X. Lin

History-dependent selection of primary cellular/dendritic spacing during unidirectional solidification in aluminum alloys

History-dependent selection of primary cellular/dendritic spacing is investigated systematically during unidirectional solidification of a series of aluminum alloys. A single crystal is formed in the sample before each experimental run, so that the influence of grain boundary on the primary spacing is avoided. The experimental results are compared with those of the two-dimensional crystal growth in the same alloy system and transparent model alloys. It is found that the primary cellular/dendritic spacing is remarkably history dependent. The average primary spacing is dependent not only on the current growth conditions, but also remarkably on the way those conditions were achieved. There exists a wide allowable range of primary spacings for a given growth condition. Experimental results are also compared with the Hunt-Lu model, which shows excellent fit between them, especially on the selection of cellular spacing. By comparing the three-dimensional experiments with the two-dimensional ones, it is also found that the allowable range of the primary spacing for the three-dimensional growth is wider than that for the two-dimensional growth.

On primary dendritic spacing during unidirectional solidification

Abstract In-situ observations of directionally solidifying interface of typical transparent model alloys, succinonitrile-ethanol and succinonitrile-acetone, have been performed on a Hunt-Jackson type temperature gradient stage and a Bridgman apparatus, respectively. Experimental results show that there exists a wide allowable range of primary spacings for both the two-dimensional and the three-dimensional dendritic arrays under a given growth condition. The upper and lower limits of the allowable range are very sharp at the same current growth condition, while the average primary spacing is remarkably related to the history of variation of both temperature gradient and growth velocity. The upper limit, λmax, and the lower one, λmin, of the allowable range as a function of growth velocity, V, obtained from the experiments of step-increment and step-decrement in growth velocity, can be generally expressed as λmax = AV−b, λmin = cV−d, where a, b, c and d are constants for given alloy and temperature gradient. The lower limit obtained experimentally by us is compared with that calculated theoretically with the Hunt-Lu model and the Warren-Langer model; it shows an excellent fit between experimental results and the Hunt-Lu model, and good agreement between experimental results and the Warren-Langer model at the high growth velocity.

Primary spacing selection of CuMn alloy under laser rapid solidification condition

In order to investigate the primary spacing selection under rapid solidification conditions, laser surface rapid resolidification experiments have been performed on Cu-26.6 wt.% Mn alloy. By taking transverse and then longitudinal sections of the directional growth dendrites/cells in the laser traces, the average primary spacing, spacing distribution range as well as the corresponding growth rate are quantitatively measured by SEM and image analyser. It has been found that there exists a wide distribution range in primary spacing under rapid solidification condition. As the growth rate increases, both the distribution range and the average spacing decrease. The maximum, λ1max, minimum, λ1min, and average primary spacing, gl1, as functions of growth rate, Vs, can be given by λ1max = 10.68V− 0.28s, λ1min = 2.42V− 0.22s, and gl1 = 510V− 0.33s, respectively. Our experimental results are compared with the current KGT model for rapid dendritic growth, and a good agreement is found.

Primary cellular/dendritic spacing selection of Al–Zn alloy during unidirectional solidification

Abstract Al–4.95xa0wt% Zn alloy is directionally solidified with Bridgman apparatus to investigate response of cellular/dendritic microstructures and primary spacing to the variation of growth velocity. The results show that, with increasing growth rate, there exist a transition from dendrite to cell and a wide distribution range in primary cellular/dendritic spacing. The maximum, λ max , minimum, λ min , and average primary spacing, λ , as functions of growth rate, V , can be given by λ max =4578 V −0.697 , λ min =1315.7 V −0.6543 , λ =3084.5 V −0.6982 , respectively. The experimental results are compared with the Hunt–Lu model, and a reasonable agreement is found.

Direct observation of phase transformation layers in the undercooled hypoperitectic Ti47Al53 alloy

Abstract The hypoperitectic Ti 47 Al 53 alloy was cyclically superheated in a containerless electromagnetic levitation apparatus in order to reach the state of supercooling. The maximum undercooling of the alloy melt was up to 250xa0K. Critical undercooling ranges for the formation of various competing phases were determined by transmission electron microscopy. The primary phase β in 0 K T K , the primary phase α in 36 K T K , and the primary phase γ in 120 K T K were determined by microstructure analysis. The transient nucleation theory was adopted to explain the phase evolution relationship in the undercooled melt on the consideration of the competing phase with the shortest incubation time having separated from the undercooled melt. Due to different structures of primary and secondary phases, clear fault ribbons from β to α (or α to γ) were detected on the boundary layer between primary and secondary phases. The reason for the formation of fault layers on the boundary layer was given with a view of the crystalline structure evolution.

Microstructure Evolution during Semi-Solid Processing of SCN-Water and Sn-Pb Alloys

Microstructure evolution was investigated during the solidification of succinonitrile-5at.% water transparent alloy and Sn–15 wt.%Pb alloy under mechanical stirring through in-situ observation and quenching, separately. The results showed that the evolution of primary microstructures under stirring experienced the growth of single grain particle and the successive agglomerating and coarsening of multi-particles when the particle size reached a certain value. The increase of stirring rate promoted the globular growth of solidification microstructure after it nucleated in the melt and increase the grain size. Thus, the microstructure during semi-solid processing could be refined by a controlled stirring and cooling process, which depended on the optimization among the stirring rate, cooling rate and temperature at which the stirring rate is changed.