Metastable phase diagrams of Cu-based alloy systems with a miscibility gap in undercooled state

Some Cu-based alloy systems with a large positive enthalpy of mixing display a eutectic or peritectic phase diagram under equilibrium conditions, but show a metastable liquid miscibility gap in the undercooled state. When the melt is undercooled below certain temperature beyond the critical liquid-phase separation temperature, it separates into two liquids with different compositions. The compositions of the two liquids change successively upon the metastable phase diagram before solidification occurs. The shape and position of the metastable miscibility gap are dependent of the alloy components and their interaction features. This study reviews the metastable phase diagrams of Cu-based alloy systems, which are derived from experiments and thermodynamic calculations.     

High‐cycle fatigue and fracture behaviours of Cu‐Be alloy with a wide strength range

The high‐cycle fatigue and fracture behaviours of Cu‐Be alloy with tensile strength ranging from 500 to 1300 MPa acquired by different treatments were studied. Fatigue crack initiation, fracture surface morphologies, S‐N curves and fatigue strength show obvious differences due to the change of microstructure. At relatively low‐strength level, some fatigue cracks originated from defects; while at high‐strength level, all the fatigue cracks initiated from cleavage facets. It was found that the fatigue ratio increases linearly and fatigue strength changes quadratically with increasing tensile strength, only considering one strengthening mechanism. Finally, the fatigue strengths of various Cu‐Be alloys were summarized.     

Interface morphology evolvement and microstructure characteristics of hypoeutectic Cu–1.0 wt%Cr alloy during unidirectional solidification

Effect of unidirectional solidification rate on microstructure of hypoeutectic Cu–1.0%Cr alloy was investigated. The microstructure evolution of Cu–1.0%Cr alloy was noticed especially during the unidirectional solidification with the different solidification rates. It is shown that eutectic (α+β) and primary α(Cu) phase grew up equably in parallel to direction of solidification. A kind of fibriform microstructure will appear when unidirectional solidification rate is up to some enough high certain values. When temperature gradient was changeless, the interface morphology evolution of the primary α(Cu) phase underwent to a series of changes from plane to cell, coarse dendrite, and fine dendrite grains with increasing the solidification rates. Primary dendrite arm spacing λ1 of α(Cu) phase increases with increasing the solidification rate where the morphology of the solid/liquid (S/L) interface is cellular. However, λ1 decreases with further increasing the solidification rate where the S/L interface morphology is changed from cell to dendrite-type. Its rule might accord with Jackson–Hunt theory model. An experience equation obtained is as follows: . On the other hand, secondary dendrite spacing λ2 of primary α(Cu) phase will thin gradually with increasing the solidification rate. Moreover, secondary dendrite will become coarse in further solidification. Another experience equation about relationship among secondary dendrite arm spacing (λ₂), temperature gradient G_L and the velocity of the S/L interface (V) is that: λ₂=−0.0003+0.0027(G_LV)^−1/3. In addition, the volume fraction of eutectic will decrease with the increase of solidification rate.     

Modeling of free eutectic growth and competitive solidification in undercooled near-eutectic alloys based on in situ measurements

Free eutectic growth and its competition with single-phase growth in solidification of undercooled near-eutectic alloys are not yet fully understood. In this paper, the historical development of eutectic growth models was reviewed. The LZ model of free eutectic growth was evaluated using recent data of eutectic growth velocities in an undercooled Ni_81.3Sn_18.7 eutectic composition. An excellent agreement was achieved between the LZ model and the data. Crystal growth velocities in off-eutectic Ni₈₃Sn₁₇ and Ni₈₀Sn₂₀ compositions were measured using a high-speed camera technique. The present data of the off-eutectic compositions and the recent data of the eutectic composition were modeled using the LZ model and the LKT/BCT model of free dendritic growth. The modeling revealed that the competition between the free eutectic growth and the single-phase growth is controlled by the highest interface temperature criterion. A coupled zone of the α-Ni-Ni₃Sn eutectic was calculated using this criterion. The coupled zone agrees well with studies of solidified structures of undercooled samples.     

Stress induced deformation in the solidification of undercooled Co80Pd20 alloys

Substantial undercooling ΔT up to 415 K was achieved for Co₈₀Pd₂₀ melt applying molten glass denucleation combined with cyclic superheating. The as-solidified structure as function of ΔT was described concisely. On this basis, dense-regular fault (DRF) ribbons were detected provided if a critical undercooling is surpassed. An integrated analysis of the DRF formation with a model calculation [16] for shrinkage-stress developed in the coherent dendrite network upon rapid solidification was then performed. This confirms that the as-formed DRF originates from a stress-induced deformation, which also plays an important role in understanding the grain refinement occurring upon rapid solidification of undercooled melts.     

Effect of aluminum content on solidification microstructure and properties of Fe‐Cr‐B‐Al alloys

The microstructures and mechanical properties of eight kinds of Fe‐Cr‐B‐Al alloys containing X wt.%Al‐0.35 wt.%C‐10.0 wt.%Cr‐1.4 wt.%B‐0.6 wt.%Si‐0.8 wt.%Mn (X = 0, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 4.0) were studied by means of optical microscopy (OM), scanning electron microscopy (SEM), X‐ray diffraction (XRD), Rockwell hardness and Vickers micro‐hardness testers. The results indicate that the as‐cast microstructure of aluminium‐free sample consists of the martensite, austenite and eutectic borocarbides, and the eutectic borocarbides are the mixture of (Fe, Cr)₂B and (Cr, Fe)₇(C, B)₃, and its hardness reaches 65 HRC. When a small amount of aluminium element (Al ? 1.0 wt.%) is added, the phase composition has no significant change, and the hardness excels 65 HRC. When the concentration of aluminium reaches 1.5 wt.%, the matrix of Fe‐Cr‐B‐Al alloy becomes pearlite and δ‐ferrite, leading to a sharply decrease of the hardness. The proportion of ferrite goes up along with increasing aluminium concentration, and the hardness of Fe‐Cr‐B‐Al alloy has slight decrease.     

Directional solidification of dendritic microstructures in microgravity and with forced melt flow

A dendritic microstructure is characteristic for many metallic alloys used in practical application. Mainly the heat and mass transport in the melt in front of the growing solid-liquid interface affects the microstructure. Purely diffusive transport conditions as realized in microgravity environment produce larger primary spacing than found in earth-grown samples. Forced melt flow generated by a rotating magnetic field results in shorter primary spacing and a significant change of the dendrites’ shapes.     

Influences of microstructure and orientation on fracture toughness of intermetallic phase Al2Cu‐based alloy under directional solidification

The influences of microstructures and orientations on fracture toughness of intermetallic phase Al₂Cu‐based alloys during different directionally solidified rates were investigated. With solidified rates increasing, the patterns of Al₂Cu phase dendrite turned from faceted V‐shaped morphology to elongated plate‐shaped morphology and discontinuous complex morphology in longitudinal section. Moreover, the deviation angle between the growth direction of Al₂Cu dendrite and the heat flow direction was increasing. Because of fine grain toughening and crack propagation path deflected by orientation, the fracture toughness of Al₂Cu‐based alloy was improved. The experimental results showed that improving the brittleness could be well achieved under higher directionally solidified rate.     

Anomalous creep behaviour of a Ni-22 at % Cu alloy and the miscibility gap

The purpose of this study is to understand the anomalous creep behaviour of Ni-22 at % Cu alloy at the suggested critical miscibility gap temperature, below 598 K (0.36T _m). The Cu-Ni system is classified as a class II solid solution at temperatures above 0.4T _m, and it is also experimentally verified by the authors that the characteristic creep behaviour of the alloy used for this work is that for a class II solid solution. However, at low temperatures, this particular alloy shows different creep behaviours, with small stress increment in the steady state, sigmodial creep deformation is observed while with large stress increases normal primary creep occurs. When unloading the stress during creep and ageing at the test temperature, no softening due to recovery is observed but the same creep rate is achieved. The activation energy of the creep for the quenched and aged specimen is anomalously high, 326 kJ mol^–1, however, for the annealed specimen it was 167 kJ mol^–1 which is the same for that of pipe diffusion. On the basis of the observed experimental results and proper analysis, it is hypothesized that, at the test temperature, the possible formation of the solute clustering is responsible for the high activation energy and stress exponent for the creep deformation. Using the mechanical testing, creep test, it is experimentally verified that Cu-Ni system has a miscibility gap at low temperature.     

Coupled problem of heat transfer,hydrodynamics, and solidification in a melt

A mathematical model is constructed which describes thermal and hydrodynamic phenomena accompanying the solidification process in a melt. The equations of hydrodynamics take into account viscoelasticity and compressibility of liquid metal. An example of calculations pertaining to solidification of an ingot is given.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 41, No. 6, pp. 1109–1118, December, 1981.     

Two‐phase thermo‐mechanical and macrosegregation modelling of binary alloys solidification with emphasis on the secondary cooling stage of steel slab continuous casting processes

As an approach towards a better modelling of solidification problems, we introduce a thermo‐mechanical and macrosegregation model that considers a solidifying alloy as a binary mixture made of a liquid and a solid phase. Macroscopic conservation laws for mass, momentum and solute are obtained by spatial averaging of the respective microscopic conservation equations. Assuming local thermal equilibrium, a single equation for the conservation of the mixture energy is then written. A single equation can be obtained for the solute as well by invoking a proper microsegregation rule. The numerical implementation in a two‐dimensional finite element code is then detailed. Lastly, some examples of simulations of academic tests as well as industrial applications for continuous casting of steel slabs are discussed. They particularly enlighten the ability of the formulation to describe the formation of central macrosegregation during the secondary cooling of slab continuous casting processes. Copyright © 2006 John Wiley & Sons, Ltd.     

Analysis of the microstructure obtained by using unidirectional solidification,tungsten inert gas weld and laser surface melt traversing techniques in Al-Mn alloys

A major challenge to solidification theory over nearly three decades has been the understanding, prediction and control of rapidly solidified microstructures. The present paper reports results of systematic and controlled conditions of rapid solidification in Al-Mn alloys, which involved measurement of undercooling, solute concentration and cell spacing for solidification front velocities, which were increased progressively, to the level needed for partitionless solidification into a microsegregation-free solid which, in principle, can be crystalline, quasicrystalline or amorphous. Comparison of the measurements with predictions of theoretical modelling give an encouraging level of agreement.Nomenclature A constant = ²/P²D² - A constant = k(ab)^1/2 - B constant = mC₀p_c/D[1–pI_v(P)] - B constant - C G(Km^–1) - C _EU eutectic composition (at %, wt %) - C ₀ alloy concentration (at %, wt %) - C _L ^* tip concentration in liquid (at %, wt.%) - C _S ^* tip concentration in solid (at %, wt %) - D diffusion coefficient in liquid (m²s^–1) - G température gradient (Km^–1) - I _V(P) Ivantsov function (P exp(P)E₁(P)) - P solute Péclet number = V_SR/2D - R tip radius (m) - T _EU eutectic temperature (K) - T _F melting point of pure substance (K) - T _G arrest growth temperature (K) - T _L liquidus temperature (K) - V _ab absolute stability velocity (ms^–1) - V _s solidification front velocity (ms^–1) - a material constant - b material constant - k distribution coefficient (C_S/C_L) - k constant - m liquidus slope (K/at %, K/wt %) - n exponent - p complementary distribution coefficient (1–k) - Gibbs-Thomson coefficient (/s_f) (Km) - s _f entropy of fusion per mole (J mol^–1K^–1) - T ₀ liquidus-solidus range at C₀(T_S–T_L) (K) - ₁ cell spacing (m) - solid/liquid interface energy - 3.1416 - _c constant = 1–(2k/[1+(2/P)²]^1/2–1+2k)     

Optimizing the microstructures and mechanical properties of Al-Cu-based alloys with large solidification intervals by coupling travelling magnetic fields with sequential solidification

Alloys with large solidification intervals are prone to issues from the disordered growth and defect formation;accordingly, finding ways to effectively optimize the microstructure, further to improve the mechanical properties is of great importance. To this end, we couple travelling magnetic fields with sequential solidification to continuously regulate the mushy zones of Al-Cu-based alloys with large solidification intervals. Moreover, we combine experiments with simulations to comprehensively analyze the mechanisms on the optimization of microstructure and properties. Our results indicate that only downward travelling magnetic fields coupled with sequential solidification can obtain the refined and uniform microstructure, and promote the growth of matrix phase -Al along the direction of temperature gradient.Additionally, the secondary dendrites and precipitates are reduced, while the solute partition coefficient and solute solid-solubility are raised. Ultimately, downward travelling magnetic fields can increase the ultimate tensile strength, yield strength, elongation and hardness from 196.2 MPa, 101.2 MPa, 14.5 % and85.1 kg mm-2 without travelling magnetic fields to 224.1 MPa, 114.5 MPa, 17.1 % and 102.1 kg mm-2,and improve the ductility of alloys. However, upward travelling magnetic fields have the adverse effects on microstructural evolution, and lead to a reduction in the performance and ductility. Our findings demonstrate that long-range directional circular flows generated by travelling magnetic fields directionally alter the transformation and redistribution of solutes and temperature, which finally influences the solidification behavior and performance. Overall, our research present not only an innovative method to optimize the microstructures and mechanical properties for alloys with large solidification intervals,but also a detailed mechanism of travelling magnetic fields on this optimization during the sequential solidification.