Lead-free, bronze-based surface layers for wear resistance in axial piston hydraulic pumps

Concerns regarding the safety of lead have provided sufficient motivation to develop substitute materials for the surface layer on a thrust bearing type component known as a valve plate in axial piston hydraulic pumps that consists of 10% tin, 10% lead, and the remainder copper (in wt. %). A recently developed replacement material, a Cu-10Sn-3Bi (wt.%) P/M bronze, was found to be unsuitable as valve plate surface layer, requiring the development of a new alloy. A comparison of the Cu-10Sn-10Pb and Cu-10Sn-3Bi powder metal valve plates showed that the differences in wear behavior between the two alloys arose due to the soft phase bismuth in the alloy that is known to cause both solid and liquid metal embrittlement of copper alloys. A lead-free alternative was developed by using infiltration of high-tin alloys into porous bronze compacts compacted at 350MPa and sintered for 2 hours at 650°C to replicate the dual phase structure of leaded bronze. The resulting engineered composite had a lower volume loss than samples of leaded and bismuth bronze under lubricated wear test conditions. It also possessed a similar coefficient of friction and abrasive/plowing mechanism of wear as the Cu-10Sn-10Pb alloys. The results of wear testing show that the alloy demonstrates wear resistant properties comparable to those of the very successful leaded-bronze alloys. Furthermore, the microstructure of the composite alloy can be engineered by increasing the compaction pressure and sintering temperature of the bronze to balance strength and percentage of the soft phase to match the end use. Also critical to the alloys performance is the control of copper-tin intermetallic formation and growth. The amount of intermetallics could be controlled by minimizing time at temperature during infiltration. In addition, it was shown that growth of the copper-tin Cu3Sn intermetallic can mitigated by the addition of manganese

Radiation Damage in Nanocrystalline Iron

There is strong evidence that grain boundaries act as recombination sites for interstitials and vacancies in a polycrystalline material. The prevailing theory is that grain boundaries act to absorb freely mobile interstitials and vacancies as well as sub-microscopic defect clusters, thereby depleting the region adjacent to the grain boundary of sufficient point defects to produce visible defect structures (e.g. stacking fault tetrahedra, voids, and dislocation loops). This theory is the basis for the design of radiation tolerant materials engineered to remove the large non-equilibrium point defect concentration created in cascades under irradiation by high energy particles by introducing a large number of grain boundary sinks. This thesis presents direct experimental evidence for a number of mechanisms operating to remove radiation damage at grain boundaries by in-situ transmission electron microscopy of free standing nanocrystalline iron films. It was found that the size of dislocation loops found in irradiated iron decreases with smaller grain size until a minimum cluster size is reached (about 2-5nm). The number density of defect clusters appears less affected by the presence of a high number of grain boundary sinks, but does vary strongly with the sink strength of the particular boundaries. If a small grain is defined by grain boundaries are capable of producing a very strong denuded zone, the cluster density can be very small. Therefore, the magnitude of this effect is dependent on the grain boundary character well into the nanocrystalline grain size regime. This work also shows that, in addition to the ability of a grain boundary to absorb sub-microscopic defects, the mobility and absorption of microscopic defect structures (i.e. defect clusters and dislocation loops) at grain boundaries has a strong influence on the response of a material to irradiation. Using in-situ TEM we examined the behavior of these microscopic DCs in nanocrystalline iron. The one-dimensional loop hop of b=1/2 DCs was found totransport DCs to close proximity to GBs where they could annihilate, suggesting a contribution to the long range flux of interstitials to GB sinks. This process had marked effects on the morphology of the irradiated microstructure in nanocrystalline iron, limiting the length of DC strings and reducing the coalescence of DCs into larger defect loops. Furthermore, the research presented in this thesis showed that when large dislocation loops are able to form they may just as easily be lost to grain boundaries: a process which enhances the effect that grain boundary sinks have on the microstructure of the irradiated material. In-situ TEM irradiations in nanocrystalline iron at temperatures from 50K to 773K show that as the temperature of the specimen is increased from cryogenic temperatures (e.g. 50K), the mobility of first b=1/2 defect clusters, then b=1/2 dislocation loops, and finally b= dislocation loops reaches sufficient levels to enable climb to grain boundaries resulting in absorption. In nanocrystalline materials with a high density of grain boundary sinks this activity results in a small downward shift in the transition temperature between a b=1/2 dominated microstructure and one that consists primarily of b= dislocation loops. At 773K, the microstructure is largely free of any dislocation loops in nanocrystalline iron, a stark change from previous work in microcrystalline iron where the b= loops remain stable. The shift in the transition temperature agrees well with initial hypotheses in literature that the nature of the loops remaining in irradiated iron depends on the relative stabilities of the dislocation loops arising due to the elastic anisotropy of iron from thermal magnetic fluctuations, and the high mobility of b=1/2 dislocation loops compared to b= dislocation loops. Using in-situ transmission electron microscopy, the activity of dislocation loops in nanocrystalline iron were directly observed and analyzed using orientation mapping. Bycomparing in-situ TEM results to molecular dynamics simulations, the process of absorption was elucidated for microscopic defect clusters, b=1/2 dislocation loops, and b= dislocation loops.Ph.D., Materials Science and Engineering -- Drexel University, 201

Lead-free, bronze-based surface layers for wear resistance in axial piston hydraulic pumps

Concerns regarding the safety of lead have provided sufficient motivation to develop substitute materials for the surface layer on a thrust bearing type component known as a valve plate in axial piston hydraulic pumps that consists of 10% tin, 10% lead, and the remainder copper (in wt. %). A recently developed replacement material, a Cu-10Sn-3Bi (wt.%) P/M bronze, was found to be unsuitable as valve plate surface layer, requiring the development of a new alloy. A comparison of the Cu-10Sn-10Pb and Cu-10Sn-3Bi powder metal valve plates showed that the differences in wear behavior between the two alloys arose due to the soft phase bismuth in the alloy that is known to cause both solid and liquid metal embrittlement of copper alloys. A lead-free alternative was developed by using infiltration of high-tin alloys into porous bronze compacts compacted at 350MPa and sintered for 2 hours at 650°C to replicate the dual phase structure of leaded bronze. The resulting engineered composite had a lower volume loss than samples of leaded and bismuth bronze under lubricated wear test conditions. It also possessed a similar coefficient of friction and abrasive/plowing mechanism of wear as the Cu-10Sn-10Pb alloys. The results of wear testing show that the alloy demonstrates wear resistant properties comparable to those of the very successful leaded-bronze alloys. Furthermore, the microstructure of the composite alloy can be engineered by increasing the compaction pressure and sintering temperature of the bronze to balance strength and percentage of the soft phase to match the end use. Also critical to the alloys performance is the control of copper-tin intermetallic formation and growth. The amount of intermetallics could be controlled by minimizing time at temperature during infiltration. In addition, it was shown that growth of the copper-tin Cu3Sn intermetallic can mitigated by the addition of manganese.</p

Classroom Response Systems in the Wild: Technical and Non-Technical Observations

Classroom Response Systems (CRS) provide lecturers a communication channel to get feedback from their students. In lessons with large audiences, CRS allow students to ask questions or state issues as the lesson continues. During the development and usage of our new CRS "Tweedback", we observed several technical and non-technical problems, which are likely to be general CRS issues. Observed problems are caused by necessary devices, their connectivity and lecturersâ?? and studentsâ?? different ways to use CRS. In this paper, we describe our observations of technical and nontechnical problems and suggest solutions, which may be applied generically to interactive feedback systems