Solidification microstructures and mechanism of grain refinement of electrolytic low titanium A1 alloys

The solidification microstructures and the mechanism of grain refinement of electrolytic low titanium Al alloys were investigated by means of the wedge-shaped sample, the directional solidification and the rapid solidification ribbon. The results show that the coarse columnar grains formed in pure Al are transformed into the equiaxed grains in electrolytic low titanium Al alloys. The grain refinement is resulted from the constitutional supercooling caused by Ti and heterogeneous nucleation of Al3Ti particles. Under the condition of normal cooling rate, the grains are refined by the increment of constitutional supercooling when the content of titanium is less than 0.2%. With the increment of content of titanium, the grains are mainly refined by heterogeneous nucleation of AI3Ti particles. The grain size is decreased with the increment of cooling rate. When the cooling rate is larger than 10^5 ℃/s, the grain size is decreased to 0.1-10μm, the grain refinement is resulted from the larger cooling velocities mainly. After directional solidification, the equiaxed grains can be formed and the Ti element is distributed at the center of the grains.     

EFFECT OF Mo AND Mo2C ON THE MICROSTRUCTURE AND PROPERTIES OF THE CERMETS BASED ON Ti（C,N）

Effect of Mo and Mo2 C on the microstructure and properties of Ti（C,N）-based cermets was investigated in this article. The results have indicated that the weight percentage of Mo from 5 to 10 can reduce Ti（C,N） grain diameter and thickness of the rim, and Ti（C,N） grain can be wetted by Ni-Cu-Mo liquid so as to get small contiguity of Ti（C,N） grain. In that way, the transverse rupture strength of Ti（C,N）-based cermets has reached 1800-1900 MPa; the fracture toughness has been due to 16-18 MPa.m1/2. But 15 wt pct Mo was not more effective on Ti（C,N）-based cermets, because the thickness of the rim becomes larger. In the circumstance of Mo2 C, 5 wt pct Mo2 C was good for microstructure and properties of Ti（C,N）-based cermets, but 11 wt pct Mo2C has resulted in larger contiguity of Ti（C,N） grain and big Ti（C,N） grain diameter so as to reduce transverse rupture strength and fracture toughness. So that, the effect of Mo on Ti（C,N）-based cermets is better than Mo2C.     

Positron Annihilation Study on TiC Powders

Titanium carbonitride (Ti(C, N)) based cermets are characterized by their high creep andwear resistance. These properties made themsuitable for cutting tool applications 11].Ti(C,N) cermets possess a very fine-grainedand stable microstyllctdre. This is attribllted tothe formation of a rim or shell around theTi(C, N) core, thereby enhancing the wetting ofthe carbonitrides to the metal binder and ililiibiting the coalescence and growth of the carbonitride grains dUring sintering. The core/ri…     

Comparative study of the fabrication of ultrafine Ti（C,N）-based cermets by spark plasma sintering and conventional vacuum sintering

Spark plasma sintering (SPS) and conventional vacuum sintering (VS) were employed to fabricate ultrafine Ti(C,N)-based cermets. The shrinkage behavior, microstructure, and porosity and mechanical properties of the samples fabricated by SPS were compared with those of the samples sintered by VS using optical microscopy, scanning electron microscopy, universal testing machine, and rockwell tester. The results are as follows: (1) The shrinkage process occurred mainly in the range of 1000-1300℃ during the VS process, and only a 0.2% linear shrinkage ratio appeared below 800℃；during the SPS process, a 60% dimensional change occurred below 800℃ as a result of pressure action. (2) By utilizing the SPS technique, it is difficult for obtaining fully dense Ti(C,N)-based cermets. Due to the much existence of pores and uncombined carbon, the mechanical properties of the sintered samples by SPS are inferior to sintered ones by VS. (3) grain size of the samples sintered by SPS is still below 0.5μm, but not by VS; because of low sintering temperature, there are no typical core/rim structures formed in the sintered samples by SPS1; the main microstrures of the sintered samples by SPS2 are a white core/grey shell sstructure, whereas by VS show a typical black core.grey shell structure.     

Preparation of Mo/Cu composites by SLS method and its post-treatment techniques

A multi-component polymer-coated molybdenum powder was chosen for selective laser sintering（SLS）. The powder was prepared by coating polymer on Mo particles and frozen by grinding techniques. The laser sintering activities and sound densification response were obtained by optimizing the process parameters. The post-treatment process of SLS samples was developed, which was high temperature sintering Mo framework combined with Cu impregnation. Then, the Mo/Cu composites are gained. The microstructure evolution of post-treatment samples was investigated by scanning electron microscopy. Mo grains frequently string together. The microstructural characterization of Mo/Cu composites is homogeneous compound structure of adhesive phase Cu linked with Mo grains. There is little ellipsoidal Mo grains singly existing around Cu phase. Between Mo grains and Cu zone, there is a medium changing zone with width of 10-20 nm. Post-treatment mechanism is Mo framework sintering of solid phase and Cu impregnation of melting/solidification. The mechanical and thermal properties concluding tensile strength, elastic modulus, elongation and linear expansion of Mo/Cu composites were studied.     

Effects of molybdenum on the microstructure and mechanical properties of Ti（C,N）-based cermets with low Ni

The effects of Mo on the microstructure and mechanical properties of Ti（C,N）-based cermets with low Ni have been studied systematically. Different contents of Mo （4-12 wt.%） were added into Tl（C,N）-based cermets. Specimens were fabricated by conventional powder metallurgy and vacuum sintered at temperatures of 1440, 1450, and 1460℃ individually. The microstructure and fracture morphology were investigated by scanning electron microscope, and the mechanical properties such as transverse strength and hardness were measured. The results show that the microstructure is uniform and the thickness of rim phase is moderate when the content of Mo is 8 wt.%; the mechanical properties of the specimens sintered at 1450℃ are better than those sintered at 1440 and 1460℃. The integrated properties of transverse strength and hardness are the best when the content of Mo is 8 wt.% and the sintering temperature is 1450℃.     

Characterization of phase transformation and microstructure of nano hard phase Ti（C,N）-based cermet by spark plasma sintering

By means of optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM), the process of densification, the characterization of phase transformation and the microstructure for spark plasma sintering (SPS) nano hard phase Ti(C, N)-based cermet were investigated. It is found that the spark plasma sintering (SPS) enables the nano hard phase Ti(C,N)-based cermet to densify rapidly, however, the full densification of the sintered samples can not be obtained. The rate of phase transformation is significantly quick.When being sintered at 1 200℃ for 8 min, Mo2C is completely dissolved, and TiN dissolves into TiC entirely and disappears. Above 1200℃, Ti(C,N) begins to decompose and the atoms of C and N separate from Ti(C,N) resul-ting in the generation of N2 and the graphite. Due to the denitrification and the graphitization, the density and the hardness of sintered samples are rather low. The distribution of grain size of the sample sintered at 1350℃ covers a wide range of 90-500 nm, and most of the grain size are about 200 nm. The hard phase is not of typical core-rim structure. Oxides on the surface of particles can not be fully removed and present in sample as titanium oxide TiO2.Graphite exists in band-like shape.     

Tungsten carbide platelet-containing cemented carbide with yttrium containing dispersed phase

A fine and platelet tungsten carbide patterned structure with fine yttrium containing dispersed phase was observed in liquid phase sintered WC-20% Co-1%Y2O3 cemented carbide with ultrafine tungsten carbide and nano yttrium oxide as starting materials. By comparing the microstructures of the alloy prepared by hot-press at the temperature below the eutectic melting temperature and by conventional liquid phase sintering, it is shown that hexagonal and truncated trigonal plate-like WC grains are formed through the mechanism of dissolution-precipitation （recrystallization） at the stage of liquid phase sintering. Yttrium in the addition form of oxide exhibits good ability in inhibiting the discontinuous or inhomogeneous WC grain growth in the alloy at the stage of solid phase sintering.     

Mechanical behaviors of NiAl-Cr(Mo)-based near eutectic alloy with Ti, Hf, Nb and W additions

Effects of Ti, Hf, Nb and W alloying elements addition on the microstructure and the mechanical behaviors of NiAl-Cr(Mo) intermetallic alloy were investigated by means of XRD, SEM, EDX and compression tests. The results show that Ni-31Al-30Cr-4Mo-2(Ti, Hf, Nb, W) alloy consists of four phases: NiAl, α-Cr solid solution, Cr2Nb and Ni2Al(Ti, Hf). The mechanical properties are improved significantly compared with the base alloy. The compression yield strength at 1 373 K is 467 MPa and the room temperature compression ductility is 17.87% under the strain rate of 5.56×10-3 s-1, due to the existence of Cr2Nb and Ni2Al(Ti, Hf) phases for strengthening and Ti solid solution in NiAl matrix and coarse Cr(Mo, W) solid solution phase at cellular boundaries for ductility. The elevated temperature compression deformation behavior of the alloy can be properly described by power-law equation: ε =0.898 σ8.47exp[-615/(RT)].     

Characterization of RuO2＋SnO2／Ti anodes with high SnO2-concentrations

Two SnO2 RuO2/Ti anodes with high SnO2-concentrations were prepared by painting, sintering and annealing through a sol-gel technique. The microstructure, morphology and grain size of coatings and the electrochemical properties of the anodes were investigated by XRD, DTA, SEM, TEM and CV. It is demonstrated that the anodic coatings consist of solid solution (Sn, X)O2 (X represents Ru or Ti) phases. The average grain size of the coatings is about less than 30 nm. When the annealing temperature increases from 450℃ to 600℃, the solid solutions decompose. The crystal of the coating is equiaxial. The morphology of TiO2 SnO2/Ti coatings is a mixture of mud-flat cracking with a kind of agglomerated structure.     

Microstructure of model cermets with high Mo or W content

The microstructure of (mol%) TiC–18TiN–24Ni–(10–29)WC and TiC–18TiN–24Ni–(5–14)Mo₂C has been investigated using X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and analytical electron microscopy (AEM). When the WC content in the raw materials was increased the W content in the outer rim of (Ti, W)(C, N) grains increased until it had a composition similar to that of the inner rim. If the WC content was high undissolved WC was present after sintering. When the Mo₂C content in the raw materials was increased, the volume fraction of inner rim increased and the Mo content in both inner and outer rim increased. Thermodynamical calculations on the Ti–W–C–N system suggest that the inner rim is formed during solid state sintering when there is an open porosity and thus a low nitrogen activity. The composition of the outer rim can be explained by the equilibrium at the sintering temperature if the volume fraction of undissolved Ti(C, N) cores is subtracted. Calculations on the Ti–Mo–C–N system show that (Ti, Mo)(C, N) decomposes into two phases with different Mo content and that the Ti(C, N) cores might be regarded as a stable phase.     

Microstructural evolution and characteristics during sintering of submicron Ti(C,N)-based cermet

1　INTRODUCTIONThebasiccompositionofTi(C ,N ) basedcer metsisTi(C ,N)andNi.IthasbeenknownthattheadditionofMoorMo2 Cisnecessarytoensuregoodwettability[1] ,andWCorothercarbidesareoftenaddedtoimprovetheplasticityofhardgrainsandhothardnessofthecermets[2 ] .Ti(C ,N) basedcermetsconsistmainlyofcarbonitride grainsandmetalbinder.The“core rim”structurewithinthehardgrainsisthetypicalmicrostructureofcermets .Thecarbonitride phaseenrichedMoandW (therimphase)hasbeenfoundtosurroundthe Ti(C ,…     

Effect of pre-alloyed raw materials on the microstructure of a (Ti,W)(C,N)–Co cermet

Four alloys manufactured from different combinations of powders (TiC + TiN + WC; Ti(C,N) + WC; (Ti,W)C + TiN and (Ti,W)(C,N)) were studied using X-ray diffractometry, optical microscopy (OM), scanning electron microscopy (SEM) and analytical electron microscopy (AEM). The alloy manufactured from binary powders had a smaller grain size and a more inhomogeneous microstructure than the other alloys. The alloys manufactured with WC contained an inner rim around Ti(C,N) cores, as well as W-rich cores. Thermodynamic calculations suggest that these are formed during solid-state sintering at 900°C in a low nitrogen activity. The outer rim had a composition that is in good agreement with calculations of the equilibrium during liquid phase sintering at 1450°C.     

Effect of Mo addition on microstructure and mechanical properties of (Ti,W)C solid solution based cermets

(Ti,W)C solid solution was synthesized by milling a mixture of C and oxides and then reducing it at 1350 °C for 2 h. The microstructure and mechanical properties of (Ti,W)C solid solution based cermets with various Mo additions were systemically studied. The dark core–gray rim carbide grains and/or gray carbide grains embedded in black Ni based binder phase were observed. The grain size of the cermets decreased with increasing Mo addition, while excessive Mo addition would result in agglomeration and inhomogeneity of the grains. The dark core was transformed from (Ti,W)C into (Ti,W,Mo)C and Mo concentration in the dark core increased with increasing Mo content. The fracture toughness of the cermets decreased with the increase of Mo content, while the hardness and TRS reached a peak value at 10 wt.% and 15 wt.% Mo additions respectively, and declined with the further increase of Mo content.     

Ostwald ripening and grain growth in Ti(C,N)-based cermets during liquid phase sintering

After liquid phase sintering, core-rim microstructure occurs in the grains of Ti(C,N)-based cermets. The mechanisms of the microstructural evolution during different sintering stages were investigated here. Our theoretical analyses show that the rim thickness of a grain is relatively independent of its initial radius when rim formation is dominated by Ostwald ripening during dissolution-reprecipitation stage, whereas it increases linearly with its initial radius when grain growth during subsequent cooling stage is the dominant mechanism. A geometric analysis via cross-sectioning technique is developed to identify dominant rim formation mechanism. Experiments with Ti(C,N)-based cermets show that rim formation is dominated by grain growth at low sintering temperatures and by Ostwald ripening at sufficiently high sintering temperatures.     

Influence of Mo on the microstructure and mechanical properties of TiC-based cermets

The microstructures and mechanics properties of TiC-based cermets composed of TiC, WC, Ni, Co, Mo, and Cr₃C₂ were investigated. The results show that Mo has a great effect on the sintering densification, microstructures, and mechanical properties. The microstructures and distribution of Mo and Ti in the TiC-based cermets were analyzed. It was indicated that a new phase with Ti, Mo, W, and C was formed on the rim of (Ti,W)C grains by means of an addition of Mo into the TiC-based cermets. The new phase with a surrounding structure was of great aid to improve the wettability of the liquid phase on the solid phase surface of TiC, decrease the porosity and refine the grains of the hard phase, which gave rise to the increase in strength and hardness. The properties of the TiC-based cermets could be further improved to some extent by adding WC, Cr₂C₃, and Co.     

Effect of VC addition on sinterability and microstructure of ultrafine Ti(C, N)-based cermets in spark plasma sintering

The effect of vanadium carbide (VC) addition on the sinterability and the microstructure of ultrafine Ti(C, N)-based cermets consolidated through spark plasma sintering (SPS) was systematically investigated using optical microscope, scanning electron microscope (SEM) with X-ray energy dispersive spectrometer (EDS), X-ray diffractometer (XRD) and transmission electron microscope (TEM). Our results reveal that the addition of VC increases the porosity of sintering body and depresses the sinterability of Ti(C, N)-based cermets. It is also found that the VC addition has a significant influence on the microstructure of ultrafine Ti(C, N)-based cermets, which inhibits the dissolution of titanium-containing compounds and the formation of inner rim phase and outer rim phase, thus preventing from grain growth. Owing to the depressed dissolution and precipitation, nitrogen liberation is mitigated, therefore resulting in less amount of graphite phase in the samples. In substance, VC changes the solubility of metallic elements in the binder, which makes more elements of Mo and W to be reserved in the binder and thus greatly decreases the content of titanium dissolved into the binder. The re-building solubility rule determines the development of phases and microstructure.     

Microstructure and mechanical properties of (Ti,W,Ta)C-xMo-Ni cermets

(Ti,W,Ta)C-xMo-20Ni (x = 0–20 wt.%) system cermets were prepared by using (Ti,W,Ta)C solid solution as hard phase, without adding secondary carbides in this work. The microstructures exhibited two kinds of carbide grains observable in the BSE mode: one had a dark core–gray rim structure; the other had a dark core–white inner rim–gray outer rim structure. Several (Ti,W)C white particles with high W content were also observed in the microstructures. Results showed that molybdenum has completely dissolved in the Ni binder and mainly was found in the rims of the carbide particles. As the Mo content increased, the wettability between (Ti,W,Ta)C particles and metallic phase was improved, and the shrinkage of cermets increased, the porosity decreased. Results also revealed that finer microstructures were obtained with the increase of Mo addition. But excess Mo addition caused the aggregation of particles. Mechanical properties such as hardness and TRS improved with the increase of Mo content, but the TRS of cermets reached a peak value at the 15 wt.% Mo addition, then declined with the further increase of Mo addition.     

Microstructural evolution during sintering of cermets studied using interrupted sintering experiments and novel 2D and 3D FESEM based techniques

During sintering of cermets the dissolution and precipitation of dissolved MeCs lead to the well-known formation of a core-rim structure. To investigate the microstructural evolution during sintering the sintering process was interrupted at different temperatures and the formation of the core-rim structure traced. EDS mappings and point scans as well as 3D-FIB tomography scans were carried out to study the distribution of added MeCs like Mo₂C or WC on ion-beam prepared samples. The studied cermet consisted of a Ti(C,N) hard phase with a mixed Co and Ni binder and added Mo₂C. The results show i.e. that within the rim area not only a high amount of MeC exists, but also that during the final stages of sintering in some grains binder inclusions within the rim area and Mo inclusion within the core appear.     

纳米TiN复合Ti(C,N)基金属陶瓷烧结过程中的相演变

本文采用XRD与SEM对纳米复合Ti(C,N)基金属陶瓷烧结过程中的相演变进行了研究,结果发现:对于缺碳体系,Mo在800℃即可以夺取TiC中的C,生成Mo2C;1 000℃时开始生成Ni2(Mo,W,Ti)4C相,其含量在1 250℃时达到最大,随着温度的进一步升高而部分分解并进入Ti(C,N)晶格,生成非化学计量的(Ti,Mo,W)(C,N),导致Ti(C,N)的相对含量升高,晶格常数减少。纳米TiN与亚微米TiN相比并没有显示出更高的烧结活性。当添加适量的C时,纳米TiN复合的Ti(C,N)基金属陶瓷与同一体系的亚微米金属陶瓷烧结过程中的相变规律基本相同。