Tin was selected as a modifying element in low-gold-content (50 at.%) bulk metallic glasses (BMGs) aiming at developing alloys with cost-effective performance. New gold-based Au–Sn–Cu–Si alloys were fabricated by injection-casting into a copper mold. The as-cast BMG Au50Sn6Cu26Si18 with 18.6-karat gold and a diameter of 1 mm possessed a lower glass transition temperature (Tg) of 82°C (355 K), a lower liquid temperature of 330°C (603 K), and a super-cooled liquid region of 31°C. The viscosity range of this BMG Au50Sn6Cu26Si18 was from 108 to 109 Pa s measured at a low applied stress of 13 kPa. To compare the viscosity with different applied stresses, its viscosity clearly increased with applied stress below Tg but not so obvious above Tg. The low viscosity of this BMG Au50Sn6Cu26Si18 at around 102°C, which is very close to the boiling temperature of water (100°C), rendered easy thermal–mechanical deformation in a boiling water-bath by hand-pressing and tweezers-bending. Such a deformation capability in boiling water is beneficial to the further applications in various fields.

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Tin-modified gold-based bulk metallic glasses

This study examined the effect of the cooling rate on the hardness and its effect on the microstructure during porcelain firing simulation of a Pd–Ag–In–Sn−Ga metal-ceramic alloy. In practice, after each firing step for porcelain bonding, the prosthesis is cooled to room temperature before proceeding to the next firing step. The cooling step is known to allow the hardness of the metal substructure to increase. The aim of the study was to determine whether controlling the cooling rate after each porcelain-firing step increases the hardness of the Pd–Ag-based metal-ceramic alloy. The results showed that the hardness of specimens cooled at a higher cooling rate increased after each firing step compared to specimens cooled at a lower cooling rate (p

Effect of cooling rate on hardness and microstructure of Pd-Ag-In-Sn-Ga alloy during porcelain firing simulation.

The thermal analyses of Au–35 mass% Ag–15 mass% Cu alloy show different phase transformations (order-disorder transition and precipitation reaction) which have not been well identified before. It is known that the physical properties of this alloy depend on the crystalline and microstructural changes. Thus, to identify all phases optical microscopy, X-ray diffraction (XRD), differential scanning calorimetry, scanning electron microscopy and high-resolution transmission electron microscopy are used. Moreover, the crystalline structure of the formed phases has been followed by XRD, elemental X-ray mapping and electronic diffraction. These techniques show for the first time, the simultaneous occurrence of discontinuous precipitation and disorder-order transition during ageing at the same temperature 415 °C. Indeed, cells with lamellae of Ag-rich α1 and Cu-rich α2 solid solutions and AuCu3-ordered phase are, respectively, formed at grain boundaries by discontinuous decomposition of Ag-rich α1 supersaturated solid solution and in the grain interior of the α0 parent Au-rich phase. The presence of a third phase—Cu-rich α2 solid solution—is revealed in the ternary Au–Ag–Cu phase diagram at 400 °C isothermal section.

Microstructural evolution of Au–35 mass% Ag–15 mass% Cu alloy during ageing at 415 °C

This study examined the effect of ice-quenching after degassing on the change in hardness of a Pd-Au-Zn alloy during porcelain firing simulations. By ice-quenching after degassing, the specimens were softened due to homogenization without the need for an additional softening heat treatment. The lowered hardness by ice-quenching after degassing was recovered greatly from the first stage of porcelain firing process by controlling the cooling rate. The increase in hardness during cooling after porcelain firing was attributed to the precipitation of the f.c.t. PdZn phase containing Au, which caused severe lattice strain in the interphase boundary between the precipitates and matrix of the f.c.c. structure. The final hardness was slightly higher in the ice-quenched specimen than in the specimen cooled at stage 0 (the most effective cooling rate for alloy hardening) after degassing. This was attributed to the more active grain interior precipitation during cooling in the ice-quenched specimen after degassing.

Effect of ice-quenching on the change in hardness of a Pd-Au-Zn alloy during porcelain firing simulation.

The effects of discontinuous precipitation and ordering on the age-hardening in Au-20Ag-30Cu (wt.%) alloy

The aim of this study was to determine the hardening effect by controlling the cooling rate during the porcelain firing process and performing an additional post-firing heat treatment in a Pd-Ag-Sn alloy. The most effective cooling rate for alloy hardening was determined by cooling the specimens at various cooling rates after oxidation treatment. A subsequent porcelain firing simulation followed by cooling at the selected cooling rate was performed. A post-firing heat treatment was then done at 600°C in a porcelain furnace. The hardening mechanism was characterized by a hardness test, X-ray diffraction, field emission scanning electron microscopy and energy dispersive X-ray spectroscopy. Alloy softening occurred during the porcelain firing process followed by cooling at a controlled cooling rate. A post-firing heat treatment allowed apparent precipitation hardening. It is advisable to perform a postfiring heat treatment at 600°C in a porcelain furnace by annealing metal substructure after porcelain fusing.

Hardening by cooling rate control and post-firing heat treatment in Pd-Ag-Sn alloy for bonding porcelain.

The age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental casting gold alloy composed of 56Au–25Ag–11.8Cu–5Pd–1.7Zn–0.4Pt–0.1Ir (wt.%) were elucidated by characterizing the age-hardening behaviour, phase transformations, changes in microstructure and changes in element distribution. The fast and apparent increase in hardness at the initial stage of the aging process at 400°C was caused by the nucleation and growth of the metastable Ag–Au-rich phase and the Cu–Au-rich phase by the miscibility limit of Ag and Cu. The transformation of the metastable Ag–Au-rich phase into the stable Ag–Au-rich phase progressed concurrently with the ordering of the Cu–Au-rich phase into the AuCu I phase through the metastable state, which resulted in the subsequent increase in hardness. The further increase in hardness was restrained before complete decomposition of the parent α0 phase due to the initiation of the lamellar-forming grain boundary reaction. The progress of the lamellar-forming grain boundary reaction was not directly connected with the phase transformation of the metastable phases into the final product phases. The heterogeneous expansion of the lamellar structure from the grain boundary caused greater softening than the subsequent further coarsening of the lamellar structure. The lamellar structure was composed of the Ag–Au-rich layer which was Cu-, Pd- and Zn-depleted and the AuCu I layer containing Pd and Zn.

https://link.springer.com/content/pdf/10.1007%2Fs13404-011-0021-x.pdf

Age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental Au-Ag-Cu-Pd-Zn alloy

Hardening mechanism associated with additional heat treatment after degassing treatment by various cooling rates in a metal-ceramic alloy of Pd-Au-Ag system

This study was intended to investigate the effect of applying liner for chemical bonding and physical surface roughness created on Yttria-toughened zirconia (YTZ core) by using a polishing tool before sintering on the bond strength between the two materials. YTZ blocks(Fine Base, K&F, Korea, 15mm×15mm ×4mm) were cut using a low-speed cutter. Plate-shaped specimen (6mm×6mm×3mm) was fabricated by sintering after giving surface roughness according to three kinds of polishing tools. Depending on whether or not to use liner, 60 specimens were divided into two groups ZP(non-liner), ZLP(liner), and the two groups were subdivided into three groups respectively in accordance with polishing tools used, totaling six groups (n=10). The surface roughness (Ra) values and shapes before sintering were observed, and shear bond strength after veneer ceramic plasticity was measured with a universal testing machine. For a test of the significance, a one-way ANOVA was performed, and Tukey"s multiple comparison test was conducted. The observation of fracture surfaces using an optical microscope revealed that the surface roughness was SZ(1.10±0.76㎛) 0.05). The research results showed that the bond strength of YTZ core and veneering ceramic was further improved by physical surface treatment before sintering, rather than by chemical bonding through liner surface treatment.

Effects of Pre-Sintering Surface Treatment and Liner Application on the Shear Bond Strength of Zirconia and Veneer Ceramic

치과용 Ag-Au-Cu-Pd-Zn-lr 합금의 상 분리와 규칙화의 이중 기구에 의한 시효경화를 규명하기 위하며 경화 거동, 상 변태, 미세 구조의 변화를 조사하여, 다음과 같은 결과를 얻었다. 시효 초기에 경도는 비교적 완만하지만 꾸준히 상승하여 최고치에 도달하였고, 그 후 경도는 급격히 하강하다가 서서히 하강했다. 시효 초기의 완만한 경도 상승은 α 0 기지에서 Ag-Au-rich α₁ 상과 준안정상인 AuCu I" 상의 핵 생성과 성장에 기인하였다. 그 후에 일어난 꾸준한 경도 상승은 준안정상인 AuCu I" 상이 안정상인 AuCu I 상으로 변태되는 것에 기인하였다. 결정 입계에서 그리고 이어서 입내에서 생성되는 층상 조직이 격자 뒤틀림을 효과적으로 해소하여 연화를 초래하였다. 입내에서 생성된 미세한 층상조직의 조대화가 입계에서 생성된 층상 조직의 조대화보다 더 뚜렷하게 경도 하강을 유발하였다.

Mi-Hyang Cho

Papers

Hardening by cooling rate control and post-firing heat treatment in Pd-Ag-Sn alloy for bonding porcelain.

Age-hardening and overaging mechanisms related to the metastable phase formation by the decomposition of Ag and Cu in a dental Au-Ag-Cu-Pd-Zn alloy

Hardening mechanism associated with additional heat treatment after degassing treatment by various cooling rates in a metal-ceramic alloy of Pd-Au-Ag system

Effects of Pre-Sintering Surface Treatment and Liner Application on the Shear Bond Strength of Zirconia and Veneer Ceramic

Age-hardening by Dual Mechanisms of Phase Separation and Ordering in a Dental Ag-Au-Cu-Pd-Zn-Ir Alloy