Time–temperature–transformation diagram and microstructures of bulk glass forming Pd40Cu30Ni10P20

Jörg F. Löffler, Jan Schroers, William L. Johnson
2000 Applied Physics Letters  
Isothermal crystallization studies were performed on the bulk glass forming alloy Pd 40 Cu 30 Ni 10 P 20 in the undercooled liquid region between the glass transition and liquidus temperature, resulting in a complete time-temperature-transformation ͑TTT͒ diagram for crystallization. The TTT diagram shows a typical "C" shape with the nose at 50 s and 680 K. Assuming steady state nucleation and a diffusion-controlled growth rate, the TTT diagram was successfully fit over the entire range of the
more » ... asurement. The microstructure after isothermal crystallization shows a modulation in Cu and P for all degrees of undercooling. The primary solidified phase is Cu 3 Pd, which forms distinct dendrites at low undercooling. From additional constant cooling experiments, the critical cooling rate to bypass crystallization was determined to be 0.33 K/s. In 1984, it was shown that Pd 40 Ni 40 P 20 1 can be produced up to a thickness of 1 cm in its smallest dimension when the material was fluxed in B 2 O 3 . However, not until the 1990's did research intensify in this new area of "bulk metallic glasses," when a new class of Zr based bulk metallic glasses 2,3 with potential applications was discovered. In particular, the alloy Zr 41.2 Ti 13.8 Cu 12.5 Ni 10 Be 22.5 ͑Vit1͒ 3 has an excellent glass forming ability that has initiated a series of crystallization studies over a wide temperature range below the melting temperature. A complete time-temperaturetransformation ͑TTT͒ diagram for crystallization was measured by containerless electrostatic levitation ͑ESL͒ over the entire temperature range of the undercooled liquid, i.e., from the melting point down to the glass transition temperature, T g . 4 This TTT diagram showed a typical "C" shape with the nose at 51 s and 850 K. The critical cooling rate to bypass crystallization was found to be 1.8 K/s. The TTT diagram could not be described by kinetic formulations as given, for example, by Uhlmann 5 and Davis. 6 This was attributed to the relatively complex crystallization behavior of Vit1, particularly at low temperatures where decomposition precedes nucleation. 7,8 The same TTT diagram as in Ref. 4, where the samples were processed without containers, was obtained from samples crystallized in graphite crucibles. 9 This method is more user friendly compared to ESL and has the advantage of a more reliable temperature reading. Recently, the new Pd based bulk amorphous glass Pd 40 Cu 30 Ni 10 P 20 ͑PCNP͒ was found, 10 which has a glass forming ability far exceeding that of Pd 40 Ni 40 P 20 and approaching or even exceeding that of Vit1. In contrast to Vit1, this Pd based alloy shows no decomposition on the nanometer scale at temperatures near T g . 11 In this letter, we report on crystallization studies of PCNP. Processing this alloy in graphite crucibles, using boron oxide flux, enabled us to measure the complete TTT diagram for crystallization in the undercooled melt and to determine the critical cooling rate. We present results of mi-crostructure analysis of the PCNP melt after crystallization at temperatures near the nose of the TTT diagram. We further give a kinetic formulation which fits the TTT diagram over the entire range of the measurement. Amorphous PCNP was produced by induction melting of Pd, Cu 73.4 P 26.6 , Ni 2 P, and P in a silica tube of 10 mm diameter, using B 2 O 3 oxide flux, and subsequent water quenching. For the crystallization studies, we used a differential thermal analysis ͑DTA͒ setup, 12 equipped with two graphite crucibles. The material was introduced to one of the crucibles, whereas the other was used as a reference. The crucibles were inductively heated in a titanium gettered argon atmosphere and their temperature was measured with type K thermocouples with an accuracy of Ϯ2 K. A control algorithm enabled us to perform isothermal anneals within Ϯ0.5 K of the setpoint. The microstructures of the solidified samples were analyzed with a Jeol JXA-733 electron microprobe equipped with five wavelength-dispersive x-ray ͑WDX͒ spectrometers, which allowed the element concentrations to be determined with an accuracy of Ϯ1 at. %. The crystallization studies were performed on four PCNP samples with masses between 120 and 300 mg, processed in ͑less than 10 mg of͒ B 2 O 3 . The samples were heated up to 1173 K, i.e., 350 K above the liquidus temperature T liq (ϭ823 K, as measured by DTA͒, and subsequently cooled down with a rate of about 25 K/s to a selected temperature T. At that temperature, they were held isothermally until crystallization was detected, i.e., until a temperature rise owing to a release of heat of fusion ͑recalescence͒ was observed. These isothermal undercooling experiments were performed at different temperatures and are summarized in a TTT diagram, which is shown in Fig. 1 for temperatures between the glass transition temperature T g ϭ582 K ͑as measured by differential scanning calorimetry with a heating rate of 10 K/min͒ and the liquidus temperature. The TTT diagram shows a C shape with the nose at 50 s and 680 K. The results of the cooling experiments, performed with rates between 1.35 and 0.20 K/s, are shown in Fig. 2 ͑the inset gives an example for the sample processing͒. Recales-a͒ Electronic
doi:10.1063/1.127084 fatcat:ksqcowxikbcfdmg5ehpugfilge