Design of new metallic glass composites and nanostructured alloys with improved mechanical properties

In this thesis two series of alloys were developed to obtain a combination of high strength, high ductility and work hardening. A new composition design strategy was proposed to create bulk metallic glass composites (BMGC) with high strength, large ductility and excellent work hardening from the brittle MgZnCa bulk metallic glass system. The volume fraction, size of both the dendrites and amorphous matrix can be effectively tuned by varying of the composition, and the yield strength, fracture strength and ductility also varies accordingly. The increase in alloying elements results in an increase in the volume fraction of amorphous matrix and a decrease in the dendrite size, which leads to higher yielding strength but lower ductility. The mechanical properties of the current Mg alloy can be interpreted by considering the BMGCs as a combination of the nanometer scale metallic glass matrix with a ductile dendritic structure. The high strength, large ductility and excellent work hardening observed in the Mg_(91.5 ) Zn_(7.5 ) Ca_1 can be attributed to the homogeneous deformation of nanometre scale amorphous matrices, which delays the formation and rapid propagation of microcracks from the interface. In addition, a series of in-situ-cast nanostructured CuZrTi alloys were successfully designed by appropriate choice of alloying elements and compositions. XRD and TEM analysis shows that the alloys consist of softer Cu solid solution and harder nano-scale Cu_51 Zr_(14 )matrix embedded with retained Cu_5 Zr_ .The Cu_90.5 Zr_(7.5 ) Ti_2alloy exhibited a yield strength of 787MPa, a fracture strength of 1221MPa and room temperature uniform tensile elongation of 5.16%, exhibiting simultaneous ultrahigh strength and large uniform elongation. During tensile tests, the relatively softer (larger) primary Cu dendrites with numerous intragranular nanoprecipiates are believed to yield first, leading to substantial dislocation accumulation due to their relatively large grain size and the uniform distribution of numerous intragranular nanoprecipiates. With further increase in loading, the ultrafine Cu solid solution in the ultrafine clusters starts yielding and dislocation multiplication commences in this ultrafine Cu. Meanwhile, the formation of deformation bands are believed to start in the primary Cu dendrites due to the already existing high dislocation density, both of which further contribute to the work hardening and uniform plastic deformation. Finally, the hard Cu_51 Zr_(14 ) matrix commences plastic deformation and upon further loading, cracks start to form from the interface, leading to the final failure

Direct observation of precipitation along twin boundaries and dissolution in a magnesium alloy annealing at high temperature

Precipitation along twin boundaries and dissolution in a cold-rolled Mg-Y-Nd alloy was directly observed for the first time during annealing at 490 °C. Precipitation occurred concurrently with recrystallization and the combined effect of precipitation and solute segregated to twin boundaries modified the recrystallization behaviour. Precipitates later dissolved into the matrix at the point where full recrystallization was nearly complete. The precipitates and higher solute concentration along original twin boundaries hindered grain growth of newly formed recrystallized grains. Even where twin boundaries had been consumed by recrystallization, the size of recrystallized grains were still controlled by the pre-existing twin boundaries

Alliance Foundation Trial 09: A randomized, multicenter, phase 2 trial evaluating two sequences of pembrolizumab and standard platinum-based chemotherapy in patients with metastatic NSCLC

INTRODUCTION: The sequence of chemotherapy and pembrolizumab may affect antitumor immune response and efficacy of immunotherapy. METHODS: This multicenter, randomized, phase 2 trial was designed to evaluate the efficacy of two sequences of chemotherapy and pembrolizumab in patients with stage 4 NSCLC. Both arms were considered investigational, and the study used a pick a winner design. The primary end point was objective response rate by independent radiologic review after eight cycles (24 wk). Patients were randomized 1:1 to arm A (chemotherapy for four cycles followed by pembrolizumab for four cycles) or arm B (pembrolizumab for four cycles followed by chemotherapy for four cycles). Patients in both arms without disease progression after the initial eight cycles continued pembrolizumab until disease progression, unacceptable toxicity, or a maximum of 2 years. RESULTS: From March 2016 to July 2018, a total of 90 eligible patients were randomized (43 patients to arm A and 47 patients to arm B). The objective response rate at 24 weeks in arms A and B was 39.5 % (95 % confidence interval [CI]: 24.9%-54.1 %) and 40.4 % (95 % CI: 26.4%-54.5 %), respectively ( CONCLUSIONS: Additional evaluation of either sequence in a phase 3 trial is not warranted

Individual effect of recrystallisation nucleation sites on texture weakening in a magnesium alloy: Part 1- double twins

Recrystallised grain nucleation, grain growth and corresponding texture evolution in a cold-rolled rare earth containing WE43 Mg alloy during annealing at 490 �C was fully tracked using a quasi-in-situ electron backscatter diffraction method. The results show nucleation sites, such as double twins, can weaken the deformed texture and for the first time provide direct evidence that recrystallised grains originating from double twins can form the rare earth texture during annealing. Precipitation and recrystallisation occurred concurrently during most of the annealing period, with precipitates forming preferentially along prior grain and twin boundaries. These precipitates effectively retard the recrystallisation due to particle pinning leading to an excessively long time for the completion of recrystallisation. A large portion of recrystallised grains were observed to have 〈0001〉 poles tilted 20e45� away from the normal direction. The RE texture emerges during the nucleation of recrystallised grains and is maintained during subsequent uniform grain growth, which results in a stable RE texture being developed as recrystallisation progresses. The uniform grain growth could be attributed to solute drag suppressing the grain boundary mobility of those grains that had recrystallised with a basal texture and precipitate pinning restricting potential orientated grain growth

Atomic structure of Mg-based metallic glasses from molecular dynamics and neutron diffraction

We use a combination of classical molecular dynamics simulation and neutron diffraction to identify the atomic structure of five different Mg–Zn–Ca bulk metallic glasses, covering a range of compositions with substantially different behaviour when implanted in vitro. There is very good agreement between the structures obtained from computer simulation and those found experimentally. Bond lengths and the total correlation function do not change significantly with composition. The zinc and calcium bonding shows differences between composition: the distribution of Zn–Ca bond lengths becomes narrower with increasing Zn content, and the preference for Zn and Ca to avoid bonding to themselves or each other becomes less strong, and, for Zn–Ca, transforms into a positive preference to bond to each other. This transition occurs at about the same Zn content at which the behaviour on implantation changes, hinting at a possible structural connection. A very broad distribution of Voronoi polyhedra are also found, and this distribution broadens with increasing Zn content. The efficient cluster packing model, which is often used to describe the structure of bulk metallic glasses, was found not to describe these systems well

Enhancing ductility and strength of nanostructured Mg alloy by in-situ powder casting during spark plasma sintering

Due to internal processing defects, bulk nanostructured Mg alloys have high strength but extremely poor ductility. A novel and facile process was designed and in-situ powder casting was initiated during spark plasma sintering. This process significantly reduced processing induced defects, enhanced inter-particle bonding and introduced significant precipitation without extra ageing treatment, leading to improvement of the compressive strength and ductility. The compressive strain of bulk sample consisting of pure cryomilled powder was 3.6% with an ultimate strength of 500 MPa, while cryomilled powder mixed with eutectic Mg-Zn alloy powder obtained a compressive strain of 6.6% and ultimate strength of 506 MPa. The ductility of the sample with mixed powder was increased by 83% without any sacrifice of strength compared to the sample consisting of only pure cryomilled powder

Thermal Stability of Cryomilled Mg Alloy Powder

In this paper, the thermal stability of cryomilled nanocrystalline (NC) AZ31 powder was evaluated by annealing at elevated temperature ranging from 350 to 450 °C. The results show the NC AZ31 powder exhibited excellent thermal stability during short anneals at 350–450 °C, and the mechanisms were investigated in detail. There were two separate growth stages with a transition point at around 400 °C. More specifically, between 350 and 400 °C, NC Mg grains were stable at approximately 32 nm, even after 1 h annealing. At 450 °C, the nano grains grew to 37 nm in the first 5 min and grew quickly to approximately 60 nm after 15 min. However, the grain growth was limited when the annealing time was increased to 60 min. The average grain size remained stable less than approximately 60 nm even after long anneals at temperatures as high as 450 °C (0.78 T/TM), indicating an outstanding degree of grain size stability. This excellent thermal stability can be mainly attributed to solute drag and Zener pinning

Basal slip mediated tension twin variant selection in magnesium WE43 alloy

Tension twinning nucleation and evolution in Mg WE43 alloy over a large sampling area was investigated using a quasi-in-situ EBSD/SEM method during interrupted compression testing. The results showed tension twins with both high and low macroscopic Schmid factor (MSF) were activated under a compressive stress of 100 MPa with a strain rate of 10−1 s−1. Basal slip in most grains dominated at this stress, so nucleation of twin variants required little interaction with non-basal slip, which was different from other studies that reported prismatic slip and/or tension twinning were required to activate some low MSF tension twin variants. The geometric compatibility factor (m') was demonstrated to be an important factor to determine tension twin variant selection assisted by basal slip. The analysis indicated m' played a critical role over MSF in tension twin variant selection during twin nucleation stage, and final twin variant types were insensitive to increasing stress, but they inherited twin variant types determined at twin nucleation stage. Moreover, which specific grain boundary of a grain with hard orientation for basal slip would nucleate which twin variant could be also validated by m' and largely depended on two factors: (a) high value of m' with 1st or 2nd rank between the tension twinning of nucleated twin variant and basal slip in adjoining grains; and (b) intensive basal slip activity in the neighbouring grains before twin nucleation

Individual effect of recrystallisation nucleation sites on texture weakening in a magnesium alloy: Part 1-double twins

Recrystallised grain nucleation, grain growth and corresponding texture evolution in a cold-rolled rare earth containing WE43 Mg alloy during annealing at 490 °C was fully tracked using a quasi-in-situ electron backscatter diffraction method. The results show nucleation sites, such as double twins, can weaken the deformed texture and for the first time provide direct evidence that recrystallised grains originating from double twins can form the rare earth texture during annealing. Precipitation and recrystallisation occurred concurrently during most of the annealing period, with precipitates forming preferentially along prior grain and twin boundaries. These precipitates effectively retard the recrystallisation due to particle pinning leading to an excessively long time for the completion of recrystallisation. A large portion of recrystallised grains were observed to have 〈0001〉 poles tilted 20–45° away from the normal direction. The RE texture emerges during the nucleation of recrystallised grains and is maintained during subsequent uniform grain growth, which results in a stable RE texture being developed as recrystallisation progresses. The uniform grain growth could be attributed to solute drag suppressing the grain boundary mobility of those grains that had recrystallised with a basal texture and precipitate pinning restricting potential orientated grain growth