Full Density Compacts Research Articles

A novel microstructure design for high-performance structural materials with high strength and high ductility

Creation of ‘Harmonic Structure (HS)’ in metals and alloys is a new material design paradigm allowing the enhancement of structural performance. In the present study, near full density compacts with HS were prepared to investigate the effect of such a bimodal microstructure on the commercially pure metals. The mechanical milling (MM) was used to obtain bimodal structure in the metal powders, i.e. ultra-fine-grained (UFG) region on their periphery, or ‘Shell’, and coarse-grained (CG) region in the centre or ‘Core’. Subsequent consolidation of the MM powders by Spark Plasma Sintering process resulted in sintered compacts with an interconnected three-dimensional network of UFG structure along with dispersed CG areas. In other words, they demonstrated heterogeneous yet well organised and fully controllable HS. The creation of bimodal harmonic structure in pure metals exhibited considerably higher yield strength and ultimate tensile strength as compared to their homogeneous coarse-grained counterparts. Moreover, the improvement in strength was achieved without compromising the uniform deformation limit and overall ductility of the materials. Finally, numerical simulations were carried out to reveal the deformation behaviour of harmonic structured materials. The results of numerical simulations are presented and discussed.

Mechanically induced self-propagating reaction and consequent consolidation for the production of fully dense nanocrystalline Ti55C45 bulk material

We employed a high-energy ball mill for the synthesis of nanograined Ti55C45 powders starting from elemental Ti and C powders. The mechanically induced self-propagating reaction that occurred between the reactant materials was monitored via a gas atmosphere gas-temperature-monitoring system. A single phase of NaCl-type TiC was obtained after 5h of ball milling. To decrease the powder and grain sizes, the material was subjected to further ball milling time. The powders obtained after 200h of milling possessed spherical-like morphology with average particle and grain sizes of 45μm and 4.2nm, respectively. The end-products obtained after 200h of ball milling time, were then consolidated into full dense compacts, using hot pressing and spark plasma sintering at 1500 and 34.5MPa, with heating rates of 20°C/min and 500°C/min, respectively. Whereas hot pressing of the powders led to severe grain growth (~436nm in diameter), the as-spark plasma sintered powders maintained their nanograined characteristics (~28nm in diameter). The as-synthesized and as-consolidated powders were characterized, using X-ray diffraction, high-resolution electron microscopy, and scanning electron microscopy. The mechanical properties of the consolidated samples obtained via the hot pressing and spark plasma sintering techniques were characterized, using Vickers microhardness and non-destructive testing techniques. The Vickers hardness, Young's modulus, shear modulus and fracture toughness of as-spark plasma sintered samples were 32GPa, 358GPa, 151GPa and 6.4MPa·m1/2, respectively. The effects of the consolidation approach on the grain size and mechanical properties were investigated and are discussed.

Fabrication of multilayered Ti–Al intermetallics by spark plasma sintering

A novel processing method, based on heat treatment and spark plasma sintering (SPS) of elemental foils, has been proposed to prepare the Ti–Al intermetallics. It is demonstrated that the full density compacts with different microstructures, i.e. Ti/Ti–Al intermetallics multilayer, Ti3Al/TiAl multilayer, or homogeneous Ti–Al intermetallics alloy, can be successfully prepared by controlling the sintering conditions. The results show that the compacts with Ti/Ti–Al intermetallics multilayered structure obtained the better mechanical properties as compared with the multilayered Ti3Al/TiAl and the homogeneous Ti–Al intermetallics compacts. The better mechanical properties of the multilayered Ti/Ti–Al intermetallics compacts were attributed to the deflection of the moving cracks by the ductile α-Ti layer. Therefore, it was found that the unique multilayered structure consisting of alternating Ti and Ti–Al intermetallics layers offers the possibility of improving the fracture toughness.

Synthesis of multinary rare-earth sulfides PrGdS 3, NdGdS 3, and SmEuGdS 4, and investigation of their thermoelectric properties

We have developed a new strategy for the synthesis of the ternary rare-earth sulfides LnGdS 3(Ln: Pr, Nd) and the quaternary rare-earth sulfide SmEuGdS 4. This strategy involves the use of a polymerized complex method and sulfurization. Multinary rare-earth oxycarbonate powders were first prepared from nitrates by the polymerized complex method. The oxycarbonates were then sulfurized with CS 2 gas to synthesize the multinary rare-earth sulfides. While the sulfurized LnGdS 3 powders were crystallized in the orthorhombic α -phase, the sulfurized SmEuGdS 4 powder was crystallized in the cubic γ -phase. The α -LnGdS 3 transformed completely into γ -LnGdS 3 after heat treatment at 1723 K. The γ -LnGdS 3 and γ -SmEuGdS 4 powders were consolidated by pressure-assisted sintering to fabricate full-density compacts. The Seebeck coefficient, electrical resistivity, and thermal conductivity of the sintered compacts were measured between 300 and 950 K. While the NdGdS 3 sintered compact showed an n-type Seebeck coefficient and low electrical resistivity, the SmEuGdS 4 sintered compact showed a p-type Seebeck coefficient and high electrical resistivity. The thermal conductivity of NdGdS 3 ranged from 0.8 to 1.2 W/K m. The highest figure of merit ( Z T = 0.29 ) was obtained for NdGdS 3 at 950 K.

Nanostructure characterization of mechanical alloyed and consolidated iron alloys

High-energy ball milling (which is readily adoptable to commercial application) was used to develop sufficient quantities of nanostructured material to produce compacts capable of being measured for macroscopic properties. Characterization of the ball-milled powders show that grain boundary properties play a significant role in the overall properties of the milled powder. Nearly full-dense compacts were produced by hot-pressing. Characterization of the strength properties of these compacts show that there was little influence of hardness, density, or alloy composition on the failure properties. The range of failure stress was large and when fitted to a Weibull distribution suggest that failure was the result of flaws or cracks resulting from the hot-pressing. Hardness data, commonly used to evaluate the strength of nanostructured materials, showed no correlation to tensile strength, but correlated highly to compression maximum stress.

Microstructure and tensile properties of compacted, mechanically alloyed, nanocrystalline Fe-AI

Data on mechanical properties of nanocrystalline materials have been limited, due in part to the difficulty in producing consolidated nanocrystalline materials of sufficient quantity for characterization and evaluation. A second problem is consolidation and retention of the nanostructure. A vacuum hot-pressing consolidation program has been developed to produce full-dense compacts from attrition milled, mechanically alloyed, nanograin micron-size particles of Fe-2 wt pet Al powder. The resulting compacts were of sufficient size to allow evaluation of microstructure, density, hardness, and tensile properties. The compacted microstructure was a composite of pure iron submicrograins and Fe-A1 nanograin clusters. Tensile strength was found to be proportional to the sample’s density squared. For full-dense compacts, tensile strength of nanocrystalline compacts approaching 1 GPa was obtained.

Processing and microhardness of bulk Cu-Fe nanocomposites

Full-density bulk nanocomposites have been developed in the immiscible Cu-Fe system through a powder metallurgy route. Elemental copper and iron powders were first mechanically alloyed to form single-phase, nanocrystalline, metastable solid solutions, Cu 100 − x Fe x (x = 0 to 100). These solid solutions were subsequently decomposed into Cu/Fe two-phase domains of various volume fractions during the hot consolidation process, forming in situ nanophase Cu-Fe composites. Full-density compacts have been produced at relatively low consolidation temperatures (< 500°C) by employing sinter forging at high applied pressure and a protective atmosphere. Such a consolidation scheme, coupled with enhanced grain size stability due to the unique microstructural evolution sequence involved, retained the grain sizes in the nanometer range for both Cu and Fe grains in the composite products. Fully dense composite specimens exhibit enhanced microhardness as compared with rule-of-mixtures predictions. This enhancement is attributed to interface strengthening at fcc-bcc interphase boundaries.

Full-density nanocrystalline Fe–29Al–2Cr intermetallic consolidated from mechanically milled powders

Fe–29Al–2Cr powders with nanoscale grain sizes were produced by mechanical milling of prealloyed intermetallic powders. A consolidation procedure employing high-pressure, low strain rate hot forging (sinter-forging) has been developed to consolidate the powders into full-density compacts. The relative density and average grain size of the compact have been studied as a function of consolidation temperature at constant pressure. Fully dense compacts (&gt;99.5% theoretical density) were produced at a relatively low temperature of 545°C with a pressure of 1.25 GPa. Transmission electron microscopy and x-ray diffraction analysis indicate that the average grain size has been maintained to the order of 30 nm in samples consolidated under these conditions. By using protective Ar atmosphere during mechanical milling and consolidation, contamination of oxygen and carbon in consolidated samples has been controlled to below a small fraction of an atomic percent. Microhardness tests of nanocrystalline Fe–29Al–2Cr samples indicate a significant strengthening effect due to grain size refinement and a monotonic hardness increase with decreasing residual porosity. Our work demonstrates the feasibility of using mechanically milled powders as the source of nanocrystalline materials for the production of fully dense, low-impurity, nanocrystalline bulk samples needed for reliable mechanical property measurements and practical applications.

ステンレス鋼粉の擬ＨＩＰ成形におけるち密化と組織形成

The pseudo-HIP method was applied to the hot compaction of two kinds of stainless steel powders with average particle sizes of 53 and 0.06 microns, respectively. The preheated powder preform, encapsulated in a glass tube, was compacted in a steel die using preheated alumina granules as a pressure-transmitting medium. The densification behavoir and microstructural development were investigated by varying compacting pressure and temperature. It has been found that full dense compacts with fine-grained microstructure can be obtained at temperature 300-400K lower than with normal sintering.

Effect of Atomization Media and Consolidation Techniques upon Physical Properties of a P/M Cobalt-Base Alloy

Inert gas produces a powder with less surface area and lower gas content than water atomization. Nitrogen as an atomization medium results ia approximately five times the nitrogen content compared to powders atomized with argon. Oxygen is present principally in the form of surface oxide, and to a minor extent as adsorbed gas on the surface of the powder particles. The latter can be removed by a vacuum treatment which also partially removes the oxide film. Results from Coldstream processing with air as the fluid for comminution is compared with inert gas processing. Coldpressing and sintering, hot extrusion, and hot forging are compared as alternative methods to consolidate powder of type X-40 into high density bodies. Densities of 92% of theoretical were obtained by coldpressing and sintering in dry hydrogen or vacuum. Both hot forging and hot extrusion using the canning-evacuation method produce full density compacts. Room temperature properties for X-40 and two other Co-base alloys were determined for these consolidation methods. Metallographic evaluation of P/M products vs conventionally-cast material reveals differences believed to produce improved reproducibility and higher reliability of the P/M product. Alloys earlier known as “nonforgeable” can be given a high degree of forgeability by starting with a prealloyed powder as the raw material. Examples of such applications are presented.