Correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

The correlation of microstructure with the hardness and wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation was investigated in this study. The mixtures of TiC, SiC, or TiC + SiC powders and CaF₂ flux were placed on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures using an electron-beam accelerator. The surface composite layers of 1.2 to 2.1 mm in thickness were formed without defects and contained a large amount (up to 66 vol pct) of precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates and a hardened matrix in the surface composite layer, improved the hardness and wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate, because 66 vol pct of TiC and Ti₅Si₃ was precipitated homogeneously in the hardened martensitic matrix. These findings suggested that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved hardness and wear properties.     

Correlation of microstructure with the wear resistance and fracture toughness of hardfacing alloys reinforced with complex carbides

A correlation was made of the microstructure, wear resistance, and fracture toughness of hardfacing alloys reinforced with complex carbides. The hardfacing alloys were deposited twice on a low-carbon steel substrate by a submerged arc welding (SAW) method. In order to investigate the effect of complex carbides, different fractions of complex carbide powders included inside hardfacing electrodes were employed. Microstructural analysis of the hardfaced layer showed that cuboidal carbides, in which a TiC carbide core was encircled by a WC carbide, and rod-type carbides, in which W and Ti were mixed, were homogeneously distributed in the bainitic matrix. In the surface layer hardfaced with FeWTiC powders, more complex carbides were formed, because of the efficient melting and solidification during hardfacing, than in the case of hardfacing with WTiC powders. As the volume fraction of complex carbides, particularly that of cuboidal carbides, increased, the hardness and wear resistance increased. In-situ observation of the fracture process showed that microcracks were initiated at complex carbides and that shear bands were formed between them, leading to ductile fracture. The hardness, wear resistance, and fracture toughness of the hardfacing alloys reinforced with complex carbides were improved in comparison with high-chromium white-iron hardfacing alloys, because of the homogeneous distribution of hard and fine complex carbides in the bainitic matrix.     

Correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation

This study is concerned with the correlation of microstructure and abrasive and sliding wear resistance of (TiC,SiC)/Ti-6Al-4V surface composites fabricated by high-energy electron-beam irradiation. The mixtures of TiC, SiC, Ti + SiC, or TiC+SiC powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then an electron beam was irradiated on these mixtures. The surface composite layers of 1.2 to 2.1 mm in thickness were homogeneously formed without defects and contained a large amount (30 to 66 vol pct) of hard precipitates such as TiC and Ti₅Si₃ in the martensitic matrix. This microstructural modification, including the formation of hard precipitates in the surface composite layer, improved the hardness and abrasive wear resistance. Particularly in the surface composite fabricated with TiC + SiC powders, the abrasive wear resistance was greatly enhanced to a level 25 times higher than that of the Ti alloy substrate because of the precipitation of 66 vol pct of TiC and Ti₅Si₃ in the hardened martensitic matrix. During the sliding wear process, hard and coarse TiC and Ti₅Si₃ precipitates fell off from the matrix, and their wear debris worked as abrasive particles, thereby reducing the sliding wear resistance. On the other hand, needle-shaped Ti₅Si₃ particles, which did not play a significant role in enhancing abrasive wear resistance, lowered the friction coefficient and, accordingly, decelerated the sliding wear, because they played more of the role of solid lubricants than as abrasive particles after they fell off from the matrix. These findings indicated that high-energy electron-beam irradiation was useful for the development of Ti-based surface composites with improved abrasive and sliding wear resistance, although the abrasive and sliding-wear data should be interpreted by different wear mechanisms.     

Correlation of Microstructure and Ballistic Performance of Multilayered Zr-based Amorphous Surface Composites Fabricated by High-Energy Electron-Beam Irradiation

In this study, multilayered, Zr-based amorphous surface composites were fabricated by high-energy electron-beam irradiation; the correlation of their microstructure, hardness, compressive properties, and fracture properties with ballistic performance was investigated. The mixture of Zr-based amorphous powders and LiF + MgF₂ flux powders was deposited on a pure Ti substrate or a plain carbon steel substrate, and then an electron beam was irradiated on this powder mixture to fabricate a one-layered surface composite. The multilayered surface composite was fabricated by an irradiating electron beam several times again onto the powder mixture deposited on the one-layered surface composite. The microstructural analysis results indicated that a small amount of fine crystalline particles was distributed homogeneously in the surface composite layer. Because the surface composite layers absorbed the ballistic impact energy by forming many cracks or microcracks, the surface composite plates were not perforated during the ballistic impact test. On the one hand, in the surface composite without containing ductile β phases, the composite layer was cracked completely and fallen off from the substrate. On the other hand, a small amount of fragmentation was found in the impacted area of the composite containing β phases because it had the sufficient hardness and fracture toughness simultaneously for effectively blocking the traveling of a projectile, thereby improving ballistic performance.     

Improvement of the hardness and wear resistance of (TiC, TiN)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

This study is concerned with the microstructural analysis and improvement of the hardness and wear resistance of Ti-6Al-4V surface-alloyed materials fabricated by a high-energy electron beam. The mixtures of TiC, TiN, or TiC + TiN powders and CaF₂ flux were deposited on a Ti-6Al-4V substrate, and then the electron beam was irradiated on these mixtures. In the specimens processed with a flux addition, the surface-alloyed layers of 1 mm in thickness were homogeneously formed without defects and contained a large amount (over 30 vol pct) of precipitates such as TiC, TiN, (Ti _x Al_1−x )N, and Ti(C _x N_1−x ) in the martensitic or N-rich acicular α-Ti matrix. This microstructural modification, including the formation of hard precipitates and hardened matrices in the surface-alloyed layers, improved the hardness and wear resistance. Particularly in the surface-alloyed material fabricated by the deposition of TiN powders, the wear resistance was greatly enhanced to a level 10 times higher than that of the Ti alloy substrate. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-based surface-alloyed materials with improved hardness and wear resistance.     

Correlation of microstructure with hardness and wear resistance of surface composites fabricated by high-energy electron beam irradiation of Fe-based metamorphic powders

In this study, surface composites were fabricated with Fe-based metamoprhic powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and wear resistance was investigated. Fe-based metamorphic powders were deposited on a plain carbon steel substrate, and then electron beam was irradiated on these powders without flux to fabricate a one-layered surface composite. A two-layered surface composite was also fabricated by irradiating electron beam again onto the powders deposited on the one-layered surface composite. The composite layers of 1.3∼1.9 mm in thickness were homogeneously formed without defects and contained a large amount (up to 48 vol pct) of hard and fine Cr₂B crystalline phases in the Cr_0.19Fe_0.7Ni_0.11 matrix. Since the hardness and wear resistance of the surface composite layers were directly influenced by hard Cr₂B phases, they were two to three times greater than those of the steel substrate. In particular, the two-layered surface composite showed a high hardness of ∼300 VHN even at 750 °C, as well as at room temperature, because Cr₂B phases and the Cr_0.19Fe_0.7Ni_0.11 matrix were hard and thermally stable.     

Microstructural Modification and property improvement of boride/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

The present study is concerned with the fabrication and microstructural analysis of boride/Ti-6Al-4V surface-alloyed materials using the irradiation of a high-energy electron beam. Mixtures of TiB₂ or MoB powders and CaF₂ flux were placed on a Ti-6Al-4V alloy substrate and subsequently irradiated using a high-energy electron beam. Specimens processed with a flux mixing ratio of 40 wt pct showed that the melted region of 1.1 to 1.5 mm in thickness was homogeneously formed without defects and contained a large amount of titanium borides (TiB). The formation of TiB in the melted region greatly improved the Vickers hardness, high-temperature Vickers hardness, and wear resistance to levels 2 or 3 three times higher than the those for the Ti alloy substrate. Also, the addition of MoB powders into the mixtures made possible the fabrication of surface-alloyed materials with various properties by controlling the kind, size, and volume fraction of TiB and the characteristics of the matrix. These findings suggested that surface alloying using high-energy electron-beam irradiation was economical and useful for the development of boride/Ti-6Al-4V surface-alloyed materials with improved properties.     

Improvement of hardness and wear resistance of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation

The objective of this study is to investigate microstructure, hardness, and wear properties of three kinds of (TiC,TiB)/Ti-6Al-4V surface-alloyed materials fabricated by high-energy electron-beam irradiation. The mixtures of Ti+C, TiC+TiB₂, and Ti+B₄C powders and CaF₂ flux were deposited on a Ti-6A1-4V substrate, and then high-energy electron beam was irradiated on these mixtures. The surface-alloyed layers of 0.9 to 1.6 mm in thickness were homogeneously formed, and contained a large amount (30 to 44 vol. pct) of hard precipitates such as TiC and TiB in the martensitic matrix. This microstructural modification improved the hardness and wear resistance of the surface-alloyed layer 2 times and 6 to 9 times, respectively, greater than that of the substrate. Particularly, the surface-alloyed material fabricated with Ti+B₄C powders had a larger volume fraction of TiB and TiC homogeneously distributed in the martensitic matrix, and thus showed the best hardness and wear resistance. These findings suggested that the surface-alloying using high-energy electron-beam irradiation was economical and useful for the development of titanium-base surface-alloyed materials with improved hardness and wear properties.     

Effects of heat treatment on wear resistance and fracture toughness of duo-cast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate effects of heat treatment on wear resistance and fracture toughness in duo-cast materials composed of a high-chromium white cast iron and a low-chromium steel as a wear-resistant part and a ductile part, respectively. Different size, volume fraction, and distribution of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium, and the volume fraction of martensite in the austenitic matrix was varied by the heat treatment. In the alloys containing a small amount of chromium, an interdendritic structure of eutectic M₇C₃ carbides was formed, and led to the improvement of wear resistance and fracture toughness. After the heat treatment, the selective wear of the matrix and the cracking or spalled-off carbides were considerably reduced since the hardness difference between carbides and matrix decreased by the increase in the matrix hardness, thereby leading to the improvement of the wear resistance. However, the fracture toughness of the heat-treated alloys was lower than that of the as-cast alloys because the matrix containing a considerable amount of martensite did not effectively prevent the crack propagation.     

稀土元素的添加对原位生成的Ti-TiC-TiB复合材料抗磨损性能的影响

采用Ti 6Al 4V 5B4C和Ti 6Al 4V 5B4C 1Nd 两种成分的原始粉末, 反应热压后原位生成了Ti TiC TiB复合材料。经过X射线检测, 证明了试验中原位生成反应5Ti+B4C 4TiB+TiC的进行。采用摩擦磨损试验机检测了两种材料的抗磨损性能。通过扫描电子显微镜和电子探针分析了材料的磨损表面。结果表明, 添加稀土元素能提高材料的硬度, 韧性和抗磨损性能。     

Correlation of microstructure with the wear resistance and fracture toughness of duocast materials composed of high-chromium white cast iron and low-chromium steel

The objective of this study is to investigate the correlation of microstructure with wear resistance and fracture toughness in duocast materials that consisted of a high-chromium white cast iron and a low-chromium steel as the wear-resistant and ductile parts, respectively. Different shapes, sizes, volume fractions, and distributions of M₇C₃ carbides were employed in the wear-resistant part by changing the amount of chromium and molybdenum. In the alloys containing a large amount of chromium, a number of large hexagonal-shaped primary carbides and fine eutectic carbides were formed. These large primary carbides were so hard and brittle that they easily fractured or fell off from the matrix, thereby deteriorating the wear resistance and fracture toughness. In the alloys containing a smaller amount of chromium, however, a network structure of eutectic carbides having a lower hardness than the primary carbides was developed well along solidification cell boundaries and led to the improvement of both wear resistance and toughness. The addition of molybdenum also helped enhance the wear resistance by forming additional M₂C carbides without losing the fracture toughness. Under the duocasting conditions used in the present study, the appropriate compositions for wear resistance and fracture toughness were 17 to 18 pct chromium and 2 to 3 pct molybdenum.     

Effect of cerium on abrasive wear behaviour of hardfacing alloy

Hardfacing alloys with different amounts of ceria were prepared by self-shielded flux cored arc welding.The abrasion tests were carried out using the dry sand-rubber wheel machine according to JB/T 7705-1995 standard.The hardness of hardfacing deposits was meas-ured by means of HR-150AL Rockwell hardness test and the fracture toughness was measured by the indentation method.Microstructure characterization and surface analysis were made using optical microscopy,scanning electron microscopy(SEM) and energy spectrum analy-sis.The results showed that the wear resistance was determined by the size and distribution of the carbides,as well as by the matrix micro-structure.The main wear mechanisms observed at the surfaces included micro-cutting and micro-ploughing of the matrix.The addition of ceria improved the hardness and fracture toughness of hardfacing deposits,which would increase the resistance to plastic deformation and scratch,thus the wear resistance of hardfacing alloys was improved.     

通过等离子渗碳/CrN覆层改性处理提高Ti-6Al-4V合金的耐磨性和抗疲劳特性

In this study, a newly developed duplex coating method incorporating plasma carhurization and CrN coating was applied to Ti-6AI-4V and its effects on the wear resistance and fatigue life were investigated. The carburized layer with approximately150μm in depth and CrN coating film with 7. 5 μm in thickness were fomled after duplex coating. Hard carbide particles such as TiC And V4C3 were formed in the carburized layer. XRD diffraction pattern analysis revealed that CrN film had predominant [111] and [200] textures. The hardness (Hv) was significantly improved up to ahout 1960 after duplex coating while the hardness value of original Ti-6Al-4V was 402. The threshold load for the modification and/or failure of CrN coating was measured to be 32 N using the acoustic emission technique. The wear resistance and fatigue life of duplex coated Ti-6Al-4V improved significantly compared to those of un-treated specimen. The enhanced wear resistance can be attributed to the excellent adhesion and improved hardness of CrN coating film for the duplex coated Ti-6Al-4V. The initiation of fatigue cracks is likely to be retarded by the presence of hard and strong layers on the surface, resulting in the enhanced fatigue life.     

Relationship between fracture toughness and crack extension resistance curves (R curves) for Ti-6Al-4V alloys

The effects of microstructure, impurity content, and testing temperature on the fracture toughness (as measured by the crack tip opening displacement (CTOD)) and microcrack extension resistance curves (R curves) of Ti-6Al-4V alloys were examined. At 0 °C, microstructure is the most influential factor in the toughness-strength relationship. Acicular microstructure specimens have a higher CTOD than specimens with equiaxed microstructures, regardless of strength (0.2 pct proof stress) and impurity content. At −196 °C, impurity content becomes a controlling factor in the toughness-strength relationship. Extra-low impurity (ELI) specimens, which have a lower impurity content, show a higher CTOD, irrespective of microstructure. Microcracks extended from the notch tip before the maximum load was reached during testing were investigated, and crack initiation (δ _i) and extension-resistance properties were evaluated by obtaining exact R curves of the microcracks. At 0 °C, specimens with different microstructures and different impurity contents have almost the same δ _i. But acicular-microstructure specimens with a higher CTOD at a given strength show a greater crack extension resistance. At −196 °C, ELI specimens, which have a higher CTOD, show a larger crack extension resistance. It is concluded that the crack extension-resistance property of the microcracks extended from the notch tip before the maximum load is a controlling factor for the fracture toughness of Ti-6Al-4V alloys.     

High-temperature mechanical behavior of Ti-6Al-4V alloy and TiC p /Ti-6Al-4V composite

Mechanical behaviors at 538 °C, including tensile and creep properties, were investigated for both the Ti-6Al-4V alloy and the Ti-6Al-4V composite reinforced with 10 wt pct TiC particulates fabricated by cold and hot isostatic pressing (CHIP). It was shown that the yield strength (YS) and ultimate tensile strength (UTS) of the composite were greater than those of the matrix alloy at the strain rates ranging from approximately 10⁻⁵ to 10⁻³ s⁻¹. However, the elongation of the composite material was substantially lower than that of the matrix alloy. The creep resistance of the composite was superior to that of the matrix alloy. The data of minimum creep strain rate vs applied stress for the composite can be fit to a power-law equation, and the stress exponent values of 5 and 8 were obtained for applied stress ranges of 103 to 232 MPa and 232 to 379 MPa, respectively. The damage mechanisms were different for the matrix alloy and the composite, as demonstrated by the scanning electron microscopy (SEM) observation of fracture surfaces and the optical microscopy examination of the regions adjacent to the fracture surface. The tensile-tested matrix alloy showed dimpled fracture, while the creep-tested matrix alloy exhibited preferentially interlath and intercolony cracking. The failure of the tensile-tested and creep-tested composite material was controlled by the cleavage failure of the particulates, which was followed by the ductile fracture of the matrix.     

Effect of hydrogen on fracture and inert-environment sustained load cracking resistance of α- β titanium alloys

The fracture toughness and resistance to inert-environment sustained load crack propagation of α-β titanium alloys are usually reduced by increased hydrogen contents. The range of hydrogen contents over which either fracture toughness or threshold stress intensity for sustained load cracking was observed to decrease with hydrogen content is small (0 to 50 ppm) for Ti-6 Al-4 V, but further increases in hydrogen content can cause an increase in cracking rates. Sustained load crack propagation is characterized by a mixture of microvoid coalescence with cleavage, usually on a plane 12 to 15 deg from {0001} of the hep α phase with some {000l} cleavage. Cleavage apparently initiates ahead of the main crack front within a grains, usually near apparent α-β interfaces. Atmospheric moisture is inert with respect to sustained load cracking, that is, it does not cause stress corrosion cracking. Sustained load cracking was demonstrated in Ti-8 Al-1 Mo-1 V, Ti-6 Al-6 V-2 Sn, and several grades of Ti-6 Al-4 V.     

Correlation of Microstructure,Hardness, and Fracture Toughness of Fe-Based Surface Composites Fabricated by High-Energy Electron Beam Irradiation with Fe-Based Metamorphic Alloy Powders and VC Powders

Iron-based surface composites were fabricated with Fe-based metamorphic alloy powders and VC powders by high-energy electron beam irradiation, and the correlation of their microstructure with hardness and fracture toughness was investigated. Mixtures of metamorphic powders and VC powders were deposited on a plain carbon steel substrate, and then the electron beam was irradiated on these powders without flux, to fabricate surface composites. The composite layers 1.3 to 1.8 mm in thickness contained a large amount (up to 47 vol pct) of hard Cr₂B and V₈C₇ particles formed in eutectic colony regions and inside colonies, respectively. The hardness of the surface composites was approximately 2 to 4 times greater than that of the substrate because of Cr₂B and V₈C₇ particles. According to the microfracture observation of the composite fabricated with mixing 30 wt pct VC powders, microcracks initiated at coarse V₈C₇ particles ins inside colonies as well as at Cr₂B particles in colony regions, and were connected with other microcracks in a zigzag shape. Thus, it showed a higher fracture toughness and hardness twice as high as the composite fabricated without mixing VC powders.     

Correlation of microstructure and fracture toughness in three high-speed steel rolls

The objective of this study is to clarify the fracture characteristics of high-speed steel (HSS) rolls in terms of microstructural factors such as matrix phase and primary carbide particles. Three HSS rolls with different chromium contents were fabricated by centrifugal casting, and the effect of the chromium addition was investigated through microstructural analysis, fracture-mechanism study, and toughness measurement. The hard and brittle primary carbides, as well as the eutectic carbides (ledeburites), were segregated in the intercellular regions and dominated overall properties. Observation of the fracture process revealed that these primary carbides cleaved first to form microcracks at low stress-intensity factor levels and that the microcracks then readily propagated along the intercellular networks. The addition of chromium to a certain level yielded microstructural modification, including the homogeneous distribution of primary carbides, thereby leading to enhancement of fracture toughness of the HSS rolls.