Formation of in situ reinforced microstructures in α-sialon ceramics: Part III. Static and dynamic ripening

2004 ◽  
Vol 19 (8) ◽  
pp. 2402-2409 ◽  
Author(s):  
Hong Peng ◽  
Zhijian Shen ◽  
Mats Nygren
Keyword(s):  
Heat Treatment ◽  
Grain Growth ◽  
Liquid Glass ◽  
Glass Phase ◽  
Ostwald Ripening ◽  
Graphite Furnace ◽  
Stoichiometric Composition ◽  
Temperature Threshold ◽  
Post Heat Treatment

Dual cation (Yb + Y)-stabilized α-sialon ceramics with either stoichiometric composition or nonstoichiometric composition that yield less than 3 vol% of an additional intergranular liquid/glass phase were consolidated by spark plasma sintering (SPS). This process allows very fast heating and cooling, thus providing a unique possibility to monitor and manipulate the kinetics of phase transformation and grain growth during sintering. Below a temperature threshold, full densification and complete α-sialon formation are accompanied by very limited grain growth. The grain growth kinetics were investigated both by post heat-treatment of SPS pre-consolidated monophasic α-sialon bodies consisting of sub-micron sized equiaxed grains in a conventional graphite furnace using extended holding times (hours) and directly rapid annealing in the SPS apparatus above the temperature threshold (within minutes). Post heat treatment in the graphite furnace yielded in situ reinforced microstructures consisting of interlocking elongated grains only in the presence of an additional intergranular liquid/glass phase. Direct annealing by SPS process yielded in situ reinforced microstructures whether or not an additional liquid/glass was involved. The former microstructures are formed via the static Ostwald ripening mechanism whereas the latter ones are generated via a dynamic ripening mechanism. This demonstrates that the dynamic ripening provides an efficient means of developing in situ reinforced microstructure in α-sialon ceramics with improved mechanical properties.

SPS Processing and Superplastic Deformation of Silicon Nitride Based Ceramics

2005 ◽  
Vol 287 ◽  
pp. 146-155 ◽  
Author(s):  
Hong Peng ◽  
Zhi Jian Shen ◽  
Mats Nygren
Keyword(s):  
Heat Treatment ◽  
Silicon Nitride ◽  
Liquid Phase ◽  
Electric Field ◽  
Grain Growth ◽  
Growth Kinetics ◽  
Graphite Furnace ◽  
Temperature Threshold ◽  
Post Heat Treatment ◽  
Grain Growth Kinetics

Recently we have introduced a novel processing concept of sialon components implying that an extra liquid phase that is thermodynamically compatible with sialon phases is introduced by increasing the O/N ratio in the general formula (Yb+Y)xSi12-(3x+n)Al3x+nOnN16- n while keeping constant the Yb, Y, Si, and Al proportions. By increasing the oxygen content from its stoichiometric value of 5.16 to 15 eq%, a series of powder mixtures were prepared and their overall compositions are located slightly above the homogeneity region of the a- sialon phase. These compositions were consolidated to full densities by hot pressing (HP) and Spark Plasma Sintering (SPS), respectively. The sintering kinetics in the HP and SPS units is compared. The grain growth kinetics were investigated both by post heat-treatment of SPS pre-consolidated monophasic a-sialon bodies consisting of sub-micron sized equiaxed grains in a conventional graphite furnace using extended holding times (hours) and in the SPS apparatus rapidly heated exceeding the temperature threshold of grain growth and using short holding times (minutes). Post heat treatment in the SPS apparatus yielded in-situ reinforced microstructures no matter if an additional liquid/glass was involved or not while corresponding microstructures could only be obtained for non-stoichiometric compositions by post heat treatment in the graphite furnace. The grain growth kinetics is discussed in terms of static and dynamic ripening mechanisms. We have recently shown that the ductility of covalent bonded silicon nitride based ceramics is dramatically enhanced in presence of a pulsed electric field. Compressive strains rates in the range of 10-2 s-1 can easily be achieved at T ³ 1500oC. The enhanced ductility is explained by that the electric field induces motion of charged species present in the grain boundary glassy/liquid phase that in turn promotes grain sliding along the grain boundaries.

Optimization of α-SiAlON Microstructure by Heat Treatment

2008 ◽  
Vol 368-372 ◽  
pp. 891-893
Author(s):  
S. Kurama
Keyword(s):  
Heat Treatment ◽  
Grain Growth ◽  
Microstructural Development ◽  
Analysis Method ◽  
Image Analysis Method ◽  
Post Heat Treatment ◽  
Heat Treated ◽  
Controlling Mechanism ◽  
Width Direction ◽  
Gas Pressure Sintering

Nd-doped α-SiAlON starting composition (Nd0.33Si9.38Al2.62O1.62N14.38) was prepared by gas pressure sintering at 1825°C for 3 hrs. In order to explore the effect of post heat treatment on the developments of elongated α-SiAlON grains, sample was heat treated at 1800°C for 4-12 hrs. It was found that post heat treatments promoted formation of the elongated α-SiAlON grains. The controlling mechanism of grain growth was determined via plotting on a graph the growth in width/length versus time graphics using Image Analysis method. Different growth rates were found between the length and width direction of the α-SiAlON crystals, resulting in anisotropic grain growth in the microstructural development.

Effect of SiO2 and Y2O3 additives on the anisotropic grain growth of dense mullite

2000 ◽  
Vol 15 (3) ◽  
pp. 718-726 ◽  
Author(s):  
T. Huang ◽  
M. N. Rahaman ◽  
T-I Mah ◽  
T. A. Parthasarathay
Keyword(s):  
Fracture Toughness ◽  
Grain Growth ◽  
Average Length ◽  
Stoichiometric Composition ◽  
Second Phase ◽  
Transmission Electron ◽  
Indentation Fracture Toughness ◽  
Grain Growth Kinetics ◽  
Mullite Powder

Mullite powder with a nearly stoichiometric composition was doped with 1.5–5 wt% SiO2 or 0.5–1.0 wt% Y2O3 and hot pressed at 1525–1550 °C to produce almost fully dense materials. The effect of the additives on the grain growth of the dense systems was investigated during subsequent annealing at temperatures above that of the eutectic (∼1590 °C) for the SiO2–Al2O3 system. The average length and width of the grains were measured by image analysis of polished and etched sections. At 1750 °C, anisotropic grain growth was relatively rapid, leading to the formation of rodlike grains. Compared to the undoped mullite, the addition of SiO2 and Y2O3 produced a small reduction in the grain growth kinetics. Transmission electron microscopy revealed that the glassy second phase was concentrated at the three-grain junctions or distributed inhomogeneously at the grain boundaries. For the materials annealed at 1750 °C, the indentation fracture toughness at room temperature increased from 2.0 to 2.5 MPa m1/2 for the undoped mullite to values as high as 4.0–4.5 MPa m1/2 for the doped mullite. The implications of the data for enhancing the fracture toughness of mullite by the in situ development of a microstructure of elongated grains are considered.

Observation of structural modulations and formation of H-carbides and ζ-silicides during α2 → γ transformation in fully lamellar γ-TiAl

2002 ◽  
Vol 753 ◽  
Author(s):  
M. Karadge ◽  
P. I. Gouma
Keyword(s):  
Heat Treatment ◽  
Electron Microscope ◽  
Stacking Faults ◽  
Silicide Phase ◽  
Post Heat Treatment ◽  
Transmission Electron ◽  
Modulated Structures ◽  
Regular Arrangement ◽  
Fully Lamellar

ABSTRACTThe present paper illustrates the experimentally observed microstructures related to α2 → γ + Ti2AlC (H-carbide) and Ti5Si3 (ζ silicide) phase transformations in fully lamellar Ti-46Al-2Cr-3Nb-0.2W-0.2Si-0.1C alloy. These transformations were observed by in-situ heat treatments in the TEM at temperatures up to 800°C and by post heat treatment examination in a high-resolution transmission electron microscope. α2 dissolution was observed to start both inside the lath and at the α2/γ interface. H-type carbides were observed to nucleate inside the α2 lath, which explains the appearance of long arrays of well-aligned structures along prior α2 laths in creep-deformed samples. ζ silicide precipitates nucleate at the α2/γ interface, this growth occurs epitaxially along the pre-existing <0001>α2 direction. Modulated structures were observed during the α2 → H, newly formed γ and at the α2 / ζ interface. All these modulated structures consist of regular arrangement of stacking faults and dislocations.

Grain Growth Impediment of Fe-Based Nanocomposites During Heat Treatment at Elevated Temperature

1995 ◽  
Vol 400 ◽  
Author(s):  
B. Huang ◽  
R.J. Perez ◽  
E.J. Lavernia
Keyword(s):  
Heat Treatment ◽  
Thermal Stability ◽  
Grain Boundaries ◽  
Grain Growth ◽  
Ostwald Ripening ◽  
Nanocrystalline Structure ◽  
Liquid Argon ◽  
High Thermal Stability ◽  
Additional Stabilization ◽  
Nanocrystalline Microstructure

AbstractCryogenic ball milling of Fe-10 wt.%Al in liquid argon is shown to produce a nanocrystalline microstructure with high thermal stability against grain growth. The experimental evidence suggests that this stability may originate from the presence of fine γ-Al2O3 particles which pin the Fe grain boundaries. In contrast, Fe3O4 particles formed during cryomilling of Fe in liquid nitrogen were not able to impede grain growth during consolidation at 823 K. The interactions between the γ-Al2O3 particles and the grain boundaries during growth may be described by Gladman's theory. Additional stabilization against grain growth may have been provided by the presence of Al at the Fe grain boundaries. Following annealing at 1223 K, the grain size of the Fe-10 wt.%Al powders cryomilled in liquid argon was 13.3 ± 7.9 nm, but it exceeded 1700 nm following annealing at 1373 K. The loss of nanocrystalline structure may be attributed to the Ostwald ripening of γ-Al2O3 particles, resulting in abnormal grain growth.

Effect of In Situ Post Weld Heat Treatment on Mechanical Behavior and Austenite–Ferrite Phase Balance of Fe-Cr-Ni/Fe-Cr Stainless Steels Upset Resistance Dissimilar Weld

2019 ◽  
Vol 142 (2) ◽  
Author(s):  
Ashkaan Ozlati ◽  
Mojtaba Movahedi
Keyword(s):  
Heat Treatment ◽  
Optimum Condition ◽  
Grain Growth ◽  
Stainless Steels ◽  
Joint Strength ◽  
High Strength ◽  
Post Weld Heat Treatment ◽  
Dissimilar Weld ◽  
Phase Balance

Abstract Effect of the in situ post weld heat treatment (PWHT) was investigated on the flash profile, austenite/ferrite phase balance, and mechanical properties of the upset resistance dissimilar weld between Fe-Cr-Ni and Fe-Cr stainless steels rods. In order to explore the effect of the heat treatment on the joint strength, two as-welded samples with low strength (116 MPa) and high strength (372 MPa) were used. The results showed that in situ PWHT was beneficial for both welded samples, though in different ways. For the weld with low strength, PWHT improved the joint strength (∼130% increase in the optimum condition compared with the as-welded sample) due to the increase in the size of the flash and the related bonded area at the joint interface. However, ferrite percent in the weld zone increased from ∼50% up to ∼70%. For the sample with the high strength, ferrite/austenite phase balance was restored at an optimum condition of PWHT. However, the joint strength decreased slightly (less than 5%) due to the grain growth in the Fe-Cr rod, i.e., the fracture location. Fracture analysis was used for justification of the variations in the joint strength. For both Fe-Cr-Ni side and Fe-Cr side of the welds, in situ PWHT generally reduced the hardness. This observation is discussed in light of the simultaneous effects of the grain growth and formation of little martensite.

Effect of post-heat treatment on microstructure and mechanical properties of laser welded Al-Cu-Mg alloy

2021 ◽  
Vol 64 ◽  
pp. 620-632
Author(s):  
Alexander Malikov ◽  
Anatoly Orishich ◽  
Igor Vitoshkin ◽  
Evgeniy Karpov ◽  
Alexei Ancharov
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Mg Alloy ◽  
Post Heat Treatment ◽  
Microstructure And Mechanical Properties

scholarly journals In Situ Grain Growth of Nanograined Magnetite under Ion Irradiation at Room Temperature and 500 ℃

2021 ◽  
Vol 27 (S1) ◽  
pp. 2640-2643
Author(s):  
Chris McRobie ◽  
Ryan Schoell ◽  
Tiffany Kaspar ◽  
Daniel Schreiber ◽  
Djamel Kaoumi
Keyword(s):  
Grain Growth ◽  
Room Temperature ◽  
Ion Irradiation
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