Recrystallization developed in the largely undercooled Ni54.6Pd45.4 alloy

The structure evolution driven by recrystallization in the bulk undercooled Ni_54.6Pd_45.4 alloy was investigated by quenching the sample at different cooling stages. It is found that recrystallization of the solid beyond the critical undercooling for grain refinement starts at the end of rapid solidification, and develops during the slow solidification and the subsequent cooling process. At the initial stage of recrystallization, nuclei successively form in the sample, leading to a decrease in grain size. At the later stage, annexation of grains dominates the recrystallization, due to which the grain size increases. As undercooling prior to solidification increases, the deformation degree in the rapidly solidified dendrite net rises, and the recalescence temperature as well as the duration at high temperature decreases. Consequently, the grain size of the recrystallization structure decreases.     

Correlation of recalescence with grain refinement of magnesium alloys

The grain refinement of Mg-Al based alloys with carbon inoculation was investigated by a computer-aided cooling curve analysis(CA-CCA) system.The results show that carbon inoculation decreases the main parameters of the recalescence regime during the initial stage of solidification.These parameters include the recalescence undercooling(Δθ_(rec)),duration of recalescence (t_(rec)),and liquid peak parameter(LPP) which is firstly introduced into magnesium alloys.The resultant grain size decreases with incre...     

The origin of anomalous eutectic structures in undercooled Ag–Cu alloy

A melt encasement (fluxing) method was used to undercool Ag–Cu alloy at its eutectic composition. The recalescence of the undercooled alloy was filmed at a high frame rate. For undercoolings <60 K, a microstructure consisting of mixed anomalous and lamellar eutectic is observed. Analysis of eutectic spacing in the lamellar eutectic reveals little dependence upon the undercooling of the bulk melt and is consistent with growth at an undercooling of 1.5 K. Depending upon undercooling, the progress of the recalescence front may be either continuous or spasmodic, wherein periods of rapid growth are separated by significant interludes in which growth totally arrests. Analysis of spot brightness profiles reveals that, during continuous growth, the recalescence is characteristic of the advancement of a planar, space-filling front, while a double recalescence occurs during spasmodic growth, the first of which is characteristic of the propagation of a dendritic, or non-space-filling, front. It is concluded that, during spasmodic growth, the propagation of two-phase, or eutectic, dendrites is observed, which subsequently remelt to form the anomalous eutectic, while the lamellar eutectic grows during post-recalescence cooling.     

Assessment of solidification characteristics of carbon-inoculated Mg-3%Al melt by thermal analysis

The Mg-3%Al melt was inoculated by carbon with different holding time. The effect of holding time on grain refining efficiency was evaluated. The solidification characteristics of the carbon-inoculated Mg-3%Al melt with different holding time were assessed by computer-aided cooling curve analysis. The results showed that Mg-3%Al alloy could be effectively refined by carbon inoculation. Slight fading phenomenon occurred with increasing the holding time to 60 min. Carbon inoculation could significantly influence the shape of cooling curves of Mg-3%Al melt. The nucleation starting and minimum temperatures increased. The recalescence undercooling and duration decreased to almost zero after carbon inoculation. The grain refining efficiency of carbon inoculation could be assessed by the shape of the cooling curve and solidification characteristic parameters including nucleation starting and minimum temperatures, recalescence undercooling and duration.     

过冷Ni-15％Cu合金组织与微观织构演化规律

采用熔融玻璃净化和循环过热方法实现Ni-15％Cu（摩尔分数）合金的深过冷。采用再辉后自然冷却和水淬两种方式研究凝固冷却对凝固组织和微观织构的影响。在小过冷度下,自然冷却条件下晶粒细化组织呈随机位向,而快淬条件下晶粒细化组织呈集中位向,但均无退火孪晶;在大过冷度下,晶粒细化组织呈随机位向,大量退火孪晶出现,再结晶和晶粒长大发生。分析表明：在小过冷度和大过冷度下的晶粒细化组织的微观织构形成过程中,对流和再结晶起重要作用。     

深过冷Ni-21.4%Si共晶合金的凝固特性研究

对熔融玻璃净化后深过冷Ni-21.4%Si（原子分数,下同）共晶合金的凝固特性进行了实验研究,并对其均质形核过冷度进行了理论预测.结果发现,采用熔融玻璃净化可使Ni-21.4%Si共晶合金获得318 K的过冷度.理论计算表明,此过冷度达到了Ni-21.4%Si共晶合金的均质形核过冷度.Ni-21.4%Si共晶合金凝固特性与过冷度△T有关:当过冷度小于250 K时,冷却曲线有2个再辉峰,其中当过冷度小于206 K时,凝固组织由Ni3Si相和规则共晶组成,当过冷度在206 K到250 K之间时,凝固组织由α-Ni相和规则共晶组成;过冷度大于250 K后,冷却曲线只有1个再辉峰,凝固组织为反常共晶.过冷度会影响初生相Ni3Si的生长方式.随着过冷度的增大,初生相Ni₃Si的生长会由小平面生长方式转为非小平面生长方式.     

Effect of strontium and solidification rate on eutectic grain structure in an AI-13 wt% Si alloy

The influence of strontium addition and solidification rate on eutectic grain structure in a near-eutectic AI-Si alloy was investigated. The characteristic temperature of eutectic nucleation (TN),minimum temperature prior to recalescence (TM),and the growth temperature (TG) during cooling were determined by quantitative thermal analysis. All characteristic temperatures were found to decrease continuously with increasing Sr content and solidification rate. Microstructural analysis also revealed that the eutectic grain size decreases with increasing Sr content and solidification rate. Such eutectic grain refinement is attributed to the increased actual under-cooling ahead of the liquid/solid interface during solidification.     

深过冷Ni—50%Cu合金的晶粒细化

Ｎｉ－５０％Ｃｕ合金随过冷度的增加发生两次明显的晶粒细化过程,第一次产生于５０－１００Ｋ的过冷度范围内,第二次发生于临界过冷度２２０Ｋ之后,实验首次发现高冷度下细化合金的晶粒内含有枝晶亚结构。理论分析指出,枝晶重熔倾向的大小可用枝晶主干中最先析出的固相在再辉过程中的无量纲过热度来衡量,该无量钢过热度随原始过冷度的增加先增后减,其最大值对应于第一次细化发生的过冷度区间。热力学计算和组织分析表明,小过     

过冷Ni-1at%Fe合金晶粒细化及再结晶机理分析

通过熔融玻璃净化与循环过热相结合的方法研究了过冷Ni-1at%Fe合金的微观组织演化及再结晶。通过再辉后快淬和空冷实验对比,证明大过冷下的细化组织是再结晶的产物。结合无量纲过热度的计算,系统考察了合金凝固组织随过冷度的演化规律。研究发现合金的凝固组织在所获得的过冷度范围内发生了两次晶粒细化,第一次细化归因于枝晶的熟化和重熔导致的高度发达树枝晶碎断,第二次细化归因于应力积累导致的再结晶。     

Bi-20%Sb合金不同熔体状态下的凝固行为及组织

前期研究发现,升温过程中Bi-20%Sb合金熔体在829～1107℃区间存在不可逆的电阻率温度异常行为。据此,分别选取700℃和900℃进行等温电阻率试验,在700℃保温电阻率随时间无变化,而在900℃保温过程中电阻率在37～54min发生了明显降低,揭示熔体结构状态发生了改变。根据这一结果进行凝固试验,发现在相同的冷却条件下经历过熔体结构转变试样的冷却速度较慢,凝固组织明显细化;两种熔体状态下的过冷度与冷却速率分别满足分段线性关系;两种熔体状态下单位面积晶粒数和冷却速率均满足良好的线性关系。凝固过冷度增加,是因为合金熔体在结构转变后原子团簇变得更加细小均匀,需要降到更低温度时才可以发生形核。冷却速度变慢,是因为热导率随着混乱度的增加而减小,经历过熔体结构转变的熔体在降温的过程中热导率减小。     

气雾化镍基高温合金粉体的过冷度和显微组织特征

雾化技术是一种获得微细球形合金粉体的有效方法,其中雾化过程中的过冷度是影响粉体性能的重要因素。本文借助DSC等实验手段,研究了粉体尺寸和冷却速度对粉体过冷度和显微组织的影响,以及粉体尺寸,冷却速度和过冷度直间的关系。结果表明,粉体尺寸和冷却速度越小,粉体冷却时的过冷度越大。同时,较大的过冷度会显著降低粉体中树枝晶的臂间距。另外,粉体尺寸越小,粉体中的胞状晶的比例越高,晶粒的尺寸也显著减小。     

反向扩散效应对过冷单相固溶体合金凝固动力学的影响(英文)

基于平均体积方法建立过冷单相固溶体合金的凝固动力学模型并研究反向扩散在凝固动力学过程中的作用。模型在过冷Ni-15%Cu(摩尔分数)合金快速凝固中的应用表明:反向扩散显著影响凝固结束温度但对再辉阶段凝固的固相体积分数影响不明显。与Herlach观点相反,凝固结束温度介于杠杆定律和Scheil方程的预测值之间,且其具体值由反向扩散、初始过冷度和冷却速率决定。     

RECALESCENCES AND SOLIDIFICATION MICROSTRUCTURE OF HIGHLY UNDERCOOLED Ni_(68)B_(21)Si_(11) ALLOY

One,two and three recalescence events in the solidification of highly undercooledNi_68B_21Si_11 alloy melt and the conditions of undercooling together with the effect onsolidification morphologies were revealed.The solidification morphologies of the undercooledalloy may be examined in the light of two aspects,i.e,growth of either Ni_3B,Ni_6Si_2B andNi_2B ternary eutectic or dendrite cluster with Ni_3B as leading phase.When the degree ofundercooling below 200 or over 310 K,the alloy solidifies to accompany one recalescence only,and the crystals grow according to the first mode of solidification.While undercooling at othertemperatures,two or three events of recalescence may occur and the dendrite cluster may so-lidify.     

Modeling the overall solidification kinetics for undercooled single-phase solid-solution alloys. II. Model application

An overall solidification kinetic model was applied to undercooled Ni–15 at.% Cu alloy. A good agreement between the model predictions and the measured cooling curves was obtained by adopting a “phenomenological” heat boundary condition. Applying numerical calculations, it was demonstrated that solute is uniformly distributed at the purely thermal-controlled growth stage; a transition from non-equilibrium to near-equilibrium solidification occurs at the mainly thermal-controlled growth stage only if sufficiently high initial undercooling is available; and a transition from recalescence to post-recalescence occurs at the solutal-controlled growth stage wherein the current model reduces to Scheil’s equation. The volume fraction solidified during recalescence is the same as the ratio of the initial undercooling to the hypercooling limit, and solidification should end generally at a temperature between the prediction of Scheil’s equation and that of the Lever rule if back diffusion is considered.     

Effect of cooling rate on eutectic cell count, grain size, microstructure, and ultimate tensile strength of hypoeutectic cast iron

This article describes a series of microstructural and strength studies performed on hypoeutectic cast iron, which was sand cast using a variety of end chills (metallic, nonmetallic, water-cooled, and subzero, respectively). The effects of cooling rate on the eutectic cell count (ECC), grain size, and the ultimate tensile strength (UTS) were evaluated. Attempts were also made to explain these effects and to correlate the UTS with ECC. It was found that subzero chilled and water-cool, chilled cast iron exhibit severe undercooling compared to normal sand cast iron. It was concluded from this investigation that nucleation conditions are completely altered but growth conditions prevail as usual. Therefore, undercooling during solidification is considered to be responsible for variation in ECC, grain size, microstructure, and tensile strength.     

A cellular automaton investigation of the transformation from austenite to ferrite during continuous cooling

A cellular automaton model has been employed to investigate the transformation from austenite to ferrite in low carbon steels during continuous cooling. An important aspect of this approach is the implementation of incorporating local concentration changes into a nucleation or growth function, which is utilized by the automaton in a probabilistic fashion. The modeling gives a visual insight into the effect of cooling conditions on this transformation. The final nucleation number, the number of ferrite grains per austenite grain, ferrite grain size and the kinetics of ferrite formation are obtained as a function of the cooling rate or the undercooling temperature.     

深过冷合金再辉与凝固组织的研究

发现深过冷Ni_(68)B_(21)Si_(11)合金液形核过程中的多次再辉现象,揭示了二次和三次再辉的过冷条件,以及对凝固组织的影响。研究结果表明,深过冷Ni_(68)B_(21)Si_(11)合金液存在两种凝固机制:一是NilB,Nj‘si,B和Ni,B的三相共晶生长;另一个是以NilB为领先相的枝晶簇生长。当过冷度低于200K或超过310K时,合金液凝固仅伴生一次再辉,其晶体生长以第一种凝固方式进行。其他过冷范围内的合金液则发生多次再辉和枝晶簇凝固。     

Numerical simulation of recalescence of 3-dimensional isothermal solidification for binary alloy using phase-field approach

A accelerated arithmetic algorithm of the dynamic computing regions was designed, and 3-dimensional numerical simulation of isothermal solidification for a binary alloy was implemented. The dendritic growth and the recalescence of Ni-Cu binary alloy during the solidification at different cooling rates were investigated. The effects of cooling rate on dendritic patterns and microsegregation patterns were studied. The computed results indicate that, with the increment of the cooling rate, the dendritic growth velocity increases, both the main branch and side-branches become slender, the secondary dendrite arm spacing becomes smaller, the inadequate solute diffusion in solid aggravates, and the severity of microsegregation ahead of interface aggravates. At a higher cooling rate, the binary alloy presents recalescence; while the cooling rate is small, no recalescence occurs.     

冷却速度对超高强马氏体钢的马氏体相变起始温度和硬度的影响

采用热膨胀法测量了马氏体试验钢在20~70 ℃/s冷却速度下的马氏体相变起始温度(Ms点温度),并研究了其对试验钢硬度的影响。结果表明,冷却速度的提高可增加奥氏体强度、提高奥氏体的热稳定性,进而降低马氏体相变起始温度。同时,更高的冷却速度容易造成晶格畸变,增大位错密度,还会提高过冷度,细化有效晶粒尺寸,使试验钢的硬度提高。     

Grain refinement of Al-Cu alloy in low voltage pulsed magnetic field

The effect of a low voltage pulsed magnetic field (LVPMF) on grain refinement of Al-Cu alloy was investigated at different solidification stages. The cooling curve was also studied to investigate the grain refinement mechanism of LVPMF. The fine grains are obtained by applying the LVPMF during the nucleation stage. The LVPMF has no obvious influence on the solidification structure when it is applied during liquid phase stage or crystal growth stage. Application of LVPMF increases the nucleation temperature of the isomorphous transformation, and also decreases the recalescence magnitude of the alloy. The refining mechanism was proposed that the LVPMF provides extra energy for nucleation, which decreases the energy barrier and the critical radius for nucleation, leading to high nucleation rate and grain refinement.