on the impact resistance of the 7055 and 7A52 aluminum alloys have been experimentally investigated by using split Hopkinson pressure bar (SHPB) and electrochemical hydrogen charging technique. It was first found that hydrogen induced stress chase after yield deformation in 7055 and 7A52 alloys. Meanwhile, hydrogen reduced the yield strengths of the two alloys. Besides, the evolution of microstructures including cracks, particles and dislocations has been quantitatively analyzed by polarizing microscope (PM), scanning electron microscope (SEM) and transmission electron microscopy (TEM). It has been found that both two aluminum alloys are sensitive to hydrogen embrittlement under high-speed deformation conditions. The hydrogen causes hydrogen compression cracks at grain boundaries, pores, and Al2Cu particles in the material, and the hydrogen cracking of both aluminum alloys after impact appeared to be layered, and the lower layer cracking was flat, which indicated that the hydrogen made the alloy embrittle to some extent. Besides, hydrogen reduces the hindering effect of copper-containing phase and large η phase on dislocation movement, which makes the dislocations distribution inside the 7055 alloy grains more uniform. The results show that 7000-series aluminum alloys are sensitive to hydrogen embrittlement under high-speed deformation conditions.

High-temperature corrosion of weld overlay coating/bulk FeCrAl exposed in O2 + H2O + KCl(s) at 600 °C – A microstructural investigation

This work investigates the impact of high-temperature corrosion behavior of the newly developed FeCrAl alloy Kanthal® EF101 bulk material and weld overlay coating in the presence of KCl(g)/KCl(s) at 600 °C. The oxide scale formed within the secondary corrosion regime after exposure and the impact of alloy microstructure on corrosion behavior was investigated using scanning transmission electron microscopy. The findings indicated the key microstructural differences is the alloy grain size which influences the formation of a protective scale. In addition, It is indicated that coating exhibited inferior performance than the bulk material, primarily attributed to the microstructural differences.