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1.
Grain refinement of aluminum alloys: Part I. the nucleant and solute paradigms—a review of the literature 总被引:1,自引:0,他引:1
Despite the extensive literature on grain refinement, there is not a consensus on the mechanism of grain refinement in aluminum
alloys. Recently, there has been a shift in understanding of the grain-refinement paradigm from purely being concerned with
the nucleation event, called here the “nucleant paradigm,” to also being concerned with the effect of solute elements, or,
the “solute paradigm,” on the final grain structure. This article is divided into two parts. In Part I, the literature underpinning
both paradigms is explained, and the validity of the paradigm shift toward the solute paradigm as a more complete understanding
of grain refinement is presented. Part II experimentally confirms the validity of the solute paradigm and details a mechanism
which explains the need for both effective nucleants and a solute of a good segregating power in order to obtain grain refinement. 相似文献
2.
To be able to determine the grain size obtained from the addition of a grain refining master alloy, the relationship between
grain size (d), solute content (defined by the growth restriction factor Q), and the potency and number density of nucleant particles needs to be understood. A study was undertaken on aluminium alloys
where additions of TiB2 and Ti were made to eight wrought aluminum alloys covering a range of alloying elements and compositions. It was found from
analysis of the data that
. From consideration of the experimental data and from further analysis of previously published data, it is shown that the
coefficients a and b relate to characteristics of the nucleant particles added by a grain refiner. The term a is related to the maximum density of active TiB2 nucleant particles within the melt, while b is related to their potency. By using the analysis methodology presented in this article, the performance characteristics
of different master alloys were defined and the effects of Zr and Si on the poisoning of grain refinement were illustrated. 相似文献
3.
Study of the mechanism of grain refinement of aluminum after additions of Ti- and B-containing master alloys 总被引:7,自引:0,他引:7
Mats Johnsson Lennart Backerud Geoffrey K. Sigworth 《Metallurgical and Materials Transactions A》1993,24(2):481-491
A highly sensitive thermal analysis technique has been used to study the mechanisms of grain refinement in high-purity aluminum.
Additions of Al-Ti-B master alloys were made both below and above the peritectic concentration in reference to the Al-rich
corner of the binary Al-Ti phase diagram (0.15 pct Ti in solution). The experiments were conducted at various times after
the addition of grain refiner. From the results, except for formation of TiB2, no effect of boron on the Al-rich portion of the binary Al-Ti phase diagram can be observed. With hypoperitectic additions
of Al-Ti-B master alloys, TiB2 particles are the most frequent nucleant for aluminum grains. Also, when Al-5Ti-lB additions are made, nucleation frequently
occurs above the equilibrium liquidus temperature. From a thermodynamic point of view, this phenomenon can occur only if regions
of the melt (which contain bondes and nucleate new grains) have a higher Ti concentration than is present in the bulk of the
liquid. A mechanism has been proposed to account for this observation. When hyperperitectic additions of grain refiner were
made, a metastable formation of Al solid was often observed to occur at 2 to 5 deg above the equilibrium peritectic temperature.
Other researchers have made this observation and proposed that a metastable aluminide phase was formed, even though no X-ray
evidence of this phase was found. The experiments reported here show that the metastable nucleation occurs on boride particles
when cooling from high temperature, which allow high (metastable) quantities of dissolved Ti to be retained in portions of
the melt. 相似文献
4.
The role of solute in grain refinement of magnesium 总被引:53,自引:0,他引:53
Y. C. Lee A. K. Dahle D. H. StJohn 《Metallurgical and Materials Transactions A》2000,31(11):2895-2906
The effect of separate solute additions of Al, Zr, Sr, Si, and Ca on grain size of Mg has been investigated. Increasing the
Al content in hypoeutectic Mg-Al alloys resulted in a continuous reduction in grain size up to 5 wt pct Al, reaching a relatively
constant grain size for higher Al contents (above 5 wt pct). The effect of Sr additions was investigated in both low- and
high-Al content magnesium alloys, and it was found that Sr had a significant grain refining effect in low-Al containing alloys
but a negligible effect on grain size in Mg-9Al. Additions of Zr, Si, and Ca to pure magnesium resulted in efficient grain
refinement. The grain refinement is mainly caused by their growth restriction effects, i.e., constitutional undercooling, during solidification, but the effect of nucleant particles, either introduced with the alloying
additions or as secondary phases formed as a result of these additions, may enhance the grain refinement. A brief review of
grain refinement of magnesium alloys is included in this article to provide an update on research in this field. 相似文献
5.
Xiaoming Wang Jie Song Wei Vian Haibin Ma Qingyou Han 《Metallurgical and Materials Transactions B》2016,47(6):3285-3290
In the grain refinement of aluminum, Al3Ti and TiB2 particles are introduced to reduce the casting grain size down to 200 micrometer level, which makes cold working possible. The particles are brought in by the addition of Al-Ti-B-type master alloys. It is generally believed that TiB2 particles are stable and nucleate α-Al grains in solidification in the presence of titanium in solution from the dissolution of Al3Ti particles in the master alloys. The titanium in solution either forms Al3Ti layers on the surface of TiB2 particles to promote the nucleation of α-Al grains or remains as solute to restrict the growth of α-Al grains in solidification. However, a consensus on a grain refinement mechanism is still to be reached due to the lack of direct observation of the three phases in castings. This paper presents finding of the TiB2/Al3Ti interfaces in an Al-Ti-B master alloy. It demonstrates a strong epitaxial growth of Al3Ti on the surface of TiB2 particles, a sign of the formation of an Al3Ti layer on the surface of TiB2 particles in grain refinement practice. The Al3Ti layer has a crystal coherency with α-Al and hence offers a substrate for heterogeneous nucleation of α-Al grains. However, the layer must be dynamic to avoid the formation of compounded Al3Ti and TiB2 particles leading to the loss of efficiency in grain refinement. 相似文献
6.
Influence of Solute Content and Solidification Parameters on Grain Refinement of Aluminum Weld Metal
Philipp Schempp Carl Edward Cross Andreas Pittner Michael Rethmeier 《Metallurgical and Materials Transactions A》2013,44(7):3198-3210
Grain refinement provides an important possibility to enhance the mechanical properties (e.g., strength and ductility) and the weldability (susceptibility to solidification cracking) of aluminum weld metal. In the current study, a filler metal consisting of aluminum base metal and different amounts of commercial grain refiner Al Ti5B1 was produced. The filler metal was then deposited in the base metal and fused in a GTA welding process. Additions of titanium and boron reduced the weld metal mean grain size considerably and resulted in a transition from columnar to equiaxed grain shape (CET). In commercial pure aluminum (Alloy 1050A), the grain-refining efficiency was higher than that in the Al alloys 6082 and 5083. Different welding and solidification parameters influenced the grain size response only slightly. Furthermore, the observed grain-size reduction was analyzed by means of the undercooling parameter P and the growth restriction parameter Q, which revealed the influence of solute elements and nucleant particles on grain size. 相似文献
7.
K. R. Ravi S. Manivannan G. Phanikumar B. S. Murty Suresh Sundarraj 《Metallurgical and Materials Transactions A》2011,42(7):2028-2039
Although the grain-refinement practice is well established for wrought Al alloys, in the case of foundry alloys such as near
eutectic Al-Si alloys, the underlying mechanisms and the use of grain refiners need better understanding. Conventional grain
refiners such as Al-5Ti-1B are not effective in grain refining the Al-Si alloys due to the poisoning effect of Si. In this
work, we report the results of a newly developed grain refiner, which can effectively grain refine as well as modify eutectic
and primary Si in near eutectic Al-Si alloys. Among the material choices, the grain refining response with Al-1Ti-3B master
alloy is found to be superior compared to the conventional Al-5Ti-1B master alloy. It was also found that magnesium additions
of 0.2 wt pct along with the Al-1Ti-3B master alloy further enhance the near eutectic Al-Si alloy’s grain refining efficiency,
thus leading to improved bulk mechanical properties. We have found that magnesium essentially scavenges the oxygen present
on the surface of nucleant particles, improves wettability, and reduces the agglomeration tendency of boride particles, thereby
enhancing grain refining efficiency. It allows the nucleant particles to act as potent and active nucleation sites even at
levels as low as 0.2 pct in the Al-1Ti-3B master alloy. 相似文献
8.
H. Li A. McLean J. W. Rutter I. D. Sommerville 《Metallurgical and Materials Transactions B》1988,19(3):383-395
Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization
behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg
1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding
time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions,
a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder
additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the lévitation melting method.
Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects
of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that
rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved
cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with
Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis.
The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor
in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed,
which can successfully explain the experimental results.
on research leave from Beijing University of Iron and Steel Technology 相似文献
9.
Jichun Dai Mark A. Easton Mingxing Zhang Dong Qiu Xiangyuan Xiong Wencai Liu Guohua Wu 《Metallurgical and Materials Transactions A》2014,45(10):4665-4678
The effect of Al additions on grain refinement of Mg-Gd-Y alloys with different solute contents at different cooling rates has been investigated. For all alloys, significant grain refinement was due to the formation of Al2(Gd x Y1?x ) nucleant particles. The number density and size distribution of Al2(Gd x Y1?x ) were affected by both solute content and the cooling rate. Grain sizes (d gs) of Mg-Gd-Y base alloys and of Mg-Gd-Y-Al alloys were related to solute content (defined by the growth restriction factor, Q), cooling rate ( \( \dot{T} \) ), and area number density (ρ ns) and size (d p) of nucleant particles that can be activated. It is found that grain sizes of Mg-Gd-Y base alloys follow the relationship \( d_{\text{gs}} = a + \frac{b}{{Q\sqrt {\dot{T}} }} \) , while grain sizes of Al-refined samples follow the relationship \( d_{\text{gs}} = \frac{a'}{{\sqrt {\rho {}_{\text{ns}}} }} + \frac{b'}{{\sqrt {\dot{T}} Qd_{\text{p}} }} \) , where a, b, a′, and b′ were constants. In addition, the grain refinement effect of Al additions was more susceptible to solute content and the cooling rate than that of Zr which is regarded as the most efficient grain refiner for Mg alloys. 相似文献
10.
In view of the continuing interest in the solidification characteristics of dilute Al-Ti alloys,1 a recent study2 of the mechanism(s) of grain refinement induced in high purity aluminum by various additions of master alloys containing Ti, B, Cr, Mo, V and Zr is reported. For alloys containing more than 0.2 wt pct Ti, TiAl3 was shown to be commonly a nucleant, in both Al-Ti and Al-Ti-B systems. In addition, it was found that the nature of the master alloys is important in determining the degree of grain refinement a given alloying addition will produce. A “saturation” effect is reported for additions of Ti, and ti-B,i.e., further additions of titanium beyond a given level do not provide any further reduction in grain size. 相似文献
11.
H. Li A. McLean J. W. Rutter I. D. Sommerville 《Metallurgical and Materials Transactions B》1988,19(2):383-395
Two series of experiments have been conducted to determine the influence of rare earth additions on the nucleation and crystallization
behavior of pure iron and 1045 steel. In the first series, additions of rare earth suicide or cerium dioxide powder to two-Kg
1045 steel ingots indicated that rare earth suicide can refine the as-cast structure of such ingots. However, if the holding
time after rare earth silicide addition is over two minutes, the grain refinement decreases. With cerium dioxide additions,
a relatively large columnar zone was obtained. In the second series, the effects of cerium metal or cerium dioxide powder
additions on the degree of undercooling obtainable in pure iron and 1045 steel were examined by the lévitation melting method.
Surface tension measurements of the levitated droplets were carried out at the same time to investigate the possible effects
of surface tension variations on the nucleation and crystallization behavior of the metals. The experimental data show that
rare earth inclusions can greatly reduce the degree of undercooling of iron and steel, and that a small amount of dissolved
cerium can further reduce the degree of undercooling of levitated droplets. The structure and reaction products obtained with
Fe-Ce levitated droplets were examined with both optical and scanning electron microscopy as well as X-ray diffraction analysis.
The experimental results clearly indicated that cerium solute redistribution during solidification is the dominant factor
in refining the as-cast structure. A nucleation and solidification model for the Fe-Ce levitated droplets has been developed,
which can successfully explain the experimental results.
on research leave from Beijing University of Iron and Steel Technology 相似文献
12.
Al-Ti-B-Er refiner was successfully prepared by CR (contact reaction process), a process based on SHS (self propagating high-temperature synthesis). The microstructure of the alloy was studied by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy-dispersive spectrometry. The results showed that Al-Ti-B-Er alloy was composed of a-Al, block-like TiAl3 and flocked TiB2. Compared with Al-Ti-B refiner, formation of TiAlEr compounds, Er modified the morphology of TiAl3 phase, and dispersed the TiB2 and TiAl3. An excellent grain refining performance was obtained when adding 1 wt.% Al-Ti-B-Er in Al-10Zn-1.9Mg-1.6Cu-0.12Zr alloy, the average grain size was about 40 µm. The refinement mechanism of Al-Ti-B-Er was also discussed. Er changed the morphology of TiAl3, TiB2 phase, the refiner would be more efficient. The decomposition of TiAlEr compounds which released Er refrained the growth of TiAl3 and made TiB2 difficult to aggregate or deposit, therefore resulted in more particles being efficient nucleation substrate. 相似文献
13.
The homogeneity of as‐cast structures is essential to the application of advanced steel materials. The relationship between as‐cast equiaxed grain structure and homogeneity was examined by means of OPA (Original Position Analysis) technology. The results indicate that macrosegregation of the solute elements was obviously suppressed in silicon steel billets with equiaxed grain structure. Furthermore, two methods to achieve an equiaxed grain structure were described. One is to use titanium‐based inoculation technology. TiN particles and TiN/ Ti2O3 complex particles can precipitate at the early stage of solidification and then play an important role of heterogeneous nucleation when the solidification condition is carefully controlled. The other method is to maintain a small temperature gradient in the mould to enhance the constitutional undercooling As a result, the as‐cast equiaxed grain structure was well developed in the experimental billets. 相似文献
14.
W. Khalifa F. H. Samuel GM-NSERC-UQAC J. E. Gruzleski H. W. Doty S. Valtierra 《Metallurgical and Materials Transactions A》2005,36(4):1017-1032
Nucleation of Fe-intermetallic phases (i.e. binary Al-Fe, α-AlFeSi, β-AlFeSi, δ-AlFeSi, and q1-AlFeSi phases) on the surface of different inclusions in six experimental Al-Si-Fe alloys was studied through a quantitative
evaluation of the number of inclusion particles that have a direct physical contact with the nucleated phase as seen through
the optical microscope. It was found that nucleation of each of the Fe-intermetallic phases was promoted on the surface of
several inclusions under the same conditions of alloy composition and cooling rates. Some inclusions exhibited high potency
for the nucleation of particular Fe-intermetallic phases under certain conditions and poor potency under other conditions.
The potent nucleants for the primary α-Al phase such as γ-Al2O3 exhibited poor potency for the nucleation of the Fe-intermetallic particles that lie within the primary phase (intragranular
particles). Reactive inclusions such as CaO and SiC are very potent nucleants for the intragranular Fe-intermetallic phase
particles. The nucleation of the Fe-intermetallic phases in Al-Si-Fe alloys obeys the general features of nucleation, in particular,
the effect of cooling rate and solute concentration on the potency of the nucleant particles: (1) it was observed that increasing
the cooling rate enhances the heterogeneous nucleation of the Fe-intermetallic phases on the surface of different inclusions,
and (2) the nucleation potency of inclusion particles in both α-Al and interdendritic regions improves with increasing solute concentration up to a certain level. Above this level, the
solute concentration poisons the nucleation sites. Nucleation of the Fe-intermetallics in the alloys studied does not seem
to be largely affected by the type of the nucleating surface. 相似文献
15.
Grain refinement of magnesium alloys 总被引:11,自引:0,他引:11
David H. StJohn Ma Qian Mark A. Easton Peng Cao Zoë Hildebrand 《Metallurgical and Materials Transactions A》2005,36(7):1669-1679
The literature on grain refinement of magnesium alloys is reviewed with regard to two broad groups of alloys: alloys that
contain aluminum and alloys that do not contain aluminum. The alloys that are free of aluminum are generally very well refined
by Zr master alloys. On the other hand, the understanding of grain refinement in aluminum bearing alloys is poor and in many
cases confusing probably due to the interaction between impurity elements and aluminum in affecting the potency of nucleant
particles. A grain refinement model that was developed for aluminum alloys is presented, which takes into account both alloy
chemistry and nucleant particle potency. This model is applied to experimental data for a range of magnesium alloys. It is
shown that by using this analytical approach, new information on the refinement of magnesium alloys is obtained as well as
providing a method of characterizing the effectiveness of new refiners. The new information revealed by the model has identified
new directions for further research. Future research needs to focus on gaining a better understanding of the detailed mechanisms
by which refinement occurs and gathering data to improve our ability to predict grain refinement for particular combinations
of alloy and impurity chemistry and nucleant particles.
This article is based on a presentation made in the symposium entitled “Phase Transformations and Deformation in Magnesium
Alloys,” which occurred during the Spring TMS meeting, March 14–17, 2004, in Charlotte, NC, under the auspices of ASM-MSCTS
Phase Transformations Committee. 相似文献
16.
W. Khalifa F. H. Samuel J. E. Gruzleski 《Metallurgical and Materials Transactions A》2004,35(10):3233-3250
Systematic inoculation experiments were carried out to study the influence of various inclusions on the nucleation of the
α-Al phase in Al-Si-Fe alloys at different cooling rates. The results showed that in dilute alloys, containing less than 1.5
pct Si+Fe, almost all the inclusion types have high percentages of occurrence within the α-Al phase, indicating that nucleation can be promoted on the surface of such inclusions. In a hypoeutectic Al-Si alloy containing
6.3 pct Si, the inclusion particles of MgO, TiB2, TiC, α-Al2O3, and SiC become mostly inactive nucleants and are pushed to the interdendritic regions because of the dominating poisoning
effect of Si. The current results were used successfully to explain the efficiency differences between the commercial grain
refiners in the hypoeutectic Al-Si alloys. Silicon is observed to preferentially segregate to the liquid-Al/inclusion interfaces
so as to lower the free energy of such interfaces. A theoretical analysis of the poisoning effect of Si showed that Si segregation
to the liquid/nucleant interface alters the interfacial energy balance so that the catalytic efficiency of the nucleant particles
is dramatically reduced. Careful analysis showed that the poisoning effect of Si in the hypoeutectic alloy is overcome when
the nucleant particles have active surface characteristics, as represented by the high catalytic potencies of γ-Al2O3, CaO, and Al4C3 particles in nucleating the α-Al phase of the hypoeutectic Al-Si alloy. Although some inclusions have comparable or higher occurrence levels than TiB2 in the α-Al phase, they cannot be used as efficient nucleants because of either their poor wettability with liquid aluminum or their
chemical reactivity, which can change the alloy chemistry. 相似文献
17.
Houshang D. Alamdari Dominique Dub�� Pascal Tessier 《Metallurgical and Materials Transactions A》2013,44(1):388-394
The mechanism of heterogeneous grain refining of aluminum by ultrafine elemental boron particles was investigated. In order to facilitate the observation of the boron-aluminum interface, a boron filament was introduced in a melt at 1013 K (740 °C) containing different levels of Ti. The Al/B interface was studied using transmission electron microscopy and different phases were identified using the electron diffraction method. The experimental results showed that boron is dissolved in pure aluminum while its dissolution is inhibited in presence of titanium solute. A thin layer of TiB2 formed at the surface of boron thickens with residence time in the melt. The mechanisms by which aluminum is crystallized on boron are discussed. 相似文献
18.
Ultrarapid cooling may result in material with extremely fine grain size and interdendritic spacing, or containing supersaturated
solutions or nonequilibrium phases. Unfortunately, ultrarapid cooling cannot be obtained in large masses of metal; thus the
interesting properties cannot be exploited commercially. The same structures can be obtained by denucleation, that is removal
or neutralization of the nucleants that act at low undercooling. A denucleated melt, regardless of its size, can be undercooled
to a temperature well below its freezing point, where it can be made to freeze rapidly on a chosen nucleant. Electro-lytically
refined aluminum crystallizes normally with a fairly coarse grain at 1 to 2°C undercooling. By removing all nucleants with
less than 5°C undercooling a grain size of less than 1 mm diam was achieved in a furnace cooled sample. In aluminum-manganese
alloys, denucleation produces refinement of the grain structure in continuously cast billets and retention in solution of
a higher percentage of manganese, in pure aluminum columnar grain and crystallization twins are found in plaster molds. By
denucleation of aluminum-iron alloys, the FeAl6 phase can be made to appear in samples cooled at commercial rates of a few °C/sec and eutectic structures can be obtained
in alloys with 4 pct Fe. In cast iron removal of nucleants with low undercooling shifts the crystallization from primary graphite
to primary austenite.
Formerly Metallurgist, Revere Copper and Brass Incorporated 相似文献
19.
Michael P. De Cicco Lih-Sheng Turng Xiaochun Li John H. Perepezko 《Metallurgical and Materials Transactions A》2011,42(8):2323-2330
Different types of nanoparticles in aluminum (Al) alloy A356 nanocomposites were shown to catalyze nucleation of the primary
Al phase. Nanoparticles of SiC β, TiC, Al2O3 α, and Al2O3
γ were added to and dispersed in the A356 matrix as nucleation catalysts using an ultrasonic mixing technique. Using the droplet
emulsion technique (DET), undercoolings in the nanocomposites were shown to be significantly reduced compared to the reference
A356. None of the nanocomposites had a population of highly undercooled droplets that were observed in the reference samples.
Also, with the exception of the A356/Al2O3 α nanocomposite, all nanocomposites showed a reduction in undercooling necessary for the onset of primary Al nucleation.
The observed nanocomposite undercoolings generally agreed with the undercooling necessary for free growth. The atomic structure
of the particles showed an influence on nucleation potency as A356/Al2O3 γ nanocomposites had smaller undercoolings than A356/Al2O3 α nanocomposites. The nucleation catalysis illustrates the feasibility of, and basis for, grain refinement in metal matrix
nanocomposites (MMNCs). 相似文献
20.