Compression-Compression Fatigue and Fracture Behaviors of Zr<SUB>50</SUB>Al<SUB>10</SUB>Cu<SUB>37</SUB>Pd<SUB>3</SUB> Bulk-Metallic Glass

Abstract

The compression-compression fatigue and fracture behaviors were studied on the Zr50Al10Cu37Pd3 bulk-metallic glasses (BMGs) under a load control, employing an electrohydraulic machine, at a frequency of 10 Hz (using a sinusoidal waveform) with an R ratio of 10, where R ¼ � min.=� max. (� min. andmax. are the applied minimum and maximum stresses, respectively). The obtained results were compared with those of the tension-tension fatigue tests. The life under the compression-compression fatigue was improved significantly. However the fatigue- endurance limits in two different stress states are comparable. The fracture morphology and the specimen surfaces were observed. The possible different fracture mechanisms under two kinds of stresses were discussed. (doi:10.2320/matertrans.MJ200780) Bulk-metallic glasses (BMGs) have been widely inves- tigated during the past 15 years due to their high strength, high hardness, and high fracture toughness, excellent corrosion, friction, and wear resistance, good magnetic properties and so on. 1,2) To realize their potential structural applications, the understanding of the mechanical properties is paramount, with fatigue properties being of critical importance for the applications involving cyclic loading. 3-6) Some results have been obtained under three-point-bend, four-point-bend, and tension-tension loading conditions. The fatigue fracture surfaces typically include the crack initiation, propagation, fast fracture, and melting regions, in which the crack-propagation area generally dominates the entire fatigue life. 7-10) However, few compression-compression fatigue studies were performed. 11,12) The stress state of the compres- sion-compression fatigue is significantly different from three- point-bend, four-point-bend, and tension-tension fatigue. Are the fatigue life and fatigue-endurance limit different for fatigue tests under various loading conditions? Are there any variations for the deformation mechanisms among different fatigue tests? All these questions are worthy of investiga- tions. In this paper, the Zr50Al10Cu37Pd7 BMG with some nano- scale phases was prepared. The stress versus cycle-life (S/N) curve was studied under the compression-compression fatigue-testing conditions. The surface and the fracture morphology after fatigue were observed, and the mechanisms of the fatigue and fracture were analyzed. The difference between compression-compression and tension-tension fa- tigue mechanisms was discussed. 2. Experiment Procedures The materials used in preparing the alloys are high purity metals: Zr (99.95%), Cu (99.999%), Al (99.999%), and Pd (99.995%). The master alloy with a nominal composition of Zr50Al10Cu37Pd3 was obtained by arc melting. The ingot was melted and flipped at least five times to ensure the chemical elements distributed homogeneously. Then, the Zr50Al10- Cu37Pd3 BMG with some nano-crystalline phase was fabricated by suction-casting the master alloy into a copper mold under a Ti-gettered Ar atmosphere. The dimension of the ingot is 6 mm in diameter and 75 mm in length, respectively. The obtained BMGs sample was centerless grounded. The diameter after centerless grounding is about 5.33 mm. The yield strength is � 1:9 GPa. The elongation of this material is about 2.5%. The structure was examined by the X-ray diffraction (XRD) with a Cu Kradiation and the high-resolution- transmission electron microscopy (HRTEM). The thin film for HRTEM is obtained by twin-injection. The twin-injection solution is 10% HClO4 + 90% C2H5OH in the volume percent. The compression-compression fatigue sample with a dimension of 5:33 mm � 10:66 mm was cut by a diamond saw. The ends of the sample was lapped and polished to ensure the parallelism. A computer-controlled Materials Test System (MTS) servohydraulic-testing machine was used for compression- compression fatigue experiments. Samples were tested at various stress ranges with an R ratio (R ¼ � min.=� max., where � min. andmax. are the applied minimum and maximum stresses, respectively) of 10 under a load-control mode, using a sinusoidal waveform at a frequency of 10 Hz. Upon failures, samples were removed and stored for later obser- vations using the scanning-electron microscopy (SEM) to identify fatigue and fracture mechanisms. 3. Results and Discussions