Review of grain ref inement methods for as-cast microstructure of magnesium alloy

As the lightest structural metal,Mg and Mg-based alloys have great potential applications in the aerospace,automotive and nuclear industries.However,such applications have been limited by low ductility and strength.Theoretically,small grain sized structure can synchronously improve its ductility and strength.Yet,universally reliable grain ref inement techniques for the magnesium alloys are still under investigation and some are in strong debating.This paper presents a brief review of development of grain ref inement methods for magnesium alloys,which would contribute to a better understanding of the factors controlling grain ref inement and provide an outlook of future research in this fi eld.

Mechanism of pulse magneto-oscillation grain refinement on pure Al

Pulse magneto-oscillation (PMO) was developed as a novel technique to refine the solidification structure of pure aluminium.Its grain refining mechanism was proposed.The PMO refinement mechanism is that the nucleus falls off from the mould wall and drifts into the melt under the action of PMO.The solidification structure of Al melt depends on the linear electric current density,and also the discharge and oscillation frequencies.The radial pressure of PMO sound wave is the major factor that contributes to the migration of nucleus into the melt.

Niobium alloying effect in high carbon equivalent grey cast iron

The effect of niobium on the formation of NbC phase and solidification structure in high carbon equivalent grey cast iron was investigated.The experimental results indicated that an increase in the niobium content is favorable to refining the graphite and eutectic cell;and the pearlite lamellar spacing is reduced.Based on the thermodynamic calculation the formation of NbC is prior to the eutectic reaction.The reduction in the pearlite lamellar spacing is mainly attributed to the decrease of eutectic temperature with the addition of niobium.Additionally,properties including hardness and wear resistance were improved after the addition of niobium.

Effect of temperature field on solidification structure of pure Al under pulse magneto-oscillation

This article discusses the effect of temperature field on the Pulse Magneto-Oscillation(PMO) induced solidification refinement of pure aluminium to provide more information for the industrial application of the PMO solidification technology.The temperature field is altered mainly by applied variable cooling conditions and pulse parameters.Experimental results show that the refinement effect in the case of full sand mould applied is weakened with the decreasing of cooling rate,however,in the alternative case,the sand mould whose sand bottom was replaced by a graphite block is favorable to the survival of equiaxed nucleus.The refinement mechanism is discussed in terms of the relationship between temperature field and the formation process of solidified structure.The formation or survival of nucleus depends on both temperature field and Joule heat produced by PMO,both low pulse frequency and high pulse current were experimentally confirmed to be effective;and PMO was demonstrated high potential in industrial application.

Effect of cooling rate on solidification structure and linear contraction of a duplex stainless steel

Cooling rate is a key factor that can drastically affect the phase transformation and thermal stress of duplex stainless steels. Therefore, in this research, different sand moulds were used to explore the influence of cooling rate on the solidification of the 2304 duplex stainless steel (DSS). The macro and micro structures of the 2304 DSS were investigated. Small equiaxed grains are obtained in chromite sand mould sample with a lower pouring temperature and a higher cooling rate, whereas coarse columnar and equiaxed grains are found in silica sand and refractory powder mould samples. The size of austenite phase is significantly increased with decreasing cooling rate, while the ferrite phase content ranging from 51.6% to 53.9% does not change obviously. In addition, the linear contraction of the 2304 DSS decreases from 2.34% to 1.09% when the mean cooling rate above 1,173 K increases from 0.99 K·s-1 to 3.66 K·s-1.

EFFECT OF NITROGEN ON GRAPHITE MORPHOLOGY IN GRAY CAST IRON

The distribution of N and morphology of graphite in gray cast iron containing N were studicd by adopting the man-made specimens of Fe-C-Si-Mn alloys with some N added. The N adsorbed along the interfaces between graphite and matrix together with solid- solutioned in graphite is believed to be the principal cause of morpholongical variation of graphite.

Size-dependent undercooling of pure Sn by single particle DSC measurements

The size-dependent solidification undercooling was investigated for single micro-sized particles of pure Sn employing differential scanning calorimeter(DSC).The particles were obtained from a solvent-encapsulation remelting and quenching(SERQ) process.Because of the basically unchanged spherical shape of the measured single particles during a series of continuous heating and cooling processes,it allows studying the independent effect of particle size on undercooling.Applying classical nucleation theory in conjunction with available thermodynamic data yields an increasing undercooling with decreasing particle size.The theoretical description is in good agreement with the experimental data.

Fast calorimetric scanning of micro-sized SnAgCu single droplet at a high cooling rate

The undercooling of the single micro-sized droplet of Sn-3.0Ag-0.5Cu(wt%)alloy has been studied via the newly developed fast calorimetric scanning technique,by which the fast heating and cooling treatment for a single droplet can be realized,with the maximum heating or cooling rate being 1×104K/s.Owing to the nearly spherical shape of the single droplet upon heating and cooling and the resul-tant geometric stability,the influence of the droplet size on the solidification process could be elimi-nated.As a result,the puzzled issue on how to separate the mutual effects of droplet size and cooling rate in the field of rapid solidification has been primarily solved,making it possible to study separately the effect of droplet size and cooling rate.Meanwhile,the in-situ observation on deep undercooling could be actualized in this condition,differing from that obtained only by theoretical calculation.The results showed that the undercooling was increased with the increasing cooling rate,and the maximum in-situ measured undercooling reached 116.9K.The undercooling of the single droplet,however,was increased abruptly when cooled at the rate of 2×103K/s.The undercooling increased slightly as the cooling rate was increased continuously to 1×104K/s,implying the infeasibility for gaining large undercooling only by increasing the cooling rate.

Deformation induced structural evolution in bulk metallic glasses

The structural behavior of binary Cu50Zr50 and ternary Cu50Zr45Ti5 bulk metallic glasses (BMGs) under applied stress was investigated by means of in-situ high energy X-ray synchrotron diffraction. The components of the strain tensors were determined from the shifts of the maxima of the atomic pair correlation functions (PDF) in real space. The anisotropic atomic reorientation in the first-nearest-neighbor shell versus stress suggests structural rearrangements in short-range order. Within the plastic deformation range the overall strain of the metallic glass is equal to the yield strain. After unloading,the atomic structure returns to the stress-free state,and the short-range order is identical to that of the undeformed state. Plastic deformation,however,leads to localized shear bands whose contribution to the volume averaged diffraction pattern is too weak to be detected. A concordant region evidenced by the anisotropic component is activated to counterbalance the stress change due to the atomic bond reorientation in the first-nearest-neighbor shell. The size of the concordant region is an important factor dominating the yield strength and the plastic strain ability of the BMGs.