Magnesium alloys have received an increasing interest in the past 12 years for potential applications in the automotive, aircraft, aerospace, and electronic industries. Many of these alloys are strong because of solid-state precipitates that are produced by an age-hardening process. Although some strength improvements of existing magnesium alloys have been made and some novel alloys with improved strength have been developed, the strength level that has been achieved so far is still substantially lower than that obtained in counterpart aluminum alloys. Further improvements in the alloy strength require a better understanding of the structure, morphology, orientation of precipitates, effects of precipitate morphology, and orientation on the strengthening and microstructural factors that are important in controlling the nucleation and growth of these precipitates. In this review, precipitation in most precipitation-hardenable magnesium alloys is reviewed, and its relationship with strengthening is examined. It is demonstrated that the precipitation phenomena in these alloys, especially in the very early stage of the precipitation process, are still far from being well understood, and many fundamental issues remain unsolved even after some extensive and concerted efforts made in the past 12 years. The challenges associated with precipitation hardening and age hardening are identified and discussed, and guidelines are outlined for the rational design and development of higher strength, and ultimately ultrahigh strength, magnesium alloys via precipitation hardening.

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Precipitation and Hardening in Magnesium Alloys

Boron carbide is characterized by a unique combination of properties that make it a material of choice for a wide range of engineering applications. Boron carbide is used in refractory applications due to its high melting point and thermal stability; it is used as abrasive powders and coatings due to its extreme abrasion resistance; it excels in ballistic performance due to its high hardness and low density; and it is commonly used in nuclear applications as neutron radiation absorbent. In addition, boron carbide is a high temperature semiconductor that can potentially be used for novel electronic applications. This paper provides a comprehensive review of the recent advances in understanding of structural and chemical variations in boron carbide and their influence on electronic, optical, vibrational, mechanical, and ballistic properties. Structural instability of boron carbide under high stresses associated with external loading and the nature of the resulting disordered phase are also discussed.

Boron Carbide: Structure, Properties, and Stability under Stress

Many of the fundamental questions regarding the solid-state chemistry of boron are still unsolved, more than 200 years after its discovery. Recently, theoretical work on the existence and stability of known and new modifications of the element combined with high-pressure and high-temperature experiments have revealed new aspects. A lot has also happened over the last few years in the field of reactions between boron and main group elements. Binary compounds such as B(6)O, MgB(2), LiB(1-x), Na(3)B(20), and CaB(6) have caused much excitement, but the electron-precise, colorless boride carbides Li(2)B(12)C(2), LiB(13)C(2), and MgB(12)C(2) as well as the graphite analogue BeB(2)C(2) also deserve special attention. Physical properties such as hardness, superconductivity, neutron scattering length, and thermoelectricity have also made boron-rich compounds attractive to materials research and for applications. The greatest challenges to boron chemistry, however, are still the synthesis of monophasic products in macroscopic quantities and in the form of single crystals, the unequivocal identification and determination of crystal structures, and a thorough understanding of their electronic situation. Linked polyhedra are the dominating structural elements of the boron-rich compounds of the main group elements. In many cases, their structures can be derived from those that have been assigned to modifications of the element. Again, even these require a critical revision and discussion.

Boron: elementary challenge for experimenters and theoreticians.

This review highlights the use and great potential of liquid metals as exotic and powerful solvents (i.e. fluxes) for the synthesis of intermetallic phases. The results presented demonstrate that considerable advances in the discovery of novel and complex phases are achievable utilizing molten metals as solvents. A wide cross-section of examples of flux-grown intermetallic phases and related solids are discussed and a brief history of the origins of flux chemistry is given. The most commonly used metal fluxes are surveyed and where possible, the underlying principal reasons that make the flux reaction work are discussed.

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The metal flux: a preparative tool for the exploration of intermetallic compounds.

Dense transition metal borides have recently been identified as superhard materials that offer the possibility of ambient pressure synthesis compared to the conventional high pressure, high temperature approach. This feature article begins with a discussion of the relevant physical properties for this class of compounds, followed by a summary of the synthesis and properties of several transition metal borides. A strong emphasis is placed on correlating mechanical properties with electronic and atomic structure of these materials in an effort to better predict new superhard compounds. It concludes with a perspective of future research directions, highlighting some recent results and presenting several new ideas that remain to be tested.

Advancements in the Search for Superhard Ultra-Incompressible Metal Borides

https://iopscience.iop.org/article/10.1088/1468-6996/11/2/023001/pdf

Raman effect in icosahedral boron-rich solids

The crystal structure of Mg51Zn20

The structures of the B6O samples prepared by oxidizing boron with ZnO at temperatures of 1350–1500°C in an argon atmosphere have been refined by the Rietveld method. The B6O samples were found to contain an amorphous phase from their X-ray diffraction profiles in addition to B6O diffraction peaks. The results indicate that the B6O samples, space group R¯3m, no.166, ahex = 0.5367(1) nm and chex = 1.2328(2) nm in the hexagonal unit cell, have oxygen deficiencies with 0.76(6) oxygen occupancy. A radial distribution function method applied to an extracted portion of the amorphous phase, suggests that each amorphous phase has nearly similar short-range order structure to that of the α-tetragonal boron type rather than to those of any other related boron phases.

Structure of B6O boron-suboxide by Rietveld refinement

The structure of MgAlB14 (space group, Imam; a = 5.848(1) A; b = 8.112(1) A; c = 10.312(1) A; Z = 4) was reinvestigated using single-crystal X-ray diffractometry. The crystal was grown from a high temperature AlB melt containing a small quantity of magnesium. A total of 1277 unique reflections (Mo Kα radiation; 2θ < 100°) was collected and used in a block diagonal leastsquares refinement to a final R value of 0.023. The boron framework in the crystal is the same as that reported by Matkovich and Economy. However, the distribution of the metal atoms is significantly different, i.e. no aluminium atoms are present at the magnesium site which is split into two positions separated by 0.39 A. The occupancies of the metals are 78% and 75% for the magnesium and aluminium sites respectively. The chemical composition determined by the structure analysis is Mg0.78Al0.75B14. Re-examination of the structure using the previously published intensity data indicates that the reported crystal probably had the same chemical composition and metal distributions as the present crystal.

Iwami Higashi

Papers

Raman effect in icosahedral boron-rich solids

The crystal structure of Mg51Zn20

Structure of B6O boron-suboxide by Rietveld refinement

Refinement of the structure of MgAlB14

Boron-rich crystals in A1-M-B (M = Li, Be, Mg) systems grown from high-temperature aluminum solutions