Microstructure and mechanical properties of a high nitrogen titanium alloy

High nitrogen titanium alloy with the chemical composition of Ti–4%Cr–0.6%N was produced by solution nitriding to nitrogen-free Ti–4%Cr alloy, and then its microstructure was controlled to have fine (α + β) dual phase structure by aging treatment to improve the ductility. As solution-nitrided specimen has a platelet hcp-martensitic structure (α′) and is characterized by hard but brittle nature that has been produced by solid solution of 0.6% of nitrogen. On the following aging treatment, fine β phase formed along the original plate boundaries, resulting in a fine (α + β) dual phase microstructure. X-ray and EELS analyses revealed that nitrogen is greatly concentrated in the tempered α′ phase. Although the hardness of as-quenched material gradually decreases during the aging treatment with increasing volume fraction of β, the hardness can be kept much higher than that of the aged Ti–4%Cr alloy without nitrogen. As a result of tensile testing, it was found that the aged Ti–4%Cr–0.6%N alloy has high tensile strength over 1 GPa with moderate ductility.     

X-ray microdiffraction and EBSD study of FSP induced structural/phase transitions in a Ni-based superalloy

Severe plastic deformation during Friction Stir Processing (FSP) of an IN738 Ni-based superalloy was studied by means of X-ray polychromatic microdiffraction, EBSD, scanning electron and optical microscopies. Modeling of the physical properties and phase composition was also performed. Several distinct zones are formed during FSP including a stir zone (SZ), a thermal-mechanical affected zone (TMAZ) and a heat affected zone (HAZ). Each zone has distinct microstructure after FSP. The initial dendrite structure is preserved in the HAZ, while strengthening γ′-phase particles partially dissolve and coagulate. Plastic deformation of the base material dendrites takes place in the TMAZ and a large number of geometrically necessary dislocations are formed. The extent of deformation increases toward the SZ and the dendrite structure is completely destroyed in the SZ and replaced by a fine submicrocrystallinne microstructure.     

Microstructural refinement of a cast hypereutectic Al–30Si alloy by friction stir processing

Hypereutectic Al-30 wt.% Si alloy was subjected to friction stir processing (FSP) to modify the cast microstructure. FSP reduces the size of undesirable coarse silicon particles, eliminates porosities, and homogenizes and refines the cast microstructure. This paper demonstrates the effect of two pass overlap friction stir processing on microstructural refinement of Al–30Si alloy, which delineates significant reduction in size and aspect ratio of silicon particles from average 200 to 2 µm and 4.93 to 1.75 µm respectively. The stir zone of two pass overlap FSP exhibits relatively homogeneous Si particle distribution. Increase in frequency of silicon particles less than 1 µm was also observed in two pass FSP stir zones. Hardness in stir zones was measured to be 75 Hv after first pass and the same changed to 85 Hv respectively after second pass. Further uniform microhardness was observed in the FSP stir zone which was not the case in as-cast Al–30Si microstructure.     

Phase separation in PM 2000™ Fe-base ODS alloy: Experimental study at the atomic level

The coarsening of the three-dimensional microstructure resulting from phase separation during ageing at 748 K of a Fe-based PM 2000™ oxide dispersion strengthened (ODS) steel has been investigated by atom probe tomography and hardness measurements. Phase separation resulted in the formation of isolated particles of the chromium-enriched α′ phase. The aluminum and titanium were found to preferential partition to the iron-rich α phase. The partitioning of aluminum is consistent with theoretical calculations. The change in the scale of the chromium-enriched α′ phase was found to fit a power law with a time exponent of 0.32 in accordance with that predicted by the classical Lifshitz, Slyozov and Wagner (LSW) theory. The solute concentrations of the coexisting α and α′ phases were estimated from concentration frequency distributions with the Langer–Bar-on–Miller (LBM) method and proximity histograms. The hardness was linearly related to the chromium content of the α′ phase.     

Structure and vibration characteristics of Ti-Al-Mo-V alloy

Ti-5Al-2.5Mo-1.4V rolled plates were subjected to solid-solution heat treatment at 870 or 930^∘C for 1 h and aged for 1–8 h at 460^∘C to investigate the relationship between the microstructure and the vibration characteristics of the alloy. According to the experimental results, the S870 solid solution matrix contains α + α′ + β structures and the S930 solid solution specimen possesses α +α′ structures (β transus is ∼ 900^∘C). Increasing the α′ phase content improves strength and hardness but reduces ductility. It also promotes internal friction and thus increases damping. During the 460^∘C aging process, the α′ phase in the β region of S870/Ah grows and transforms into a finer needle structure and the primary α phase (bounded by prior β grain boundary) of S930/Ah grows within the α′ matrix. Both S830/Ah and S930/Ah reveal similar tendencies in mechanical properties with increased aging time. When the aging time exceeds 1 h, S870/Ah, having a large quantity of β phase, has a better vibration damping ratio than S930/Ah (containing a great quantity of primary α phase).     

On the role of process variables in the friction stir processing of cast aluminum A319 alloy

The surfaces of cast A319 alloy plates of nominal composition (wt.%): Al – 5.2 Si – 2.51 Cu were subjected to single stir Friction Stir Processing (FSP) with a view to decreasing the grain size and porosity level and improving the mechanical properties. Three traverse feed rates and five tool rotational speeds were employed. For certain combinations of the variables, the processed alloy displayed an increase in value of around 50% in tensile strength and 20% in microhardness compared with those of the as-cast alloy. The ductility of the processed alloy had increased by a factor which ranged from 1.5 to 5.Optical and scanning electron microscopy revealed that FSP reduces the size of the second phase particles, which contributes to the improvements in mechanical properties.     

Effect of Low Feed Rate FSP on Microstructure and Mechanical Properties of Extruded Cast 2285 Aluminum Alloy

Friction stir processing （FSP）, a variation of FSW （friction stir welding） is an emerging surface engineering technology that can locally eliminate casting defects and refine microstructures, thereby improving the mechanical properties of material. FSP can also produce fine grained microstructures through the thickness to impart superplasticity. The technology involves plunging a rapidly rotating, non consumable tool, comprising a profiled pin and larger diameter shoulder, into the surface and then traversing the tool across the surface. The pin and the shoulder friction heat the surface which alters the grain structure in the processed area thereby improving the mechanical properties. This paper presents the effects of FSP on microstructure and mechanical properties of extruded cast 2285 aluminum alloy at three different feed rates viz. 10, 12 and 15 mm/min. With the increase in the feed speed the material was observed to have increased impact strength. FSP also increased the tensile and yield strengths with increases in hardness and ductility values also. The observation has been listed in detail and pictorially represented.     

Friction stir processing: An effective technique to refine grain structure and enhance ductility

Commercial 5083 Al rolled plates were subjected to friction stir processing (FSP) with a tool rotational speed of 430 rpm and a traverse feed rate of 90 mm/min. This treatment resulted in a fine grained microstructure of 1.6 μm and an average misorientation angle of 24°. Ductility was measured using tensile elongations at a temperature of 250 °C at three strain rates, and demonstrated that a decrease in grain size resulted in significantly enhanced ductility and lower forming loads. The ductility of the friction stir processed material was enhanced by a factor ranging from 2.6 to 5 compared to the ductility of the as received material, in the range of the strain rates tested. The strain rate sensitivity of the processed material is 0.33 while for the as received, it is 0.018. The deformation mechanism, in the fine-grained specimens is mainly controlled by solute drag creep, though the contribution of grain boundary sliding to the deformation process cannot be overlooked. Both mechanisms led to significant flow localization and simultaneous cavity formation.     

Effect of friction stir processed microstructure on tensile properties of an Al-Zn-Mg-Sc alloy upon subsequent aging heat treatment

An as-cast Al-Zn-Mg-Sc alloy was friction stir processed varying tool related parameters, yielding microstructures with different grain sizes (0.68, 1.8 and 5.5 μm). Significant increases in room temperature ductility were obtained in these materials with reasonable enhancement in strength. It is demonstrated that the type of microstructure produced by friction stir processing (FSP) has a significant influence on the choice of post-FSP heat treatment design for achieving improved tensile properties. It is also found that the ultrafine grained FSP material could not achieve the desired high strength during the post-FSP heat treatment without grain coarsening, whereas the micro-grained FSP materials could reach such strength levels (>560 MPa) under conventional age hardening heat treatment conditions.     

Friction stir processing of a D2 tool steel layer fabricated by laser cladding

Friction stir processing of a D2 tool steel layer fabricated by laser cladding was carried out to modify its as-cast microstructure. The microstructural evolution in the stir zone was observed and micro-Vickers hardness was measured. A maximum hardness of 857 HV was obtained by friction stir processing at 200 rpm. This value is higher than that of the clad layer when it is subjected to austenitizing heat treatment without friction stir processing. This superior property is attributed to the decrease of volume fraction of retained austenite after friction stir processing. The friction stir processing condition with low heat input led to high volume fractions of fine M₇C₃ carbide particles and martensite.     

Effect of friction stir processing on erosion–corrosion behavior of nickel–aluminum bronze

In the present investigation, effects of Friction Stir Processing (FSP) on Erosion–Corrosion (E–C) behavior of Nickel–Aluminum Bronze (NAB) were studied by weight-loss measurements and surface characterization using an impingement jet test system. After FSP, the initial coarse microstructure of the cast NAB was transformed to a fine structure, and the porosity defects were eliminated. In addition, different FSP structures were produced by each rotation rate. Microhardness measurements showed a marked increase in FSP samples depending upon the FSP parameters. E–C tests were carried out by erodent at kinetic energies about 0.45 μJ and in 30°, 60° and 90° impact angles to simulate actual service conditions. The maximum weight-loss was observed in FSP samples and Scanning Electron Microscopy (SEM) results showed signs of brittle fracture mechanism in FSP samples. By gravimetric analysis, the degree of synergy was evaluated at 0.45 μJ kinetic energy at normal impact angle and negative synergy result implies the presence of a protective film on all sample surfaces.     

Microstructural evolution and mechanical properties of ultrafine grained Al3Ti/Al–5.5Cu composites produced via hot pressing and subsequent friction stir processing

In situ Al₃Ti/Al–5.5Cu composites fabricated by powder metallurgy and subsequent forging were subjected to multiple pass friction stir processing (FSP) with and without active cooling. The forged sample exhibited lower strength and ductility due to the presence of coarse Al₃Ti clusters with a size range of 50–100 μm and coarse matrix grains. Four-pass FSP in air resulted in the refinement and redistribution of the Al₃Ti clusters, and the generation of micron matrix grains, thereby increasing the strength and ductility of the composites. Furthermore, coarse Al₂Cu particles dissolved and re-precipitated due to a relatively long duration of thermal exposure. Additional two pass FSP with rapid water cooling (FSP-water) dissolved most of the Al₂Cu into the matrix and retained the solutes in solution due to the short duration of thermal exposure. Meanwhile, ultrafine matrix grains with a high density of dislocations were obtained. These microstructural changes led to significant increase in strength and a decrease in ductility in the FSP-water sample. After aging, the FSP-water sample exhibited further increased yield strength and ultimate tensile strength due to the precipitation of metastable Al₂Cu phases. However, the ductility did not decrease due to the decrease of dislocation density after aging.     

Barium titanate reinforced nickel aluminium bronze surface composite by friction stir processing

Friction stir processing (FSP) is a surface modification technique, which can be used for the fabrication of surface composites. In the present work, a surface composite was prepared by introducing a piezoelectric ceramic powder (PZT; BaTiO₃) to a nickel aluminium bronze (NAB) metal matrix using FSP. BaTiO₃ powder was placed in holes drilled at the centre of a NAB plate and FSP was carried out. Microstructural characterisation of the surface composite was carried out using optical microscopy and scanning electron microscopy. The microhardness and tensile behaviour of the surface composite were investigated, together with the cavitation erosion behaviour. The results are discussed in light of the microstructural modification.     

Microstructural and mechanical properties of friction stir welded Cu–30Zn brass alloy at various feed speeds: Influence of stir bands

In this study, the effect of various feed speeds on microstructure and mechanical properties of friction stir welded Cu–30Zn brass alloy is investigated. Rotation speed was fixed at 950 rpm and feed speed varied in the range of 190–375 mm/min. Examination of the microstructure showed very fine grains with some deformed grains in the stirred zone and some coarser grains in the thermo-mechanically affected zone and base metal. A unique deformation pattern, namely “stir band” in the stirred zone region was identified and its density increased by increase in feed speed. Results showed that the grain size profile was independent of feed speed and the hardness values decreased by increase in feed speed. Increase in feed speed led to a slight improvement of yield strength and ultimate tensile strength, associated to continuous spring-like morphology of stir bands acting as a strengthening structure. However, ductility reduces considerably from 57 to 27%. Moreover, it is observed that during tensile test, fracture cracks originate exactly adjacent to the stir bands.     

搅拌摩擦加工法制备碳纳米管增强铝基复合材料

为了制备晶粒细小、 组织均匀的复合材料, 提高材料的力学性能, 用搅拌摩擦加工法制备碳纳米管增强铝基复合材料, 并对不同碳纳米管含量的复合材料的微观结构、 拉伸性能及断口形貌进行分析。结果表明: 碳纳米管添加到铝基体中, 搅拌摩擦中心区晶粒细小, 碳纳米管与基体之间结合良好, 未发现明显的缺陷; 碳纳米管对基材有明显的强化作用, 铝基复合材料抗拉强度随着碳纳米管含量的增加而提高; 碳纳米管体积分数为7%时, 抗拉强度达到201 MPa, 是基材的2.2倍; 复合材料在宏观上呈现脆性断裂特征, 微观上呈现韧性断裂特征, 其断裂机制以CNTs/Al界面脱粘、 基体撕裂和增强体断裂为主。      

Investigating effects of process parameters on microstructural and mechanical properties of Al5052/SiC metal matrix composite fabricated via friction stir processing

Friction stir processing (FSP) is a novel process for refinement of microstructure, improvement of material’s mechanical properties and production of surface layer composites. In this investigation via friction stir processing, metal matrix composite (MMC) was fabricated on surface of 5052 aluminum sheets by means of 5 μm and 50 nm SiC particles. Influence of tool rotational speed, traverse speed, number of FSP passes, shift of rotational direction between passes and particle size was studied on distribution of SiC particles in metal matrix, microstructure, microhardness and wear properties of specimens. Optimum of tool rotational and traverse speed for achieving desired powder dispersion in MMC was found. Results show that change of tool rotational direction between FSP passes, increase in number of passes and decrease of SiC particles size enhance hardness and wear properties.     

Microstructure and mechanical property of laser melting deposition (LMD) Ti/TiAl structural gradient material

This article presents fabrication, microstructure and mechanical properties study of Ti/TiAl functional gradient material. Ti–47Al–2.5V–Cr/Ti–6Al–2Zr–Mo–V gradient material was successfully fabricated by the laser melting deposition (LMD) manufacturing process. Microstructure and chemical composition was characterized by OM, SEM, TEM and EPMA. The Vickers hardness and room-temperature tensile property was evaluated on longitudinal direction. Results showed that fully lamellar (FL) microstructure consisted of γ-TiAl and α₂-Ti₃Al was formed on the Ti–47Al–2.5V–Cr side, while coarse basket weave microstructure was formed on the Ti–6Al–2Zr–1Mo–1V side. No cracking was found in the gradient zone after aging at 800 °C for 48 h. The room-temperature tensile strength of the as-deposited specimen is up to approximately 1198.8 MPa in the longitudinal direction, while the tensile elongation is approximately 0.4%, indicating a typical brittle fracture.     

Microstructure–properties relationship of TLP-bonded GTD-111 nickel-base superalloy

Relationship between microstructure and mechanical properties of transient liquid phase-bonded joints of GTD-111 nickel-base superalloy using a Ni–Si–B interlayer was investigated. Shear strength and hardness profile of the joints were discussed with respect to the bond microstructure. In the bonding condition, in which isothermal solidification has not been accomplished completely, eutectic constituent which has the highest hardness in the bond region is the preferential failure source. Additionally, it was found that when isothermal solidification is completed, the extent of γ′ formation in the bond region is the controlling factor for joint strength. Shear strength of bonds after post-bond heat treatment was close to that of the base metal.     

Effect of grain refinement and crystallographic texture produced by friction stir processing on the biodegradation behavior of a Mg-Nd-Zn alloy

The application of a single pass of friction stir processing(FSP) to Mg-Nd-Zn alloy resulted in grain refinement, texture evolution and redistribution of second phases, which improved corrosion resistance.In this work, an as-rolled Mg-Nd-Zn alloy was subjected to FSP. The microstructure in the processed zone of the FS-400 rpm alloy exhibited refined grains, a more homogenous grain size distribution, less second phases, and stronger basal plane texture. The corrosion behavior assessed using immersion tests and electrochemical tests in Hank's solution indicated that the FS-400 rpm alloy had a lower corrosion rate, which was attributed to the increase of basal plane intensity and grain refinement. The hardness was lowered slightly and the elongation was increased, which might be attributed to the redistribution of the crushed second phases.     

AZ31镁合金薄板的无针搅拌摩擦加工

目的 研究有针、无针搅拌摩擦加工对AZ31镁合金薄板的微观组织和力学性能的影响。方法 通过搅拌摩擦加工技术（FSP）以不同的转速对AZ31镁合金薄板进行加工,采用拉伸试验机、金相显微镜、UMT摩擦磨损试验机、维氏硬度机对无针搅拌加工后的AZ31镁合金加工表面的晶粒形貌、拉伸性能、磨损性能和硬度进行研究分析,并与同转速有针搅拌进行比较。结果 无针条件下与同转速下的有针搅拌相比,焊缝表面更加细密、美观,无针加工下焊缝的抗拉强度最大为242MPa,测得的维氏硬度最大为97.6HV,且焊缝的平均摩擦因数最小,为0.31。结论 无针时FSP镁合金焊缝的抗拉强度随刀具转速的提高而增大;焊缝的硬度与镁合金母材相比有明显的提高,且随刀具转速的提高,维氏硬度值逐渐降低;无针条件下获得的AZ31镁合金焊缝的平均摩擦因数随刀具转速的增加而增大,与同转速时有针条件下获得的焊缝的平均摩擦因数相比,无针时获得的焊缝平均摩擦因数明显更高;无针时获得的焊缝表面的晶粒尺寸随转速的增加而增大。